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Volume 27A, No. 7, July 1996 This Month Featuring: Symposium on Dynamic Behavior of Materials Part II; Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Welding & Joining; Solidification; Materials Processing; and Composite Materials. View July 1996 Table of Contents.
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SYMPOSIUM ON DYNAMIC BEHAVIOR OF MATERIALS PART II
Bauschinger Effect in Haynes 230 Alloy: Influence of Strain Rate and Temperature
ANIRUDDHA THAKUR, KENNETH S. VECCHIO, and SIA NEMAT-NASSER
Quasistatic and dynamic Bauschinger behavior in HAYNES 230 alloy is examined.
At low strain rate (10-3/s), the as-received 230 alloy does not show a drop in
flow stress, i.e., no Bauschinger effect is displayed. At high
strain rate (103/s), a drop in flow stress of 240 MPa was observed upon stress
reversal. In contrast, the precipitation-strengthened condition exhibited a
Bauschinger effect in both low and high strain rate stress-reversal
experiments. The magnitude of the Bauschinger effect was found to increase with
increasing strain rate, forward strain, and decreasing temperature. The
substructure evolution accompanying the forward loading cycles was investigated
by transmission electron microscopy and is related to the back stresses that
developed. The increased Bauschinger stress drop observed at high strain rate
and/or low temperature was correlated to an increased degree of planar slip
under these conditions.
Reaction Sintering of Shock-Compressed Ti+C Powder Mixtures
JONG-HEON LEE, NARESH N. THADHANI, and HENRY A. GREBE
Shock compression was used to make dense compacts of Ti and C elemental powder
mixtures for subsequent reaction sintering in near-net form. The reaction
sintering experiments were performed in an induction-heated hot press at
temperatures below the melting point of Ti, with hold times of less than a few
hours. The unique combination of defect states and packing characteristics
introduced during shock compression results in significant enhancement in the
solid-state chemical reactivity of the powder mixtures. Consequently, the
reaction behavior of the powders is altered, and the reaction mechanism is
dominated by solid-state diffusion, resulting in a microstructure reminiscent
of solid-state processes. Reaction-sintered TiCx compacts, with bulk density in
the range of 3.9 to 4.2 g/cm3 (80 to 85 pct TMD of TiC), were produced in
near-net form. The compacts had a highly refined microstructure (<6-µm
grain size) and microhardness in the range of 1360 to 1934 KHN. In this
article, reaction sintering mechanisms involving solid-state diffusion in Ti+C powder mixture compacts will be described, along with characteristics of the
titanium carbide produced by the combined shock modification and reaction
sintering approach.
Mechanistic Processes Influencing Shock Chemistry in Powder Mixtures of the
Ti-Si, Ti-Al, and Ti-B Systems
T.E. ROYAL, S. NAMJOSHI, and N.N. THADHANI
Shock-recovery experiments were performed on Ti-Si, Ti-Al, and Ti-B powder
mixtures to produce compacts of reacted and unreacted states to characterize
the reaction product microstructure as well as the shock-compressed
configuration of unreacted constituents. Microstructural and X-ray diffraction
(XRD) peak broadening analyses were performed on unreacted compacts to
determine the configurational changes occurring during shock compression of
powders and to quantify the differences in the deformation response of the
reactants in each system. The results of the present work demonstrate that the
mechanistic processes leading to shock-induced reactions are dominated by
differences in the shock-compression response of the powder mixture reactants.
It was established that the propensity for initiation of shock-induced chemical
reaction decreases from Ti-Si to Ti-B to Ti-Al powder mixtures, irrespective of
the differences in the thermodynamic characteristics of these systems. The
differences in the mechanical properties of the reactants influence the
shock-compression response (deformation or fracture and flow behavior) and
mixing of reactants, and therefore, the configuration changes prior to
initiation of shock-induced chemical reactions.
Aspects of Dynamic Recrystallization in Shaped Charge and Explosively Formed
Projectile Devices
C. FENG, L.E. MURR, and C.-S. NIOU
Under a shock wave, a shaped charge (SC) liner or an explosively formed
projectile (EFP) device transforms into a jet and a slug. At various
laboratories, it was found that the transformation was closely related to
extensive plastic flow occurring at high strain rates. Along with the shape
transformation, there is evidence of changes in hardness, strength, grain
configuration, microstructure, and resultant texture in the slug when compared
to the starting liner. This article examines the effect of material parameters
on jet and slug formation. Based on microstructure (metallography and
transmission electron microscopy) and texture data, an analysis is made of the
recovery and recrystallization processes which are believed to be closely
associated with the deformation. Grain rotations and shifts in texture are
described from the starting liners to the recovered slugs. While fully
recrystallized grains are found in a ``soft'' recovered copper EFP slug, only
partial recrystallization is achieved in a tantalum counterpart. The difference
in extent of recrystallization of copper and tantalum EFP slugs is analyzed. An
attempt is made to explain the difference in terms of the deformation
structures between the two types of metal. Experimental evidence tends to
support a grain rotation concept for recrystallization phenomena.
