METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 28A, No. 5, May 1997 This Month Featuring: Alloy Phases, Transformations, Mechanical Behavior, Environment, Surface Treatment, Solidification, Composite Materials. View May 1997 Contents.

ALLOY PHASES

TRANSFORMATIONS

Ball-Milling-Induced Crystallization and Ball-Milling Effect on Thermal Crystallization Kinetics in an Amorphous FeMoSiB Alloy
F.Q. GUO and K. LU
Microstructure evolution in a melt-spun amorphous Fe_77.2Mo_0.8Si₉B₁₃ alloy subjected to high-energy ball milling was investigated by means of X-ray diffraction (XRD), a transmission electron microscope (TEM), and a differential scanning calorimeter (DSC). It was found that during ball milling, crystallization occurs in the amorphous ribbon sample with precipitation of an -Fe solid solution, and the amorphous sample crystallizes completely into a single -Fe nanostructure (rather than - Fe and borides as in the usual thermal crystallization products) when the milling time exceeds 135 hours. The volume fraction of material crystallized was found to be approximately proportional to the milling time. The fully crystallized sample with a single -Fe nanophase exhibits an intrinsic thermal stability against phase separation upon annealing at high temperatures. The ball-milling effect on the subsequent thermal crystallization of the amorphous phase in an as-milled sample was studied by comparison of the crystallization products and kinetic parameters between the as-quenched amorphous sample and the as-milled partially crystallized samples. The crystallization temperatures and activation energies for the crystallization processes of the residual amorphous phase were considerably decreased due to ball milling, indicating that ball milling has a significant effect on the depression of thermal stability of the residual amorphous phase.

X-ray Study of Phase Transformations in Martensitic Ni-Al Alloys
P.L. POTAPOV, S.Y. SONG, V.A. UDOVENKO, and S.D. PROKOSHKIN
The formation of the Ni₅Al₃ and Ni₂Al phases in Ni-Al alloys with L1o = B2 thermoelastic martensitic transformation has been studied by X-ray analysis. Ni₅Al₃ can form from both the L1o and B2 structures, but the kinetics of L1o Ni₅Al₃ and B2 Ni₅Al₃ reactions are significantly different. A homogeneous mechanism for the former reaction and a mechanism of precipitation and growth for the latter are proposed. Ni₂Al forms from the B2 structure by the complex rearrangement of atoms. The initial stage of this reaction proceeds very rapidly and involves segregation of Ni atoms into Ni-rich zones leading to a Ni depletion in the surrounding regions. The nucleation of Ni₂Al retards the Ni₅Al₃ formation, so preaging in the B2 region affects the kinetics of the L1o Ni₅Al₃ reaction on further aging in the L1o region. The microstructural mechanism for this effect is suggested.

MECHANICAL BEHAVIOR

Modeling and Microstructure Analysis of Fatigue Initiation Life Extension by Reductions in Microporosity
J.H. ELSNER, E.P. KVAM, and A.F. GRANDT, Jr.
Fatigue initiation lifetimes were demonstrated to be related to the size and density of microporosity in the midplate region for three differently processed variations of aluminum alloy 7050-T7451. Metallographic and fractographic examination of double edge notched fatigue specimens was performed to characterize microstructural inhomogeneities. A greater size and density of micropores were found for those materials that had failed at shorter fatigue lifetimes. The specific crack-initiating feature was identified for each of the known cracks. Linear elastic fracture mechanics modeling of crack initiation from each of the micropore-induced cracks showed that this could be adequately accounted for by a stress intensity factor analysis proposed by Trantina and Barishpolsky.

Strain Distribution in Copper Tensile Specimens Prestrained in Rolling
J.V. FERNANDES and M.F. VIEIRA
Sequences of orthogonal rolling-tension experiments were performed on polycrystalline copper sheets. The effect of strain path change on subsequent yield and flow behavior has been investigated. Optical microscopy and transmission electron microscopy (TEM) were used to clarify the physical mechanisms occurring during the second deformation. The observed increase in yield stress in reloading was related to the change of slip systems corresponding to the glide of dislocations with a Burgers vector, which had not been active during prestrain. The transient observed in the work- hardening behavior after the path change corresponds to the appearance of disorganization in the dislocation microstructure. It was shown that no special feature of slip behavior inside the grains can be related to the nonhomogeneous surface deformation observed at the beginning of reloading. Also, the plastic instability of prestrained samples corresponding to the maximum load in tension does not seem to be directly controlled by the developed local substructure. The nonuniform deformation observed in reloading was studied using a simplified macroscopic two-zone model. It takes into account the presence of geometrical defects in the samples and considers the importance of the mechanical behavior. The macroscopic results, concerning the delay of starting deformation in some regions, are explained by the model, which allows formulation of an analytical condition necessary for deformation to spread through the length of the sample before necking takes place.

