METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 27A, No. 1, January 1996 This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Surface Treatment; Solidification; Materials Processing; Composite Materials. View January 1996 Table of Contents.

ALLOY PHASES

The Effect of Iron and Manganese on the Recrystallization Behavior of Hot-Rolled and Solution-Heat-Treated Aluminum Alloy 6013
RICHARD A. JENISKI, JR., BUNCHA THANABOONSOMBUT, and T.H. SANDERS, JR.
The influence of manganese and iron additions on the recrystallization behavior of a hot-rolled Al-Mg-Si-Cu alloy has been investigated. In the as-cast ingot, some manganese remains in supersaturated solid solution and, during the preheating step, precipitates as fine (0.1 to 0.3 µm) dispersoids Al₁₂Mn₃Si and (AlFeMnSi), depending on the local iron content. The presence of the dispersoids increases the alloy's resistance to recrystallization. The remaining manganese along with iron forms coarse (30 to 100 µm) (AlFeMnSi) phases in the interdendritic channels upon solidification. During rolling, these coarse phases are broken up and the matrix surrounding the phases is highly strained. The highly strained regions around the precipitates are preferential sites for the nucleation and growth of new, strain-free, recrystallized grains. Decreasing the iron content decreases the amount of coarse (AlFeMnSi) constituent phase and increases the recrystallization resistance of the alloy for a given concentration of manganese. For iron-free alloys, coarse -Al₁₂Mn₃Si phase forms during solidification. The increased recrystallization resistance is attributed to the lower probability for particle-stimulated recrystallization. Increasing the manganese content beyond what is normally found in 6013 increases the volume fraction of both the dispersoids and the coarse constituent phases. However, the increased volume fraction of the dispersoids in the iron-containing alloys more than compensates for the increase in volume fraction of coarse constituent particles, resulting in an increase in the recrystallization resistance with increased manganese content. In this research, alloys containing varying amounts of manganese and iron with levels of magnesium, silicon, and copper consistent with the nominal composition of 6013 were hot deformed and solution heat treated to produce microstructures ranging from fully recrystallized to unrecrystallized.

TRANSFORMATIONS

Mechanical Alloying of Nb-Al Powders
ZHIXUE PENG, C. SURYANARAYANA, and F.H. (SAM) FROES
The effect of mechanical alloying (MA) on solid solubility extension, nanostructure formation, amorphization, intermetallic compound formation, and the occurrence of a face-centered cubic (fcc) phase in the Nb-Al system has been studied. Solid solubility extension was observed in both the terminal compositions and intermetallic compounds: 15 pct Nb in Al and 60 pct Al in Nb, well beyond the equilibrium and even rapid solidification levels (2.4 pct Nb and 25 pct Al, respectively) and increased homogeneity range for the NbAl₃ phase. Nanostructured grains formed in all compositions. In the central part of the phase diagram, amorphization occurred predominantly. Only NbAl₃, the most stable intermetallic, formed during MA; in most cases, a subsequent anneal was required. On long milling time, an fcc phase, probably a nitride, formed as a result of contamination from the ambient atmosphere.

Experimental Investigation of the Transformation Texture in Hot-Rolled Ferritic Stainless Steel Using Single Orientation Determination
D. RAABE and M. YLITALO
Two ferritic stainless steels (16.5 mass pct Cr) were hot-rolled using seven subsequent passes. The first sample was rolled within the range 1280 °C to 750 °C, i.e., the deformation started in the ferritic region. The second sample was rolled within the range 1080 °C to 770 °C, i.e., the deformation started in the ferritic-austenitic region. In both cases, up to 40 vol pct of the ferrite transformed into austenite during hot rolling. During the last passes, the austenite transformed into cubic martensite. After hot rolling, these former austenitic regions were identified using a selective etching technique and examined using single orientation determination in the scanning electron microscope. The regions which remained ferritic throughout the hot-rolling process were investigated as well. Whereas the texture of the martensite considerably depended on the hot-rolling conditions, especially on the temperature and on the intervals between the rollings, the texture of the ferrite was less affected. The textures of the martensite were interpreted in terms of the crystallographic transformation rules between austenite and martensite. The textures of the ferrite were discussed in terms of recovery and recrystallization.