ALLOY PHASES
On the Stable Mg-Zn-Y Quasicrystals
Z.P. LUO, H.X. SUI, and S.Q. ZHANG
By the conventional air-cooled casting method, bulk ingots with a large
fraction of stable Mg-Zn-Y icosahedral quasicrystals, both simple and
face-centered, were obtained. The quasicrystal structures were directly
confirmed by high-resolution electron microscopy (HREM) observations. The
electron diffraction patterns from the quasicrystals were studied by computer
simulations. A coexisting crystalline phase of the quasicrystals was identified
as the Mg7Zn3 phase, which was proved to be the (1/1) approximant of the
quasicrystals. The quasicrystals were stable during a continuous heating
process. However, at high temperature, oxidation occurred, and thus, Y2O3 and
MgO products were formed. Oxidation at high temperatures is characteristic for
the Mg-Zn-Y alloys and differs from Al-base alloys.
Microstructural Aspects of the Dissolution and Melting of Al2Cu Phase in
Al-Si Alloys during Solution Heat Treatment
A.M. SAMUEL, J. GAUTHIER, and F.H. SAMUEL
The dissolution and melting of Al2Cu phase in solution heat-treated samples of
unmodified Al-Si 319.2 alloy solidified at ~10°C/s were studied using
optical microscopy, image analysis, electron probe microanalysis (EPMA), and
differential scanning calorimetry (DSC). The solution heat treatment was
carried out in the temperature range 480°C to 545°C for solution times of
up to 24 hours. Of the two forms of Al2Cu found to exist, i.e., blocky and
eutectic-like, the latter type is more pronounced in the unmodified alloy (at
~10°C/s) and was observed either as separate eutectic pockets or
precipitated on preexisting Si particles, 
-iron phase needles, or the blocky
Al2Cu phase. Dissolution of the (Al + Al2Cu) eutectic takes place at
temperatures close to 480°C through fragmentation of the phase and its
dissolution into the surrounding Al matrix. The dissolution is seen to
accelerate with increasing solution temperature (505°C to 515°C). The
ultimate tensile strength (UTS) and fracture elongation (EL) show a linear
increase when plotted against the amount of dissolved copper in the matrix,
whereas the yield strength (YS) is not affected by the dissolution of the Al2Cu
phase. Melting of the copper phase is observed at 540°C solution
temperature; the molten copper-phase particles transform to a shiny,
structureless phase upon quenching. Coarsening of the copper eutectic can occur
prior to melting and give rise to massive eutectic regions of (Al + Al2Cu).
Unlike the eutectic, fragments of the blocky Al2Cu phase are still observed in
the matrix, even after 24 hours at 540°C.
Communication: Stable and Metastable Ordered Phases in Microcrystalline Alloys Ni (Fe, Mn, Ti)
V.I. GOMANKOV, V.G. FEDOTOV, V.V. SOSNIN, O.M. ZHIGALINA, AND V.V. SUMIN
TRANSFORMATIONS
Splitting Phenomena Occurring in the Martensitic Transformation of Cr13 and
CrMoV14 Stainless Steels in the Absence of Carbide Precipitation
C. GARCIA DE ANDRÉS, J.A. JIMÉNEZ, and L.F. ÁLVAREZ
Previously unknown splitting phenomena were detected in the martensitic
transformation of XCr13 and XCrMoV14 stainless steels using high resolution
dilatometric analysis. These splittings, which are denominated
MS0 in this article, indicate the martensitic
subtransformation of areas of austenite rich in carbon and carbide-forming
elements. In contrast to other types of splitting known until now, the
MS0 occur in the absence of carbide precipitation during
cooling. From the experimental results obtained in this study, it can be
concluded that the splittings resulted from concentration gradients produced in
the austenite as a consequence of the partial or total dissolution of M23C6 carbides during heating.