Modeling Charpy Impact Energy Property Changes Using a Bayesian Method
R. MOSKOVIC, P.L. WINDLE, and A.F. SMITH
Surveillance schemes that monitor the effect of neutron irradiation on reactor pressure vessel materials employ Charpy impact specimens that are periodically withdrawn and tested as a function of test temperature. The resulting Charpy impact absorbed energy curves have been modeled by a three- parameter relationship with the same functional form as the Burr distribution function. The parameters of the Burr distribution function have been represented as a function of irradiation variables: dose and temperature. The method of maximum likelihood estimation has been used to obtain estimates of model parameters together with their standard errors. A method is presented for the evaluation of uncertainties in Charpy impact energy curves and temperature shifts resulting from irradiation damage. A sampling approach, based on Bayesian inference and employing Markov chain Monte Carlo (MCMC) simulation, has been used to quantify the uncertainties. The main idea of MCMC sampling is to generate the distributions of the model parameters by successive random sampling from the multivariate normal distribution. Illustrative results are presented for irradiated submerged arc weld metal.

Effects of Aluminum Content on the Mechanical Properties of a 9Cr-0.5Mo-1.8W Steel
H. NAOI, M. OHGAMI, X. LIU, and T. FUJITA
The effects of aluminum content on mechanical properties of a 9Cr-0.5Mo-1.8W steel have been investigated. It was found that aluminum addition had a beneficial effect on toughness, but significantly reduced the creep resistance of the steel, especially on the long-term side. Examination of the microstructure and precipitation characteristics revealed that almost all of the aluminum added existed as AIN-type nitrides after normalizing and tempering. The undissolved AIN in high aluminum steels resulted in a dramatic refinement of prior austenite grains, which contributed to the improvement of toughness and was also partially responsible for the decreased creep rupture strength. The formation of AIN suppressed the precipitation of VN-type nitride; AIN also provided formation sites for Nb(C, N) and M₂₃C₆ type, which had an equivalent effect to the coalescence of these precipitates on AIN, resulting in the reduction of precipitate density and, therefore, decreased creep resistance.

Isotropic and Kinematic Hardening in a Dispersion-Strengthened Aluminum Alloy
A.P. REYNOLDS and J.S. LYONS
The room temperature mechanical behavior of a dispersion strengthened aluminum alloy was examined in tension, compression, and in fully reversed loading. The alloy, 8009, is characterized by a high volume fraction of 50-100 nm dispersoid (25%) and 0.5 mm grain size. In tension, 8009 exhibits low strain to UTS and large post uniform elongation; in compression, near steady state deformation is observed after 2-3% strain. The Bauschinger effect was quantified as a function of prestrain in the forward direction. The experimental reverse loading curves were compared to those expected for ideal isotropic hardening and ideal K1 type kinematic hardening. The alloy exhibits nearly pure kinematic hardening of the K1 type. Based on the microstructure and the fully reversed loading behavior, the monotonic deformation behavior is explained.

Tensile Properties of a Thermomechanically Processed Ductile Iron
C.K. SYN, D.R. LESUER, and O.D. SHERBY
A ductile cast iron was continuously hot-and-warm-rolled or one-step-forged from a temperature in the austenite range (900°C to 1100°C) to a temperature below the A₁ temperature. Various amounts of reduction were used (from 60 to more than 90 pct). Tensile properties including tensile strength and total elongation were measured along the directions parallel and transverse to the rolling direction and along the direction transverse to the forging direction. The tensile ductility and strength both increase with a decrease in the amount of hot-and-warm working (HWW). Compared with the results obtained by other investigators, the present results showed higher strengths and ductilities over the same range of reduction in thickness. The improvement in properties is related to the lower temperature of the postprocessing heat treatment given in this study (600°C) compared to other studies (900°C). The low temperature of heat treatment leads to a structure of fine graphite in a matrix of ferrite and carbides, whereas the high temperature of heat treatment leads to coarse graphite in a matrix of carbide-free ferrite. The delay in failure from the presence of the small graphite constituent results in an increase in tensile ductility with an accompanying increase in tensile strength.

