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Volume 26A, No. 8, August 1995 This Month Featuring: The 1994 Institute of Metals Lecture, Alloy Phases, Transformations, Mechanical Behavior, Welding & Joining, Surface Treatment, Solidification, Composite Materials. View August 1995 Contents.
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" and ' Phases in AlMgSi Alloys by Transmission Electron Microscopy
" and ' phases in 6xxx aluminum alloys using transmission electron microscopy (TEM). Age-hardened extruded profiles of an AlMgSi alloy, having different coarseness in the precipitate structure, were examined. By darkfield (DF) imaging, " needles aligned in a <100 > viewing direction were brought into contrast. Thickness was determined within 10 pct using a spot contamination method. After subsequent corrections, the number density of precipitates could then be quantified, usually within 5 pct. As expected, the average number density of the " precipitates correlates well with the material's tensile strength. A density of ' was obtained by a similar method. The densities represent maximum values since precipitate-free areas are not considered. Tensile strength is quite sensitive to the ratzo between the number densities of ' and ". By determining the volume of an average " or ' needle, corresponding volume fractions of the phases can be determined. Ouantification of these phases could benefit calculations involving mechanical properties of such alloys.
Structural Evolution in Mechanically Alloyed Al-Fe Powders
D.K. MUKHOP ADHYAY, C. SURYANARAYANA, and F.H. (SAM) FROES
The structural evolution in mechanically alloyed binary aluminum-iron powder mixtures containing 1, 4, 7.3, 10.7, and 25 at. pct Fe was investigated using X-ray diffraction (XRD) and electron microscopic techniques. The constitution (number and identity of phases present), microstructure (crystal size, particle size), and transformation behavior of the powders on annealing were studied. The solid solubility of Fe in Al has been extended up to at least 4.5 at. pct, which is close to that observed using rapid solidification (RS) (4.4 at. pct), compared with the equilibrium value of 0.025 at. pct Fe at room temperature. Nanometer-sized grains were observed in as-milled crystalline powders in all compositions. Increasing the ball-to-powder weight ratio (BPR) resulted in a faster rate of decrease of crystal size. A fully amorphous phase was obtained in the Al-25 at. pct Fe composition, and a mixed amorphous phase plus solid solution of Fe in Al was developed in the Al-10.7 at. pct Fe alloy, agreeing well with the predictions made using the semiempirical Miedema model. Heat treatment of the mechanically alloyed powders containing the supersaturated solid solution or the amorphous phase resulted in the formation of the Al3Fe intermetallic in all but the Al-25 at. pct Fe powders. In the Al-25 at. pct Fe powder, formation of nanocrystalline Al5Fe2 was observed directly by milling. Electron microscope studies of the shock-consolidated mechanically alloyed Al-10.7 and 25 at. pct Fe powders indicated that nanometer-sized grains were retained after compaction.
The Intergranular Microstructure of Cast Mg-Zn and Mg-ZnRare EarthAlloys
L.Y. WEI, G.L. DUNLOP, and H. WESTENGEN
The solidification path and microstructure of cast Mg-9Zn and Mg-8Zn-1.5MM (misch metal) alloys have been investigated by a combination of thermal analysis and analytical electron microscopy. The addition of 1.5 wt pct MM had a strong influence on the as-cast microstructure with the introduction of new "ternary" interdendritic phases and structural modification of known binary phases. The temperature ranges for formation of these phases from the melt were identified, their crystal structures determined, and their compositions analyzed. Products from eutectoidal decomposition of the interdendritic phase in the binary Mg-9Zn alloy were also identified.
, which implies that the fcc phase orders antiferromagnetically before transforming to the hcp phase. Since hcp remains paramagnetic down to lower temperatures, the ordering reaction in fcc leads to a relative stabilization of this phase, which is reflected in a drastic, nonlinear decrease of Ms.