TRANSPORT PHENOMENA

MECHANICAL BEHAVIOR

Non-Schmid Effects on the Behavior of Polycrystals-m with Applications to Ni₃Al
MING DAO, BING JEAN LEE, and ROBERT J. ASARO
The elastoviscoplastic single crystal constitutive model incorporating non-Schmid effects developed by Dao and Asaro (Mater. Sci. Eng. A, 1993, vol. 170, pp. 143-60) is introduced into Asaro and Needleman's (Acta Metall., 1985, vol. 33, pp. 923-53) Taylor-like polycrystal model as well as Harren and Asaro's (J. Mech. Phys. Solids, 1989, vol. 37, pp. 191-232) finite element polycrystal model. The single crystal non-Schmid effects, strain hardening, latent hardening, and rate sensitivity, are all described on the individual slip system level, while polycrystal mechanical properties on macroscale are predicted. In general, it is found that non-Schmid effects can have important influences on the "constant offset plastic strain yield surfaces," stress-strain behavior, texture development, and shear band formation. Finite element calculations show that with moderate non-Schmid effects, localized deformation within a polycrystal aggregate tends to initiate earlier and form sharper and more intense shear bands. Heavy shear banding is found to produce less pronounced textures, which is consistent with existing experimental evidence on Ni₃Al. Examples with Ni₃Al demonstrate that the kind of non-Schmid effects existing in Ni₃Al can increase the generalized Taylor factor to values much higher than 3.06, raise the polycrystal strain hardening rate much higher than that which would be obtained using Schmid's rule, and influence the deformation texture.

The Effect of Hydrogen on the Fracture of Alloy X-750
DOUGLAS M. SYMONS and ANTHONY W. THOMPSON
The effect of hydrogen on the fracture of a nickel-base superalloy, alloy X-750, was investigated in the HTH condition. The effect of hydrogen was examined through tensile testing incorporating observations from scanning electron microscopy and light microscopy. The ductility at 25 °C, as measured by elongation to failure for tensile specimens, was reduced from 21 pct for noncharged specimens to 7.3 pct for 5.7 ppm hydrogen and to 3.5 pct for 65 ppm hydrogen. The elongation to failure was a function of the strain rate and test temperature. For hydrogen-charged specimens, the elongation decreased as the strain rate decreased at a constant temperature, while for a constant strain rate and varying temperature, there was a maximum in embrittlement near 25 °C and no embrittlement at -196 °C. For the noncharged specimens, the elongation monotonically increased as temperature increased, while there was no noticeable effect of strain rate. Prestraining prior to charging dramatically decreased elongation after hydrogen charging. When the strain rate was increased on the prestrained specimens, more plastic deformation was observed prior to failure. Failure did not occur until the flow stress was reached, supporting the proposition that plasticity is required for failure. The intergranular failure mechanism in alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. The void initiation strain, as determined from tensile data and from sectioning unfractured specimens, was observed to be much lower in the hydrogen-charged specimens as compared to noncharged specimens. The reduced ductility may be explained by either a reduction of the interfacial strength of the carbide-matrix interface or a local hydrogen pressure at the carbide-matrix interface.

Pearlite in Ultrahigh Carbon Steels: Heat Treatments and Mechanical Properties
ERIC M. TALEFF, CHOL K. SYN, DONALD R. LESUER, and OLEG D. SHERBY
Two ultrahigh carbon steel (UHCS) alloys containing 1.5 and 1.8 wt pct carbon, respectively, were studied. These materials were processed into fully spheroidized microstructures and were then given heat treatments to form pearlite. The mechanical properties of the heat-treated materials were evaluated by tension tests at room temperature. Use of the hypereutectoid austenite-cementite to pearlite transformation enabled achievement of pearlitic microstructures with various interlamellar spacings. The yield strengths of the pearlitic steels are found to correlate with a predictive relation based on interlamellar spacing and pearlite colony size. Decreasing the pearlite interlamellar spacing increases the yield strength and the ultimate strength and decreases the tensile ductility. It is shown that solid solution alloying strongly influences the strength of pearlitic steels.

