METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 28A, No. 7, July 1997 This Month Featuring: Alloy Phases; Transformations; Mechanical Behavior; Surface Treatment; Solidification; Materials Processing. View July 1997 Contents.

ALLOY PHASES

TRANSFORMATIONS

Amorphization Reaction of Ni-Ta Powders during Mechanical Alloying
PEE-YEW LEE, JU-LUNG YANG, CHUNG-KWEI LIN, and HONG-MING LIN
This study examined the amorphization behavior of Ni^xTa^100-x alloy powders synthesized by mechanically alloying (MA) mixtures of pure crystalline Ni and Ta powders with a SPEX high energy ball mill. According to the results, after 20 hours of milling, the mechanically alloyed powders were amorphous for the composition range between Ni₁₀Ta₉₀ and Ni₈₀Ta₂₀. A supersatuated nickel solid solution formed for Ni₉₀Ta₁₀, as well. X-ray diffraction analysis reveals two different types of amorphization reactions. Through an intermediate solid solution and by direct formaiton of amorphous phase. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, the crystallization temperature of amorphous Ni-Ta powders increased with increasing Ta content. In addition, the activation energy of amorphous Ni-Ta powders reached a maximum near the eutectic composition.

Microstructural Evolution during Thermomechanical Processing of a Ti-Nb Interstitial-Free Steel Just below the Ar₃ Temperature
I.A. RAUF and J.D. BOYD
Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar₃. For cooling rates in the range 0.1°C s^-1 to 130°C s^-1, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850°C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb)_xS_y, precipitates form at 850°C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb)_xS_y, precipitates retard recrystallization and grain coarsening of PF deformed at 850°C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.

Shock-lnduced Martensitic Transformations in Near-Equiatomic NiTi Alloys
A.M. THAKUR, N.N. THADHANI, and R.B. SCHWARZ
Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M_s) temperatures. The NiTi-I alloy (M_s +27°C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of "stress-assisted" martensite. The NiTi-II alloy (M_s -45°C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spell initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of "strain-induced" martensite, were seen clustering around the spell region. No stress-assisted martensite was formed in this alloy due to its very low M_s temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.

MECHANICAL BEHAVIOR

Corrosion Fatigue Crack Growth Behavior of a Squeeze-Cast Al-Si-Mg-Cu Alloy with Different Precrack Histories
KAZUAKI SHIOZAWA and SHUMING SUN
Fatigue experiments have been performed on a squeeze-cast Al-Si-Mg-Cu alloy as a function of precrack history. The precracked conditions were that the compact tension specimen was precracked with a relatively long through-thickness crack (about 6 mm) in air, in aqueous 3 pct NaCI solution, and in air followed by hydrogen precharging. It was found that a relatively long through-thickness crack can grow more rapidly than would be predicted by a traditional K involving three stages under either a corrosion fatigue test after precracking in air or a hydrogen precharging experiment followed by fatigue testing in air. The experimental evidence confirms that a hydrogen-assisted damage mechanism is mainly responsible for the rapid growth phenomenon of a relatively long crack in a corrosive environment compared to the result of fatigue testing in air after hydrogen precharging. The amount of hydrogen production in chemical-microstructure interaction processes in a corrosion fatigue experiment and the effectiveness of hydrogen transport to the region ahead of the crack tip determine the degree of hydrogen-assisted fatigue crack growth, which is controlled by the microstructure of the alloy and the chemical attack on a sharp and fresh crack tip.

Plastic Deformation of Hafnium under Uniaxial Compression
G. SUBHASH, G. RAVICHANDRAN, and B.J. PLETKA
The plastic behavior of polycrystalline hainium (Hf) was investigated over a range of strain rates under uniaxial compression. Hafnium exhibited considerable ductility and a moderately rate-sensitive plastic behavior. The stress-strain response consisted of initial yielding followed by parabolic hardening. Microstructural observations on quasistatically deformed specimens revealed that yielding occurred by dislocation activity and that hardening was dominated by twinning on {} planes and by slip/twin interactions. A considerable reduction in dislocation and twinning activity was observed in specimens deformed at high strain rates. Failure occurred by shear localization and void growth and coalescence within the shear bands. Measurement of the temperature rise during high strain rate deformation was also made. From these measurements, the fraction of work converted to heat as a function of strain was determined and found to decrease with increasing strain.

Deformation and Fracture Behavior of Two Al-Mg-Si Alloys
L. ZHEN and S.B. KANG
Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively, in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these alloys.