Atomic Structure of Interphase Boundary Enclosing Bcc Precipitate Formed in Fcc Matrix in a Ni-Cr Alloy
T. FURUHARA, K. WADA, and T. MAKI
The atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane ~(112)fcc//(
)bcc) contains a regular array of structural ledges whose terrace is formed by the (lll)fcc//(110)bcc planes. A structural ledge has an effective Burgers vector corresponding to an a/12[
]fcc transformation dislocation in the fcc 
bcc transformation. The side facet (and presumably the growth ledge riser) of the bcc lath contains two distinct types of lattice dislocation accommodating transformation strains. One type is glissile dislocations, which exist on every six layers of parallel close-packed planes. These perfectly accommodate the shear strain caused by the stacking sequence change from fcc to bcc. The second set is sessile misfit dislocations (~10 nm apart) whose Burgers vector is a/3[111]fcc = a/2[110]bcc. These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which are controlled by diffusion of substitutional solute atoms, result in the virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile dislocations simultaneously. Such a growth mode assures the retention of atomic site correspondence across the growing interface.
Effects of Self-Accommodation and Plastic Accommodation in Martensitic Transformations and Morphology of Martensites
NANJU GU, XIAOYAN SONG, JIANXIN ZHANG, FUXING YIN, and RUIXIANG WANG
The effects of self-accommodation and plastic accommodation in martensitic transformations and the displacement vector for lattice deformation are discussed. The authors propose that the formation of an invariant habit plane is connected with the self-accomodation between different martensitic variants and results in the formation of internal twinned martensites; the plastic accommodation, rather than self-accommodation, occurs between parent and new phases when the strength is low or the dislocation density is high for the parent phase and the invariant habit plane is difficult to form, resulting in the formation of dislocation martensites.
Predicting the Onset of Transformation under Noncontinuous Cooling Conditions: Part 1. Theory
T.T. PHAM, E.B. HAWBOLT, and J.K. BRIMACOMBE
On cooling a steel, the temperature at which a new phase forms is an important parameter in the genesis of final microstructure. For diffilsional transformation processes, prediction of this temperature, until now, has relied upon empirical equations which are based on the cooling rate or degree of undercooling. Modern steel processing procedures involve a range of continuous and step cooling schedules, and these equations are not appropriate in many situations. A method that is capable of predicting the onset of transformation during the accelerated cooling of steel, regardless of how complex a thermal path may be, was derived. The concept of an "ideal" isothermal transformation curve for the start of transformation was introduced based on the assumption of the consumed fractional incubation time being additive. Mathematical relationships between the experimental time to the start of transformation and the ideal incubation time were quantified, and methods for deriving an ideal time-temperature-transformation (TTT) curve from experimental data were established in this study. Details of theoretical derivations are presented in the first article of this two-part series. Application of this prediction method to an austenite-to-pearlite transformation process was studied to show the validity of the derivation. Experimental procedures and results of this application are given in PartII.
Predicting the Onset of Transformation under Noncontinuous Cooling Conditions: Part 11. Application to the Austenite Pearlite Transformation
T.T. PHAM, E.B. HAWBOLT, and J.K. BRIMACOMBE
A detailed review of the additivity principle with respect to the incubation of the austenite decomposition was summarized in Part I of this two-part series and led to the concept of an "ideal" timetemperature-transformation (TTT) diagram. This curve is eharaeteristic of the chemistry and austenite grain size in the steel and allows nonisothermal behavior to be described assuming additivity holds. The derivation of mathematical relationships between the ideal and experimental cooling data was presented in the first article. In this second article, an ideal curve for the austenite-to-pearlite transformation was derived from cooling data.The applicability of the ideal m curve for predicting the start of transformation under continuous cooling conditions was assessed for a range of cooling rates. Experiments were conducted under both isothermal and varying temperature conditions, including an industrial cooling schedule, using a Gleeble Thermal Simulator. Reasonable agreement was found between the predictions and the observed transformation start temperatures; predictions were consistent and compared favorably against other methods which have been frequently used to estimate the transformation start temperature for nonisothermal conditions.
