METALLURGICAL AND MATERIALS TRANSACTIONS A
	CONTENTS and ABSTRACTS Volume 29A, No. 12, December 1998 This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Welding & Joining; Solidification; Materials Processing; Composite Materials.

ALLOY PHASES

Al-Si alloys are materials that have been developed over the years to meet the increasing demands of the automotive industry for smaller, lighter-weight, high-performance components. An important alloy in this respect is the 319 alloy, wherein silicon and copper are the main alloying elements, and magnesium is often added in automotive versions of the alloy for strengthening purposes. The mechanical properties are also ameliorated by modifying the eutectic silicon structure (strontium being commonly employed) and by reducing the harmful effect of the -Al₅FeSi iron intermetallic present in the cast structure. Magnesium is also found to refine the silicon structure. The present study was undertaken to investigate the individual and combined roles of Mg and Sr on the morphologies of Si, Mg₂Si, and the iron and copper intermetallics likely to form during the solidification of 319-type alloys at very slow (close to equilibrium) cooling rates. The results show that magnesium leads to the precipitation of Al₈Mg₃FeSi₆, Mg₂Si, and Al₅Mg₈Cu₂Si₆ intermetallics. With a strontium addition, dissolution of a large proportion of the needle-like -Al₅FeSi intermetallic in the aluminum matrix takes place; no transformation of this phase into any other intermetallics (including the Al₁₅(Fe,Mn)₃Si₂ phase) is observed. When both Mg and Sr are added, the diminution of the -Al₅FeSi phase is enhanced, through both its dissolution in the aluminum matrix as well as its transformation into Al₈Mg₃FeSi₆. The reactions and phases obtained have been analyzed using thermal analysis, optical microscopy, image analysis, and electron microprobe analysis (EMPA) coupled with energy-dispersive X-ray (EDX) analysis.

TRANSFORMATIONS

Recrystallization Behavior of Boron-Doped Ni₇₆Al₂₄ [pp. 2893-2902]
SANDIP GHOSH CHOWDHURY, A.K. JENA, and R.K. RAY
Considerable hardness recovery and almost complete restoration of order were observed during annealing of 85 pct cold-rolled boron-doped Ni₇₆Al₂₄ prior to recrystallization. Recrystallization kinetics were investigated over a wide range of temperatures at small intervals of transformed volume fraction. The variation of transformed volume fraction with time showed two unusual distinct regions characterized by separate Avrami exponents and activation energies. The exponent decreased from 2.2 at 800°C to 0.7 at 950°C for X > 0.6 and had a temperature-insensitive value of 3 for X > 0.6. The activation energy below 900°C was 145 kJ/g atom for X < 0.6 and 110 kJ/g atom for X > 0.6, and that above 900°C was about 435 kJ/g atom. Equations for the kinetics have been derived based on concurrent recovery in the untransformed regions during recrystallization. The predictions of the equations agree exceedingly well with the experimental results and yield separate values of the activation energies for interface migration and recovery.

Clusters in Carbon Martensite: Thermodynamics and Kinetics [pp. 2903-2912]
L. DABROWSKI
An original method of evaluation of the cluster population in carbon martensite has been developed. Using this method, it is shown that Kurdjumov's model of carbon redistribution within the different octahedral site sublattices can quantitatively account for both observed normal and abnormal tetragonality in carbon martensite. It is also shown that the existence of the internal strains in martensite constitutes a necessary and sufficient condition for the energetic preference of tetrahedral over the cubic lattice. The presence of the residual tetragonal distortion in the quasi-cubic phase of k-martensite is associated with the presence of the mixed clusters formed of the atoms belonging to O_c sublattice as well as to remaining ones. By using a computer simulation of the dynamical behavior of carbon martensite approaching the thermodynamical equilibrium, it was found that the ultimate state of this system is strongly beyond the thermal equilibrium. Even after long-term aging, the free energy is far beyond the minimum value allowed for this system. The reason for such a behavior and the possible aging processes proceeding in this system are discussed at the molecular level. All of the ordering parameters are affected by the aging process. The evolution proceeds in the distinctly different time intervals for different parameters. At first, the long-range ordering parameter that determines the tetragonality of martensite evolves and reaches the stable value. In the next stage, the formation and then disintegration of two-particle clusters occurs. Disintegration of two-particle clusters coincides with the stage when three-particle cluster formation occurs at a high rate. Three-particle clusters also disintegrate when some time elapses. The same pattern repeats regarding four-, five-, six-, seven-, and eight-particle clusters. To simplify the calculations, the nine-particle clusters are assumed to be the largest possible and are identified with an existence of superstructure. The formation of 100 pct of nine-particle clusters with no contribution of free atoms in an alloy ceases all aging processes. The evolution of these processes is illustrated graphically in the time range from 16 seconds to 1500 years, as estimated on the basis of experimental data.