Communication: An Examination of Creep Data for an Al-Mg Composite
YONG LI and TERENCE G. LANGDON

ENVIRONMENT

SURFACE TREATMENT

Microscopic Investigation of Sensitized Ni-Base Alloy 600 after Laser Surface Melting
YUN SOO LIM, JEONG HUN SUH, IL HYUN KUK, and JOUNG SOO KIM
A study using a transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer was carried out to investigate microstructural and compositional changes in sensitized Ni-base Alloy 600, the surface of which had been melted by a CO₂ laser beam. Cr-rich carbides, having formed along grain boundaries during sensitization treatment, were dissolved completely or partially in the heat-affected zone (HAZ) close to or far from the laser-melted zone (LMZ), respectively, accompanied by the disappearance of Cr depletion at the boundaries. The microstructure of the LMZ consisted of fine cells in the grains having epitaxially solidified from the HAZ, and Cr concentration was observed to increase along the cell and grain boundaries in the LMZ. In addition, the cell walls in the LMZ were decorated with dislocations, and very fine particles, found to be a TiN type, were distributed randomly along the cell walls with tangled dislocations around them. The high dislocation density in the cell walls is attributed to the residual tensile stress developed during the rapid solidification.

SOLIDIFICATION

Solidification of Hypereutectic Al-38 Wt Pct Cu Alloy in Microgravity and in Unit Gravity
HONG YU, K.N. TANDON, and J.R. CAHOON
Solidification in microgravity aboard the space shuttle Endeavour resulted in a dramatic change in the morphology of the primary Al₂Cu phase compared to ground-based solidification in unit gravity. An Al-38 wt pct Cu ingot directionally solidified at a rate of 0.015 mm/s with a temperature gradient of 1.69 K/mm exhibited large, well-formed dendrites of primary Al₂Cu phase. Ingots solidified under similar conditions in unit gravity contained primary Al₂Cu phase with smooth, faceted surfaces. The primary Al₂Cu phase spacing in the microgravity ingot was much greater than that in the unit gravity ingot, 670 µm compared to 171 µm. It is suggested that thermosolutal mixing in the unit gravity ingot reduces the buildup of an Al-rich layer at the solid/liquid interface, which increases the stability of the interface resulting in smooth, faceted particles of Al₂Cu phase. It is also suggested that the large difference in primary phase spacings is due mostly to the difference in morphology rather than changes in parameters that might influence dendrite ripening mechanisms. The presence or absence of gravity had no effect on the interlamellar spacing of the inter-Al₂Cu phase eutectic. The ingot solidified in microgravity exhibited almost no longitudinal macrosegregation, in agreement with the theory of inverse segregation in the absence of thermosolutal convection. The ingot solidified in unit gravity exhibited considerable longitudinal macrosegregation, with the chilled end having about 6 wt pct more Cu than the average composition. It is not clear whether the segregation results from thermosolutal convection during solidification or from sedimentation during melting.

COMPOSITE MATERIALS

Microstructure and Properties of Spray-Deposited 2014 + 15 Vol Pct SiC Particulate-Reinforced Metal Matrix Composite
J.S. ZHANG, X.J. LIU, H. CUI, X.J. DUAN, Z.Q. SUN, and G.L. CHEN
A metal matrix composite (MMC) of 2014 aluminum alloy reinforced with 15 vol pct SiC particulate was produced by the spray-forming-deposition process. The as-deposited preform revealed a high density and a homogeneous reinforcement distribution. Reactive products were not found on interfaces between the reinforcement and the matrix. Compared to the control alloy, the composite showed accelerated aging after solutionizing at 502°C, while aging was retarded after solutionizing at 475°C. Analysis indicated that the activation energy was almost the same for the aging process after different solutionizing treatments. This suggested that while the thermal barrier for the aging process was the same, other factors affecting the aging process should be considered. For example, the effective concentration of the precipitate forming elements possibly decreased after incompletely solutionizing at 475°C. After heat treatment, the composite showed a tensile strength similar to the control alloy. The wear resistance of the composite improved considerably. The aging behavior of the composite was also studied using the nanoindentation technique. Steep gradient distribution of elastic modulus and hardness around the reinforcement SiC particulate was observed. Theoretical analysis showed that this could be attributed to the gradient distribution of precipitates, resulting from a gradient distribution of dislocation density around the SiC particulates caused by residual thermal misfit stresses.