Optimization of Cold and Warm Workability in 304 Stainless Steel Using Instability Maps
S. VENUGOPAL, S.L. MANNAN, and Y.V.R.K. PRASAD
The deformation characteristics of stainless steel type AISI 304 under compression in the temperature range 20 °C to 600 °C and strain-rate range 0.001 to 100 s^-1 have been studied with a view to characterizing the flow instabilities occurring in the microstructure. At strain rates less than 5 s^-1, 304 stainless steel exhibits flow localization, whereas dynamic strain aging occurs at intermediate temperatures and below 0.5 s^-1. At room temperatures and strain rates less than 10 s^-1, martensite formation is observed. To avoid the preceding microstructural instabilities, cold and warm working should be carried out at strain rates greater than 5 s^-1. The continuum criterion, developed on the basis of the principles of maximum rate of entropy production and separability of the dissipation function, predicts accurately all the preceding instability features.

Effect of Creep Strain on Microstructural Stability and Creep Resistance of a TiAl/Ti₃Al Lamellar Alloy
J.A. WERT and M.F. BARTHOLOMEUSZ
Creep of a TiAl/Ti₃Al alloy with a lamellar microstructure causes progressive spheroidization of the lamellar microstructure. Microstructural observations reveal that deformation-induced spheroidization (DIS) occurs by deformation and fragmentation of lamellae in localized shear zones at interpacket boundaries and within lamellar packets. Deformation-induced spheroidization substantially increases the interphase interfacial area per unit volume, demonstrating that DIS is not a coarsening process driven by reduction of interfacial energy per unit volume. Creep experiments reveal that DIS increases the minimum creep rate (_min) during creep at constant stress and temperature; the activation energy (Q_c) and stress exponent (n) for creep are both reduced as a result of DIS. Values of n and Q_c for the lamellar microstructure are typical of a dislocation creep mechanism, while estimated values of n and Q_c for the completely spheroidized microstructure are characteristic of a diffusional creep mechanism. The increase in min associated with DIS is thus attributed primarily to a change of creep mechanism resulting from microstructural refinement.

SURFACE TREATMENT

SOLIDIFICATION

MATERIALS PROCESSING

Modeling Texture Change during the Static Recrystallization of Interstitial Free Steels
L. KESTENS and J.J. JONAS
A recrystallization model was developed based on the principal features of oriented nucleation and selective growth. The nucleation mechanism that is adopted assumes that nucleation takes place in deformed grains with relatively high Taylor factors. The selective growth law is based on the assumed high mobility of {110} plane matching (PM) boundaries and also includes a variant selection rule. The two submodels enable the full recrystallization process to be simulated. The theory is applied to the annealing of a Ti-Nb alloyed interstitial free (IF) steel. It is demonstrated that the nucleation and growth algorithms can successfully account for the annealing textures observed, including the presence of a strong {111}<211> component. An important feature of the model is that it permits the role of variant selection to be assessed; it also shows the effect on the texture of the low mobility of low-angle grain boundaries (LAGBs).

Communication: Modeling of Dynamic Material Behavior: A Critical Evaluation of the Dissipator Power Co-content Approach
F. MONTHEILLET, J.J. JONAS, and K.W. NEALE

Communication: Author's Reply
Y.V.R.K. PRASAD

COMPOSITE MATERIALS

NiTi and NiTi-TiC Composites: Part II. Compressive Mechanical Properties
K.L. FUKAMI-USHIRO, D. MARI, and D.C. DUNAND
The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC particulates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stress-induced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively.