Communication: Crystallization of Amorphous Phase in Sputter-Deposited Ti-Al Alloy Thin Films
R. BANERJEE, S. SWAMINATHAN, J.M.K. WIEZOREK, R. WHEELER, AND H.L. FRASER
Communication: An Analysis of Static Recrystallization During Continuous, Rapid Heat Treatment
S.L. SEMIATIN, I.M. SUKONNIK, AND V. SEETHARAMAN
TRANSPORT PHENOMENA
Pressure Dependence of Anomalous Diffusion of Zirconium in 
-Titanium
HIDEKI ARAKI, YORITOSHI MINAMINO, TOSHIMI YAMANE, TETSUYA NAKATSUKA, and
YOSHINARI MIYAMOTO
The impurity diffusion coefficients of Zr in -Ti have been determined at temperatures from 1173 to 1773 K under pressures of 0.1 MPa and 1.0, 2.1, and 3.0 GPa using Ti/Ti-3.06 at. pct Zr diffusion couples. The Arrhenius plots of the diffusion coefficients exhibit strong curvature, and their degree becomes larger with increasing pressure. The activation volumes, 
V, evaluated from the isothermal pressure dependence of the diffusion coefficient are 0.22 to
0.36 
V, where V is the atomic volume of the host metal. The values of
V/V are very close to the ratio of 0.33 for self-diffusion in -Ti
reported by Jeffery and the ratios of 0.28 to 0.41 for W and Sn diffusion in
-Ti reported by Araki et al., whereas they are considerably smaller than
that normally expected for diffusion in bcc metals via ``simple'' monovacancy
mechanism. The increase in degree of curvature in the Arrhenius plot by
pressure and the small values of V/V can be interpreted in terms of the
model of phonon-assisted diffusion jumps via monovacancies.
Internal Friction in Hydrogen-Charged CrNi and CrNiMn Austenitic Stainless Steels
V.G. GAVRILJUK, H. H;auANNINEN, S.YU. SMOUK, A.V. TARASENKO, and K. ULLAKKO
Relaxation and hysteretic phenomena caused by hydrogen in Cr18Ni15, Cr25Ni20,
and Cr18Ni16Mn10 steels have been studied by using a low-frequency internal
friction (IF) technique. Five IF peaks were observed in the temperature range
of 80 to 450 K; three of them are of relaxation nature and two others have a
hysteretic character. The enthalpies of activation have been evaluated by means
of thermoactivation analysis. Short-range migration of hydrogen atoms has been
found to be responsible for the relaxation peaks, while the hysteretic peaks
have been attributed to the outgassing processes accompanied by cracking. It
follows from the data on orientation dependence of the relaxation strength and
values of the activation enthalpies that relaxation has a Snoeklike nature and
is caused by reorientation of complexes of hydrogen atoms with substitutional
solutes causing noncubic defects, the symmetry of which is not higher than
orthorhombic. Study of the composition effects has led to the conclusion that
different substitutional solutes contribute to different components of the
relaxation spectra in accordance with their influence on hydrogen diffusivity.
Effect of electron irradiation on hydrogen-induced relaxation was studied and
explained in terms of short-range atomic order. No indication of
hydrogen-induced Snoek-Köster (SK) relaxation was observed in accordance
with the data available evidencing absence of SK relaxation in face-centered cubic (fcc) metals having low values of stacking fault energy.
Control of Iron Nitride Layers Growth Kinetics in the Binary Fe-N System
L. TORCHANE, P. BILGER, J. DULCY, and M. GANTOIS
This study is within the framework of a research program dedicated to defining
the optimal conditions for the nitriding of iron and steels at atmospheric
pressure by using various mixtures, NH3-N2-H2 and NH3-Ar. After studying the
mechanisms of phase formation and mass transfer at the gas-solid interface, a
mathematical model is developed in order to predict the nitrogen transfer rate
in the solid, the nitride layer growth rate, and the nitrogen concentration
profiles. In order to validate the model and to show its possibilities, it is
compared with thermogravimetric experiments, analyses, and metallurgical
observations (X-ray diffraction, optical microscopy, and electron microprobe
analysis). The results obtained allow us to demonstrate the sound correlation
between the experimental results and the theoretical predictions. By applying
the model to the iron-nitrogen binary system, when the 
/
'/
configuration referred to the Fe-N phase diagram is formed, we have
experimentally determined the effective diffusion coefficient of nitrogen in
the phase. The latter is constant for a composition of the nitride
between 8 and 9.5 wt pct nitrogen. All the results obtained show that it is
possible, by means of dynamic gas flow regulation, to eliminate the incubation
period and to control the thickness, composition, and structure of the compound
layer at the beginning of the treatment.