Electron Diffraction Study of Aging Processes in Fe-1.83 Wt Pct C Martensite at Room Temperature
S.B. REN, T. TADAKI, K. SHIMIZU, and X.T. WANG
Aging processes occurring in an Fe-1.83 wt pct C martensite at room temperature have been studied by electron diffraction, assisted by transmission electron microscopy observation. Fourier analysis of diffuse scattering from the martensite suggested that a spinodal decomposition occurred therein. The spinodally modulated microstructure was composed of a dominant wave with a wavelength of about 1 nm and nondominant waves with longer wavelengths and was observed not to grow even after aging for several months. The reason for the nongrowth may be that the coarsening was stopped by strong elastic interactions between carbon-enriched and carbon-depleted regions. Furthermore, the tetragonality of the martensite was observed to remain constant during aging, suggesting that carbon redistribution during spinodal decomposition occurred within one set of the octahedral interstitial sites Oz only.
Numerical Modeling of Diffusion-Controlled Phase Transformations in Ternary Systems and Application to the Ferrite/Austenite Transformation in the Fe-Cr-Ni System
J.M. VITEK, S.A. VITEK, and S.A. DAVID
An implicit finite-difference analysis has been used to model the diffusion process in a two-phase ternary system. The system was of finite length, and the interface between the two phases, 
and , was allowed to move as the a phase grew or dissolved. Equilibrium at the interface was assumed. For simplicity, the diffusion coefficients were assumed to be independent of composition, and the cross-coefficient diffusion terms were assumed to be negligible. The details of the computational process are described. The accuracy of the approach and the major sources of error are examined in detail. The numerical results are in excellent agreement with analytical predictions under the limited conditions for which analytical solutions are available. The computational model was applied to the detailed study of ferrite growth/dissolution in the iron-chromium-nickel ternary system to examine ferrite stability in austenitic stainless steel welds. The starting compositions of the ferrite and austenite were typical of these phases in the as-welded condition, and the isothermal equilibration was followed at 700°C to 1300°C. It was found that the transformation to the equilibrium state could be divided into two major stages: the first corresponded to rapid diffusion in the ferrite phase. and the second was due to the more sluggish diffusion within the austenite. The path toward equilibrium was often indirect; sometimes the ferrite grew first before eventually shrinking to the final equilibrium size, whereas at other temperatures the ferrite initially dissolved before growing at a later stage. The results are compared to the limited available experimental data, and behavior that was found experimentally was accurately reproduced by the computations. The results provide hitherto unavailable data on the kinetics of the equilibration process at elevated temperatures near the solidus temperature and also provide insight into details of the transformation behavior.
Communication: The Effect of Annealing Temperature on the Recrystallization Kinetics of Commercially Pure Aluminum
N.H. LIN, C.P. CHANG, and P.W. KAO
, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake.
Viscoelastic Spectra of Cd0.67Mg0.33 in Torsion and Bending
L. STEVEN COOK and RODERIC S. LAKES
In both torsion and bending, the alloy exhibited a viscoelastic relaxation that could be modeled as a Debye peak superimposed on a power-law low-frequency background. In torsion, the relaxed and unrelaxed shear moduli were 9 and 12.2 GPa, respectively, maximum loss tangent was 0.12. In bending, relaxed and unrelaxed Young's moduli were 15 and 35 GPa, respectively; maximum loss tangent was 0.11. Behavior was linear to at least 125 microstrain. These results are significant in that they represent a unique combination of stiffness and loss in a monolithic material.