Communication: On the Evaluation of Efficiency Parameters in Processing Maps
S.V.S. NARAYANA MURTY. M.S. SARMA, and B. NAGESWARA RAO

SURFACE TREATMENT

SOLIDIFICATION

Modeling of Micro- and Macrosegregation and Freckle Formation in Single-Crystal Nickel-Base Superalloy Directional Solidification
M.C. SCHNEIDER, J.P. GU, C. BECKERMANN, W.J. BOETTINGER, and U.R. KATTNER
The formation of macrosegregation and freckles by multicomponent thermosolutal convection during the directional solidification of single-crystal Ni-base superalloys is numerically simulated. The model links a previously developed thermodynamic phase equilibrium subroutine with an existing code for simultaneously solving the macroscopic mass, momentum, energy, and species conservation equations for solidification of a multicomponent alloy. Simulation results are presented for a variety of casting speeds and imposed thermal gradients and for two alloy compositions. It is found that for a given alloy composition, the onset of convection and freckle formation occurs at a critical primary dendrite arm spacing, which agrees well with previous experimental findings. The predicted number and shape of the freckle chains in the unstable cases also agree qualitatively with experimental observations. Finally, it is demonstrated how the onset and nature of convection and macrosegregation vary with alloy composition. It is concluded that the present model can provide a valuable tool in predicting freckle defects in directional solidification of Ni-base superalloys.

Motion and Remelting of Dendrite Fragments during Directional Solidification of a Nickel-Base Superalloy
J.P. GU, C. BECKERMANN, and A.F. GLAMEI
The formation of spurious grains during the directional solidification of a Ni-base superalloy is studied by modeling the movement and remelting of dendrite fragments originating in channels inside the mush. Such channels exist because of thermosolutal convective instabilities during solidification and persist as freckle chains in the solidified material. The fragment model is linked to a phase equilibrium subroutine for multicomponent Ni-base superalloys, as well as to a previously developed solidification and convection code. A parametric study is performed to investigate the effects of initial fragment location and size on the fragment paths and survivability in the melt for one of the channels predicted in a typical directional solidification simulation. It is found that only a small window of initial conditions exists which leads to spurious grain formation. This window corresponds to medium-sized fragments originating near the mouth of the channel. Other fragments either remelt completely or sink into the channel. The need for an accurate fragment generation model is discussed.

The Occurrence and Periodicity of Oscillating Peritectic Microstructures Developed during Directional Solidification
KATHRYN L. ZEISLER-MASHL and THOMAS A. LOGRASSO
The layered microstructures that can form during plane-front directional solidification in peritectic systems were characterized quantitatively as a function of growth velocity using a Sn-Cd alloy. Layers were formed for an alloy composition outside of the two-phase peritectic region in the absence of longitudinal macrosegregation. The layers did not extend over the entire sample cross sections, so that the layered regions had a different composition than the alloy. Each of the two solids was found to be interconnected and continuous in three dimensions. The layer lengths and individual layer compositions did not vary with solidification distance. The average layer compositions were not a function of growth velocity and were approximately those at the peritectic temperature. This research was compared to the current model by Trivedi. which is based upon cyclic accumulation and depletion of solute in the liquid ahead of the interface linked to repeated nucleation events. The dependence of layer length on growth velocity predicted by the model was not obtained experimentally. The differences between results and predictions are related to the continuity of the two solids and the nonuniform cross-sectional composition in the Sn-Cd samples, which contradict assumptions of the model. A formation mechanism involving competitive lateral growth between the two solids at the solid-liquid interface would be more consistent with the current research.

Communication: Computational Modeling of NbC/Laves Formation In INCQNEL 718 Equiaxed Castings
L. NASTAC and D.M. STEFANESCU

MATERIALS PROCESSING

Effect of Milling Temperature on Mechanical Alloying in the Immiscible Cu-Ta System
J. XU, J.H. HE, and E. MA
Elemental powder blends with atomic composition of Cu_100-xTa_x (x = 10, 30, 50, 70, and 90) were ball milled in a SPEX mill at several temperatures (room temperature (RT), liquid nitrogen temperature (LN₂T), -80 and 95°C) to examine the effect of milling temperature on the extent of alloying and microstructural refinement. For the Cu-rich powders (10 < x < 50), high-energy ball milling to steady state at all temperatures produced a mixture of nanocrystalline Cu and Ta with no observable extension of mutual solid solubility. Compared with milling at RT, cryomilling (LN₂T) caused further refinement of Cu crystallites, while the same steady-state grain size was reached for Ta crystallites. On the Ta-rich side (50 < x < 90), ball milling at all temperatures led to refined Cu and Ta grain sizes. Partial amorphization seemed to be present, which apparently increased in extent with increasing contamination from the milling media upon extended milling. Very similar results were obtained for milling at RT and LN₂T. It was concluded that high-energy ball milling at LN₂T did not drastically enhance the amorphization reaction between Cu and Ta nor extend their mutual solubility. The limited power of cryomilling to alloy immiscible elements such as Cu-Ta is explained as a consequence of the inability to fully suppress, during energetic collisions, the atomic mobility responsible for phase separation even when the milling is conducted at the nominal LN₂T. The temperature dependence of milling-induced microstructural refinement and alloying is analyzed in terms of the dynamics of the generation and annihilation of the nonequilibrium vacancies in an externally driven system. It is predicted that externally forced mixing as well as diffusion assisted by high-energy ball milling can be merely weakly temperature dependent between RT and LN₂T. As a result, the extension of solubility by using cryomilling is feasible only in limited systems, and this process cannot be expected to alloy all immiscible elements.