MECHANICAL BEHAVIOR
Increased Ductility in High Velocity Electromagnetic Ring Expansion
MARINA ALTYNOVA, XIAOYU HU, and GLENN S. DAEHN
Thin rings have been rapidly expanded using large, transient magnetic fields to
study the effect of deformation velocity on strains to failure of ductile
metals. A classical electrodynamics analysis similar to one developed
previously by Gourdin was employed to estimate sample velocities. Within
expansion velocities studied (50 to 300 m/s), the experimental results show
that ductility of Al 6061 and OFHC Cu increases monotonically with increasing
velocity. In each case, sample strain at failure is almost twice as great at
300 m/s as in the static condition. Comparison to a one-dimensional
rigid-viscoplastic dynamic finite element method analysis suggests that
inertial effects are mainly responsible for enhanced ductility over a wide
range of velocity.
Microstructure and Tensile Behavior of Nitrogen-Alloyed, Dual-Phase Stainless Steels
H. BERNS, J. KLEFF, G. KRAUSS, and R.P. FOLEY
Two alloys of high-nitrogen stainless steel have been heat treated to produce
dual-phase microstructures. The first alloy, N10CrNiMo17 1, a Ni-containing
stainless steel, was processed conventionally. The second alloy, N20CrMo17, a
Ni-free stainless steel, was processed to obtain a higher nitrogen content by
pressurized electroslag remelting. The martensite in N10CrNiMo17 1 was
homogeneously distributed in the ferrite and obtained a near-constant volume
fraction as a function of intercritical annealing temperature. Microprobe
analysis and microhardness measurements of the martensite constituent suggested
that up to 0.4 pct N was dissolved in the austenite before quenching. Austenite
formation, martensite transformation, undissolved nitrides, and retained
austenite were evaluated by transmission electron microscopy (TEM). The
Ni-containing alloy exhibited classic dual-phase tensile behavior in that
continuous yielding was observed together with good combinations of ultimate
tensile strength and total elongation. The martensite constituent in alloy
N20CrMo17 was concentrated within bands. Comparison of tensile properties of
the two alloys at similar volume fractions and hardness levels of martensite
and ferrite showed that the microstructure containing banded martensite had
inferior combinations of strength and ductility. The degradation of tensile
ductility was accompanied by a fracture mode transition from microvoid
coalescence to transgranular cleavage. The deformation and fracture behavior of
both alloys were related to the microstructure.
Characterization and Mechanical Properties of Ultrahigh Boron Steels Produced by Powder Metallurgy
J.A. JIMÉNEZ, G. GONZÁLEZ-DONCEL, and O.A. RUANO
The present work is part of an investigation into the use of rapid
solidification and powder metallurgy techniques to obtain iron-boron alloys
with good mechanical properties. Two Fe-B binary alloys and two ultrahigh boron
tool steels were gas atomized and consolidated by hot isostatic pressing (HIP)
at temperatures ranging from 700°C to 1100°C to have a fine
microstructure. Optimum properties were achieved for the binary alloys at low
consolidation temperatures, since the solidification microstructure from the
original powders is eliminated and, at the same time, fine microstructures and
low porosity are obtained in the alloys. At high temperatures and low strain
rates, three of the four alloys exhibited low stress exponents, but only the
Fe-2.2 pct B alloy showed tensile elongations higher than 100 pct. At low
temperatures, only the Fe-2.2 pct B alloy deformed plastically. This alloy
showed values of tensile elongation and ultimate tensile strength that were
strongly dependent on testing and consolidation temperatures.
Effect of Phase Composition and Hydrogen Level on the Deformation Behavior of Titanium-Hydrogen Alloys
O.N. SENKOV and J.J. JONAS
Compression tests were carried out on a series of titanium-hydrogen alloys
(containing up to 31 at. pct H) over the temperature range 500°C to 1000
°C within the , + , and phase fields. The effect of hydrogen
content was studied on the flow stress, rate of work hardening, and mechanical
anisotropy. Hydrogen in solid solution produces significant softening of the
phase and ( + ) phase mixture, while it induces hardening of the
phase. The occurrence of strain softening was detected in the ( + ) phase
field as well, the extent of which depends on temperature, strain rate, and
hydrogen concentration. The work required to deform the Ti-H alloys is
sensitive to the hydrogen concentration and has the lowest value at
interstitial levels that correspond to the ( + )/ phase boundary at the temperature of interest.
Dynamic Strain Aging and Hydrogen-Induced Softening in Alpha Titanium
O.N. SENKOV and J.J. JONAS
Compression tests were carried out on samples of commercial-purity titanium
charged with up to 4.7 at. pct hydrogen. Strain rates of 10-3 to 1s-1 were employed and testing was limited to the phase
field at temperatures of 773 to 973 K. The dependences of the flow stress on
strain, strain rate, and temperature were determined. A plateau or bulge
appeared in the temperature and strain-rate dependences of the flow stress, and
the work-hardening rate also showed peaks. Serrations were observed on some of
the stress-strain curves. All these features indicated that dynamic strain
aging (DSA) was occurring. Analysis of the results (together with data from
other authors) indicates that there are three ranges of DSA behavior in this
material within the experimentally investigated temperature range; these appear
to be associated with the diffusion of iron, carbon, and oxygen, respectively.