Modeling the Hot Consolidation of Ceramic and Metal Powders
R.E. DUTTON, S. SHAMASUNDAR, and S.L. SEMIATIN
Modeling of the consolidation of ceramic and metal powders by sintering, hot pressing, and hotisostatic pressing (HIP) was conducted using a continuum yield function and associated-flow rule modified to incorporate microstructure effects such as grain growth, pore size, and pore geometry. It was shown that consolidation behavior can be described over the entire range of densities through two parameters, the stress intensification factor and Poisson's ratio, which are readily measured using uniaxial upset tests. Both parameters are functions of relative density, whose exact dependence varies from one material to another. Furthermore, it was demonstrated that in sinter forging of ceramics, an "apparent" Poisson's ratio depending on stress level (relative to the sintering stress) gives a quantitative measure of the competition between sintering and creep deformation. The accuracy of the microstructure-sensitive yield function was established through finite-element modeling (FEM) simulations of the isothermal sintering of a soda-lime glass, sinter forging of alumina, and die pressing of an alpha-two titanium alurninide alloy.
Analysis of Steady-State Creep and Creep Fracture of Directionally Solidified Eutectic /'- Alloy
J. LAPIN and F. DELANNAY
The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-1.6V-1.2Re (wt pct) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress is n = 7.5 ± 0.3, and the apparent activation energy for creep of the eutectic /'- composite is determined to be Q = 517 ± 11 kJ mol-1. When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creep Qc is calculated to be between 412 and 424 kJ mole-1 and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation.
Initiation ancl Growth of Small Fatigue Cracks in a Ni-Base Superalloy
Z. MEI, C.R. KRENN, and J.W. MORRIS, JR.
This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (Kmax). Small surface cracks initiated predominantly at (Nb,Ti)xCy inclusion particles. and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of 
K above the large-crack threshold, Kth, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (
th) and the threshold cyclic stress intensity (Kth) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime.
A Simple Technique to Generate In-Plane Forming Limit Curves and Selected Applications
K S RAGHAVAN
A simple technique to generate in-plane forming limit curves has been developed This technique is based on the Marciniak biaxial stretch test using a single punch/die configuration, but the specimen and washer geometries have been modified in order to achieve failure in both drawing and stretching deformation modes The experimental technique is described, and the advantages of using this inplane method over the conventional out-of-plane dome method are discussed It is shown that (a) sheet thickness has an intrinsic influence on forming limits that is not related to small bending strain variations with thickness or to deformation in the presence of friction and curvature, (b) plastic anisotropy (
value) does not substantially affect forming limits, and (c) in-plane forming limits are slightly lower (5 to 6 pct) than out-of-plane forming limits near plane strain; these differences are smaller than previously reported values (12 to 15 pct) in the literature
Mechanical Properties of Isothermally Aged High-Nitrogen Stainless Steel
JOHN W. SIMMONS
The effects of nitride (Cr2N) precipitation on the tensile, impact, and hardness properties of a typical high-nitrogen, low-carbon austenitic stainless steel (SS), nominally Fe-19Cr-5Mn-5Ni-3Mo-0.024C-0.69N, were determined. Annealed and cold-rolled (20 pct reduction in thickness) specimens were isothermally aged 700°C and 900°C for times ranging from 0.1 to 10 hours. Only grain boundary Cr2N precipitation occurred in annealed materials aged at 700°C. Precipitation at 900°C occurred sequentially at grain boundaries, by cellular precipitation, and, finally, by transgranular precipitation within the matrix. Nitride precipitation had little effect on yield and ultimate strengths but reduced tensile ductility and impact toughness. Embrittlement occurred due to grain boundary separation (700°C and 900°C) and fracture through cellular precipitate regions, initiated at nitrides (900°C). Prior deformation increased precipitation kinetics and had a controlling influence on nitride morphology, enhancing grain boundary and transgranular Cr2N and retarding cellular precipitation. Nitride structures produced in cold-rolled materials were just as detrimental to material plasticity as those produced in annealed materials, but prior deformation increased the kinetics of embrittlement. Due to strain recovery, the yield and ultimate strengths of cold-rolled materials decreased with aging time and temperature.