NiTi and NiTi-TiC Composites: Part III. Shape-Memory Recovery
K.L. FUKAMI-USHIRO and D.C. DUNAND
The transformation behavior of near-equiatomic NiTi containing 0, 10, and 20 vol pct TiC particulates is investigated by dilatometry. Undeformed composites exhibit a macroscopic transformation strain larger than predicted when assuming that the elastic transformation mismatch between the matrix and the particulates is unrelaxed, indicating that the mismatch is partially accommodated by matrix twinning during transformation. The thermal recovery behavior of unreinforced NiTi which was deformed primarily by twinning in the martensite phase shows that plastic deformation by slip increases with increasing prestrain, leading to (1) a decrease of the shape-memory strain on heating, (2) an increase of the two-way shape-memory strain on cooling, (3) a widening of the temperature interval over which the strain recovery occurs on heating, and (4) an increase of the transformation temperature hysteresis. For NiTi composites, the recovery behavior indicates that most of the mismatch during mechanical deformation between the TiC particulates and the NiTi matrix is relaxed by matrix twinning. However, some relaxation takes place by matrix slip, resulting in the following trends with increasing TiC content at constant prestrain: (1) decrease of the shape-memory strain on heating, (2) enhancement of the two-way shape-memory strain on cooling, and (3) broadening of the transformation interval on heating.

Subcritical Crack Growth at Bimaterial Interfaces: Part I. Flexural Peel Technique
ZHEHUA ZHANG and J.K. SHANG
A flexural peel (FP) technique was developed to study the crack growth behavior along a bimaterial interface. The technique was based on a sandwich specimen where one arm of the specimen was peeled away from the interface under a combined tensile and shear mode. An approximate linear elastic fracture mechanics solution was derived analytically from the J integral formulation. The solution was compared to finite element calculations based on the crack closure method and experimental measurements. From the finite element analysis, ratios of the mode-I and mode-II components of the crack tip field were determined for a wide range of modulus and thickness ratios.

Subcritical Crack Growth at Bimaterial Interfaces: Part II. Microstructural Effects on Fracture Resistance of Metal/Ceramic Interfaces
GANG LIU and J.K. SHANG
The flexural peel technique was used to study the fracture resistance of two model Al₂O₃/Al interfaces. The bimaterial interface was formed by bonding high-purity Al₂O₃ with molten Al-5 pct Cu alloy under pressure. The specimens were then heat treated so that the Al-Cu alloy reached peak-aged and extended-overaged conditions. The fracture resistance curve was established for two interfaces with either the peak-aged or overaged Al alloy. The fracture resistance of the interface with the peak-aged Al-Cu alloy was higher in terms of both the initiation and peak toughness. While the peak toughness of the interface scaled with the yield strength of the metal, the initiation toughness differed by a factor of 8. The difference in the initiation toughness is discussed in terms of the disparity in the interfacial microstructure.

Subcritical Crack Growth at Bimaterial Interfaces: Part III. Shear-Enhanced Fatigue Crack Growth Resistance at Polymer/Metal Interface
ZHEHUA ZHANG and J.K. SHANG
Fatigue crack growth along an Al/epoxy interface was examined under different combinations of mode-I and mode-II loadings using the flexural peel technique. Fatigue crack growth rates were obtained as a function of the total strain energy rate for G_II/G_I ratios of 0.3 to 1.4, achieved by varying the relative thickness of the outerlayers for the flexural peel specimen. Fatigue crack growth resistance of the interface was found to increase with increasing G_II/G_I ratio. Such a shear-enhanced crack growth resistance of the interface resulted in a gradual transition of crack growth mechanism from interfacial at the low G_II/G_I ratio to cohesive at the high G_II/G_I ratio. Under predominantly mode-I loading, the damage in the polymer took the form of crazing and cavitation. In contrast, laminar shear occurred under highly shear loading, resulting in a larger amount of plastic dissipation at the crack tip and improved fatigue crack growth resistance.

Communication: On the In Situ Formation of TiC and Ti₂C Reinforcements in Combustion-Assisted Synthesis of Titanium Matrix Composites
S. RANGANATH and J. SUBRAHMANYAM