Alloying with hydrogen decreases the magnitude of the DSA attributable to these
elements and displaces the phenomenon to higher temperatures and/or to lower
strain rates. The dependence on strain rate and temperature of the relative
softening attributable to hydrogen addition was determined. The results
indicate that hydrogen-induced softening is related to the occurrence of DSA in
this temperature range. Possible explanations for this relationship are
discussed.
Characterization of Superplastic Deformation Behavior of a Fine Grain 5083 Al Alloy Sheet
R. VERMA, P.A. FRIEDMAN, A.K. GHOSH, S. KIM, and C. KIM
Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct
Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial
tension over the temperature range of 500°C to 565°C. Strain rate
sensitivity values >0.3 were observed over a strain rate range of 3X10-5 s-1 to 1X10-2 s-1, with a maximum value of 0.65 at 5X10-4 s-1 and 565°C. Tensile elongations at constant strain rate
exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained
under constant crosshead speed and variable strain rates. A short but rapid
prestraining step, prior to a slower superplastic strain rate, provided
enhanced tensile elongation at all temperatures. Under the two-step schedule, a
maximum tensile elongation of 600 pct was obtained at 550°C, which was
regarded as the optimum superplastic temperature under this condition. Dynamic
and static grain growth were examined as functions of time and strain rate. It
was observed that the dynamic grain growth rate was appreciably higher than the
static growth rate and that the dynamic growth rate based on time was more
rapid at the higher strain rate. Cavitation occurred during superplastic flow
in this alloy and was a strong function of strain rate and temperature. The
degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic
pressure during deformation, which produced a tensile elongation of 671 pct at
525°C.
Multiple Matrix Cracking in a Fiber-Reinforced Titanium Matrix Composite under High-Cycle Fatigue
D.P. WALLS, J.C. McNULTY, and F.W. ZOK
An experimental study of multiple matrix cracking in a fiber-reinforced
titanium alloy has been conducted. The focus has been on the effects of stress
amplitude on the saturation crack density and the effects of crack density on
hysteresis behavior. Comparisons have been made with predictions based on unit
cell models, assuming the sliding resistance of the interface to be
characterized by a constant interfacial shear stress. In addition, independent
measurements of the sliding stress have been made using fiber pushout tests on
both pristine and fatigued specimens.
A Comparison of Fracture Behavior of Low Alloy Steel with Different Sizes of Carbide Particles
G.Z. WANG and J.H. CHEN
The fracture behaviors of low alloy steels with similar grain sizes but
different sizes of carbide particles were investigated using precracked and
notched specimens. The results indicate that in precracked specimens (COD),
steel with coarser carbide particles has a lower toughness than steel with
finer carbide particles over a temperature range from -196°C to -90°C. However, in notched specimens (four-point bending (4PB) and Charpy V),
these two steels shows similar toughness at low temperature where specimens are
fractured by cleavage without fibrous cracking. In the transition temperature
range, the steel with coarser carbide particles conversely shows a little
higher toughness due to the longer extension length of the fibrous crack. This
phenomenon indicates that in precracked specimens, the second-phase particles
play a leading role in cleavage fracture, while in notched specimens, the grain
size dominates the fracture behavior.
PHYSICAL CHEMISTRY
Gibbs Energies of Formation of Chromium Carbides
S. ANTHONYSAMY, K. ANANTHASIVAN, I. KALIAPPAN, V. CHANDRAMOULI, P.R. VASUDEVA RAO, C.K. MATHEWS, and K.T. JACOB
The carbon potentials corresponding to the two-phase mixtures Cr + Cr23C6,
Cr23C6 + Cr7C3, and Cr7C3 + Cr3C2 in the binary system Cr-C were measured in
the temperature range 973 to 1173 K by using the methane-hydrogen gas
equilibration technique. Special precautions were taken to prevent oxidation of
the samples and to minimize thermal segregation in the gas phase. The standard
Gibbs energies of formation of Cr23C6, Cr7C3, and Cr3C2 were derived from the
measured carbon potentials. These values are compared with those reported in
the literature. The Gibbs energies obtained in this study agree well with those
obtained from solid-state cells incorporating CaF2 and ThO2(Y2O3) as solid
electrolytes and sealed capsule isopiestic measurements reported in the
literature.