The Influence of the Sulfur/Oxygen Ratio in the Environment on the Creep and Creep Damage Behavior of a HeatResistant Steel
M.F. STROOSNIJDER, V. GUTTMANN, and J.H.W. de WIT
Investigations have been carried out on the creep behavior of a 32Ni-27Cr-0.07Ce heat-resistant steel in several S-O bearing environments at 700°C. It was found that the creep strength is reduced if severe corrosion, as was found in the case of the highest S/O ratio, causes a reduction of load-bearing cross section. Deformation-induced fingerlike corrosion paths along grain boundaries, which form an alternate corrosion pattern, were less damaging, even when the penetration was deep. Creep ductility was strongly reduced in all environments regardless of the S/O ratio. The formation of sulfide-oxide corrosion products causes rapid crack initiation at the surface and promotes crack propagation.
Forming Properties and Springback Evaluation of Copper Berillium Sheets
A.A. TSENG, K.P. JEN, T.C. CHEN, R. KONDETIMMAMHALLI, and Y.V. MURTY
Copper beryllium (CuBe) alloys possess excellent strength and conductivity. They have become the most important materials used for producing high reliability connectors and interconnections for electrical and electronic applications. As demand for high connection density in electrical and electronic products grows, springback behaviors become increasingly critical in fabricating these miniaturized contact components from sheet base materials. In the present article, a study of the springback behavior of CuBe sheets under different heat treatments is presented, with the goal of providing reliable information needed for fabricating more intricate connection parts. Both experimental and analytical techniques were adopted. The tensile tester was first used to study the springback related tensile properties. The govermng tensile parameters on springback were identified, and their variations for sheets with different heat treatments were studied. It was found that a bilinear constitutive relationship can best characterize the stress strain behavior of the CuBe alloy. A closed form solution based on this bilinear relationship was formulated to predict the springback for the CuBe sheets at bending conditions. A V-shaped bend tester having an interchangeable punch to accommodate multiple radii was designed and built to evaluate the springback properties of CuBe sheets. A good correlation was found between the analytical predictions and experimental data. A parametric study, as an example, was also performed to provide the springback information needed for designing complicated connectors.
Communication: Mechanical Properties of Molybdenum Alloyed Liquid Phase-Sintered Tungsten-Based Composites
PRESTON B. KEMP and RANDALL M. GERMAN
phase and eutectic consisting of pseudohexagonal close-packed (hcp) M7C3 (M = Cr, Fe, W) carbides and fcc phase. The hypereutectic microstructures consisted of hcp M7C3 primary carbides and eutectic similar to that in the hypoeutectic microstructures. The formation of hypoeutectic or hypereutectic microstructures was infiuenced by the alloy composition particularly the C concentration which depends on the amount of powder delivered into the melt pool and the extent of substrate melting. The enhancement of the lattice parameter of the phase is associated with the significant dissolution of alloying elements and lattice strains resulting from rapid quenching. The higher hardness of the hypereutectic microstructures is principally attributed to the formation of hcp M7C3 primary carbides. The relatively lower hardness of the hypoeutectic microstructures is related to the presence of phase in the primary dendrites excessive dilution from the base material and relatively low concentrations of Cr and C. The results provide insight into the significance of laser-processing conditions on the composition and hardness of Fe-Cr-W-C alloy coahngs and associated solidification characterishcs.
Pressureless Infiltration of Aluminum Metal-Matrix Composites
Y. KAJIKAWA, T. NUKAMI, and M.C. FLEMINGS
Pressureless infiltration of ceramic preforms by molten aluminum is described. The preforms are SiC with varying amounts of particulate Al, Ti, and Ni. Infiltrants employed are pure Al and Al-12.5 wt pct Si. It is shown that a pressure differential within the preform is required for infiltration. and measurements are made of pressure changes in the preforms during infiltration. Results indicate that atmospheric pressure is essential for infiltration but that capillarity may play a role as well.
Influence of Cr and W Alloying on the Fiber-Matrix Interfacial Shear Strength in Cast and Directionally Solidified Sapphire NiAI Composites
R. ASTHANA, R. TIWARI, and S.N. TEWARI
Sapphire-reinforced NiAI matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAI specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAI and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAI by designing dual-phase microstructures in NiAI alloys reinforced with sapphire fibers.
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