WELDING & JOINING
Microstructural Development in NiAl/Ni-Si-B/Ni Transient Liquid Phase Bonds
W.F. GALE and S.V. OREL
A transmission electron microscopy (TEM) based investigation of microstructural
development during transient liquid phase bonding of near-stoichiometric NiAl
to commercial purity nickel is presented in this article. The work described
employed Ni-4.5 wt pct Si-3.2 wt pct B (BNi-3) melt-spun interlayers. The
precipitation of both Ni-Al based phases and borides within the joint and
adjacent substrate regions is discussed. The article considers martensite
formation (within the NiAl substrate) and the precipitation of L12 type phases
(both within the joint and at the interface with the NiAl substrate). The
relative roles of the two substrate materials (NiAl and Ni) in the isothermal
resolidification process are identified. The preferential formation of Ni3B
boride phases in the Ni substrate near the original location of the Ni
substrate-joint interface is discussed and contrasted with the absence of
similar events in the NiAl substrate.[el18]
SOLIDIFICATION
Influence of Temperature Transients on the Hot Workability of a Two-Phase Gamma Titanium Aluminide Alloy
V. SEETHARAMAN and S.L. SEMIATIN
The hot deformation behavior, microstructure development, and fracture
characteristics of a wrought two-phase -titanium aluminide alloy
Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were investigated
with special reference to the influence of temperature transients immediately
preceding plastic deformation. Specimens were soaked at 1321°C or 1260°C, cooled directly to test temperatures of 1177°C and 1093°C, and
upset under conditions of constant strain rate and temperature. Plastic flow
behavior and microstructure evolution occurring in tests involving prior
temperature transients were compared with those occurring in specimens which
were directly heated to the test temperature and upset under identical
deformation conditions. Flow curves associated with prior exposure at 1321°C
exhibited very sharp peaks and strong flow softening trends compared to those
obtained under isothermal conditions, i.e., involving no temperature
transients. During cooling from 1321°C, the metastable phase undergoes
limited or complete decomposition into /2 + lamellae, depending on
the final temperature (1177°C/1093°C). Subsequent hot deformation leads
to partial globularization of the lamellae together with extensive kinking and
reorientation of lamellae. In contrast, isothermal deformation at 1177°C/1093°C preserves the fine, equiaxed microstructure, through
dynamic recrystallization of the grains. Cracking observed in specimens deformed at
1093°C and 1.0 s-1 after exposure at 1321°C has been attributed
to the low rate of globularization as well as the occurrence of shear
localization. Plastic flow behavior observed in this work is compared with that
observed in several single-phase and two-phase gamma titanium aluminide alloys
in order to identify mechanism(s) responsible for flow softening.
The Improved Microstructures and Properties of 7075 Alloys Produced by a Water-Cooling Centrifugal Casting Method
JIEN-WEI YEH, SHANG-HAW JONG, and WEN-PIN LIU
A centrifugal water-cooling casting method was used to cast a 7075 alloy with
the aim of refining the grain and inclusion size and improving mechanical
properties in the wrought condition. Conventional ingot casting methods were
also used and investigated for comparison with the centrifugal casting method.
The results show that by the centrifugal casting method, a small equiaxed grain
size, 17 µm, is obtained in as-cast condition. Only 50 minutes are required
for material homogenization. After rolling to obtain sheet, a grain size of 15X8X6 µm and an inclusion size of 2 to 3 µm are achieved.
Fine-grained centrifugal-cast 7075 alloy exhibits higher strength than the
ingot-cast one in the early stages of aging but poorer in the latter stages.
However, its ductility and combination of strength and ductility is superior to
the ingot-cast ones at all aging times. The reduction in strength in the latter
aging stages for the fine-grained structure arises from its higher volume
fraction of soft precipitate free zones. The improved ductility is attributed
to the higher fraction of transgranular fracture, higher transgranular fracture
strain, and intergranular fracture strain. Fine-grained 7075 alloy also
displays significant improvements in the exfoliation corrosion resistance.
These improvements are related to the increased density of attacking sites on
the surface and the increased turns for crack propagation along grain
boundaries.
Communication: The Rayleigh Instability and the Origin of Rows of Droplets in the Monotectic Microstructure of Zinc-Bismuth Alloys
B. MAJUMDAR AND K. CHATTOPADHYAY
MATERIALS PROCESSING
Recrystallization in Oxide-Dispersion Strengthened Mechanically Alloyed Sheet Steel
R.C. KLUG, G. KRAUSS, and D.K. MATLOCK
Systematic annealing at temperatures between 1300°C and 1380°C was
applied to sheets of INCOLOY MA-956, an oxide-dispersion strengthened (ODS),
mechanically alloyed, iron-base steel containing (in mass percent) 20.8Cr,
5.0Al, 0.5Y2O3, and 0.5Ti. The billets, comprised of hot iso[chstatically
pressed (``hipped''), mechanically alloyed powder, were hot- and cold-rolled to
produce a 0.5-mm-thick sheet with a strong (100)<110> deformation texture.
Light and transmission electron microscopy established that recrystallization
initiated by nucleation at the sheet centerline. Initial rapid growth of the
centerline-nucleated grains, designated stage I, resulted in plate-shaped
grains oriented parallel to the rolling plane at the sheet centerline.
Subsequent growth, designated stage II, was developed by planar growth fronts
through the sheet thickness at a slower rate. The final product was a very
coarse grain structure, sometimes with only a single grain through the sheet
thickness. The recrystallization kinetics were typified by an incubation time,
a temperature dependance characterized by an activation energy of 506 kJ/mole,
and a decreasing rate of boundary migration with increasing time at
temperature. The microstructural evolution is discussed in terms of the
influences of deformation texture, residual stress, dislocation substructure,
and oxide dispersion on the recrystallization process.
Effect of Holding Time in the (
+ 
) Temperature Range on Toughness of Specially Austempered Ductile Iron
TOSHIRO KOBAYASHI and SHINYA YAMADA
Austempered ductile iron (ADI) finds wide application in the industry because
of its high strength and toughness. The QB' process has been developed to
produce a fine microstructure with high fracture toughness in ADI. This process
involves reaustenitizing a prequenched ductile iron in the (+)
temperature range followed by an isothermal treatment in the bainitic
transformation temperature range. In the present work, the effect of holding
time in the (+) temperature range on the structure and un-notched
toughness of ADI has been studied. Prior to the austempering treatment, the
as-cast ductile iron was heat treated to obtain martensitic, ferritic, and
pearlitic matrix structures. In the case of prequenched material (martensitic
matrix), the un-notched impact toughness increased as a function of holding
time in the (+) temperature range. The reaustenitization heat treatment
also resulted in the precipitation of fine carbide particles, identified as
(Fe,Cr,Mn)3C. It was shown that the increase in holding time in the (+)
temperature range leads to a reduction in the number of carbide particles. In
the case of a ferritic prior structure, a long duration hold in the (+)
temperature range resulted in the coarsening of the structure with a marginal
increase in the toughness. In the case of a pearlitic prior structure, the
toughness increased with holding time. This was attributed to the decomposition
of the relatively stable carbide around the eutectic cell boundary with longer
holding times.
Milling Dynamics: Part II. Dynamics of a SPEX Mill and a One-Dimensional Mill
D. MAURICE and T.H. COURTNEY
The dynamics of mechanical milling can be described in either local or global
terms. Local descriptions consider aspects of powder deformation, fracture, and
welding that transpire when powder is entrapped between grinding media during a
typical media collision. Global descriptions consider heterogeneous aspects of
milling in specific grinding devices; for example, the distribution of impact
energies and powder segregation within a mill. In part I of this series, we
described facets of attritor global dynamics. In this article, we do the same
for a SPEX mill, (SPEX is a trademark of SPEX Industries, Edison, NJ), and for a hypothetical one-dimensional vibratory mill. In a following companion article,
we compare the dynamics of an attritor with a SPEX mill and, by extension,
other kinds of mills. Specific attention is paid to the relative efficiencies
of these devices and the characteristics of powder processed in them.
Milling Dynamics: Part III. Integration of Local and Global Modeling of Mechanical Alloying Devices
D. MAURICE and T.H. COURTNEY
In previous articles, we have attempted to describe the global dynamics of an
attritor and a SPEX mill, two devices commonly used for mechanical alloying
(MA). In this, the last of this series, we compare and contrast these devices
with respect to a number of features. These include alloying kinetics and the
properties, scale, and structure of processed powder. In addition, we
illustrate how a ``local'' description of MA is useful in examining the
interaction between material properties and process parameters, the latter of
which is often imposed by the characteristics of the device used to achieve
MA.
Communication: An Optical Method for Determining the Surface Orientation of Crystals
LAURI PIRTTIAHO AND JACK BLAKELY
COMPOSITE MATERIALS
Effect of Magnesium on the Aging Behavior of Al-Zn-Mg-Cu/Al2O3 Metal Matrix Composites
MING-CHUN CHOU and CHUEN-GUANG CHAO
The effect of magnesium content on the aging behavior of Al-Zn-Mg-Cu alloy
reinforced with alumina (Al2O3) was studied by using the differential scanning
calorimetry (DSC) technique and hardness measurement. The magnesium contents
were studied in the range from 1.23 to 2.97 wt pct. The addition of magnesium
was found to increase the coherent Guinier-Preston (GP) zones in composites.
The apparent formation enthalpy of GP zones of composites (0.1 Vf) was 0.932
cal/g for 1.23 wt pct magnesium content and 1.375 cal/g for 2.97 wt pct
magnesium content. The precipitation time to achieve the maximum hardness in
the composites depends on the magnesium content. The time changed from 12 to 48
hours as the magnesium content increased from 1.23 to 2.97 wt pct. Both Vickers
microhardness and Rockwell hardness increased with increasing magnesium
content. The maximum hardness occurred in the composites that contained maximum
amounts of GP zones and 
' precipitates. However, the microhardness of the
composites was always lower than that of monolithic alloys due to the alumina
fibers which caused the suppression of GP zones and ' formation in the
composites.
Growth Behavior of Microstructurally Short Cracks in the 6061 Aluminum Alloy
with and without 22 Vol Pct SiC Whiskers
HIROYUKI TODA and TOSHIRO KOBAYASHI
Short crack growth behavior of the 6061 Al alloy with and without SiC whiskers
was investigated. Fluctuations in the growth rate of short cracks converge with
growth of the cracks and become substantially constant between 25 and 40 µm
in the metal matrix composite (MMC) and 110 and 183 µm in the unreinforced
alloy. This is attributed to the release of the short cracks from the
microstructural effects, i.e., the interaction with reinforcement structure in
the MMC and grain boundaries in the unreinforced alloy. Furthermore, there
exists slowing down of short crack advance in the MMC, and this was explained
from rapid development of crack closure obtained in this study.
The Control of Grain Size and Distribution of Particles in a (6061 Alloy)m/(Al2O3)p Composite by Solutionizing Treatment
S.K. VARMA, JAVIER PONCE, MICHAEL SOLIS, SHANE ANDREWS, and DANIEL SALAS
The grain growth during an isothermal treatment at a solutionizing temperature
of 540°C has been studied in a composite containing 6061 aluminum alloy
matrix with Al2O3 particles. The grain growth law is generally applicable to the composites containing 0.10, 0.15, and 0.20 volume fraction of the Al2O3
particles (VFAP). It has been observed that the grain growth process involves
the disintegration of the agglomerated particles first and then particles
coalesce at longer solutionizing times in the composite containing 0.20 VFAP.
The process of coalescence has not been observed up to a heating time of 20
hours at this temperature in the composites containing 0.10 and 0.15 VFAP. The
transmission electron microscopy (TEM) study indicates the generation of a
large number of dislocations in both the matrix and the area adjacent to the
particles. The dislocation densities at these two locations in the composites
increase with an increase in VFAP and the particle size. The microhardness
measurements confirm the microstructural observations, and the hardness values
for the composite and the matrix appear to be more sensitive to the particle
distribution and the particle size compared to the grain size.
Interface Effects on the Micromechanical Response of a Transversely Loaded Single Fiber SCS-6/Ti-6Al-4V Composite
S.G. WARRIER, D.B. GUNDEL, B.S. MAJUMDAR, and D.B. MIRACLE
The ability of a fiber-matrix interface to support a transverse load is
typically evaluated in straight-sided composite specimens where a stress
singularity exists at the free surface of the interface. This stress
singularity is often the cause of crack initiation and debonding during
transverse loading. In order to develop a fundamental understanding of the
transverse behavior of the fiber-matrix interface, it is necessary to alter the
crack initiation site from the free surface to an internal location. To achieve
this objective, a cross-shaped specimen has been recently developed. In this
study, based on the experimentally observed onset of nonlinearity in the
stress-strain curve of these specimens and finite element analysis, the bond
strength of the SCS-6/Ti-6Al-4V interface was determined to be 115 MPa. The
micromechanical behavior of these specimens under transverse loading was
examined by finite element analysis using this interface bond strength value
and compared with experimental observations. Results demonstrate that the
proposed geometry was successful in suppressing debonding at the surface and
altering it to an internal event. The results from numerical analysis
correlated well with the experimental stress-strain curve and several simple
analytical models. In an attempt to identify the true bond strength and the
interface failure criterion, the present study suggests that if failure
initiates under tensile radial stresses, then the normal bond strength of the
SCS-6/Ti-6Al-4V composites is about 115 MPa; under shear failure, the
tangential shear strength of the interface is about 180 MPa.
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