Nonclassical Decomposition Products of Austenite in Fe-C-Cr Alloys [pp. 2913-2924]
A.A. KAYA and D.V. EDMONDS
Unusual austenite decomposition products in two Fe-0.4C alloys containing chromium additions of 3.5 and 10 wt pct have been studied. Detailed transmission electron microscopy (TEM) has been carried out on partially transformed specimens in order to determine the identities and morphologies of the phases and the mode of formation. The most descriptive terms for these novel products are spiky pearlite and acicular ferrite/carbide aggregates. The spiky pearlite is distinguished by its non-nodular transformation front and by the presence of individual segments or units composed of ferrite-sheathed carbides. The acicular aggregates appear as dark-etching, macroscopic plate-shaped structures that are formed from the successive nucleation of these single ferrite/carbide subunits, which are crystallographically related to the austenite grain in which they grow, with a predominant orientation. The uniqueness of these structures has been reinforced by the detection of customary pearlite in both of the alloys and by the presence of classical upper and lower bainites in the low-chromium alloy. It is proposed that the structures develop as a result of the oriented coupled growth of the individual ferrite/carbide segments identified by the study.

The Effect of Geometrical Assumptions in Modeling Solid-State Transformation Kinetics [pp. 2925-2931]
YVONNE VAN LEEUWEN, SIMONE VOOIJS, JILT SIETSMA, and SYBRAND VAN DER ZWAAG
In the quest for the ideal transformation model describing the austenite decomposition in steel, emphasis shifts from empirical to physical models. This has resulted in the widely used description of the transformation by means of the interface velocity between the parent phase and the newly formed phase, a description which yields reliable predictions of the transformation behavior only when combined with a realistic austenite geometry. This article deals with a single-grain austenite geometry model applied to transformations in which the interface velocity is constant throughout the transformation, e.g., certain types of massive transformations. The selected geometry is a regular tetrakaidecahedron, combining topological features of a random Voronoi distribution with the advantages of single-grain calculations. The simulations show the influence of the ferrite-nucleus density, the relative positions of the ferrite nuclei inside the austenite grain, and the grain-size distribution. From simulations with a constant interface velocity, the transformation behavior for a tetrakaidecahedron is in agreement with transformation kinetics in terms of the Johnson-Mehl-Avrami (JMA) model. Using the tetrakaidecahedron geometry, one can simulate transformation curves that can be experimentally obtained by calorimetry or dilatometry, in order to study the quantities affecting the transformation behavior.

TRANSPORT PHENOMENA

Liquidlike Sintering Behavior of Nanometric Fe and Cu Powders: Experimental Approach [pp. 2941-2949]
O. DOMINGUEZ, Y. CHAMPION, and J. BIGOT
Nanometric Fe and Cu powders were sintered in vacuum, He, and H₂ atmospheres after uniaxial cold pressing. The shrinkage behavior of samples was studied using three different dilatometric techniques: constant heating rate, isothermal annealing, and the Dorn method. Density greater than 90 pct was obtained at sintering temperatures of 900°C. In nanometric powders, densification and grain coarsening occurred in a narrow temperature interval. Despite the low oxide content in the starting powders (1.5 to 4 wt pct), the reducing atmosphere plays a relevant role in the sintering process. The self-diffusion activation energies obtained for nanometric Fe were 116 and 60 kJ/mole in vacuum and H₂, and those obtained for nanometric Cu were 70 and 43 kJ/mole in He and H₂. According to the present results, the activation energies obtained from both nanometric powders in H₂ could be associated with those for self-diffusion in liquid Fe (65 kJ/mole) and Cu (41 kJ/mole).

A Diffusion-Kinetic Model for Predicting Solder/Conductor Interactions in High Density Interconnections [pp. 2951-2956]
K.J. RÖNKÄ, F.J.J. VAN LOO, and J.K. KIVILAHTI
A combined thermodynamic and diffusion-kinetic approach is very viable for developing microjoining and interconnection materials and processes, in particular, whenever thinner metallizations, coated overlayers, or smaller solder-joint volumes are encountered in very high density electronics. A diffusion-kinetic model based on the utilization of integrated diffusion coefficients and mobilities is introduced and discussed for calculating the layer growth of intermetallic compounds between metal conductors and tin-based solders and is exemplified with a relatively simple ternary Cu/SnBi system. The model has also been used for calculating the local nominal composition of the effective joint or contact region. Moreover, the mobilities of Sn and Cu in Cu₆Sn₅ and Cu₃Sn intermetallic compounds are determined, and the role of both stable and metastable phase diagrams is discussed in predicting the appearance of possible reaction products as well as the driving forces for the dissolution, diffusion, and precipitation processes.

MECHANICAL BEHAVIOR

Experimental and Theoretical Studies of the Superposition of Intergranular and Macroscopic Strains in Ni-Based Industrial Alloys [pp. 2967-2973]
T.M. HOLDEN, C.N. TOMÉ, and R.A. HOLT
Measurements of the strain response to applied stress in polycrystalline MONEL-400 by neutron diffraction are modeled with the elastoplastic self-consistent (EPSC) theory. The strains in the different crystallographic orientations of grains, which are generated in the tensile test experiments, are shown to be caused by the anisotropy of elastic and plastic deformation with respect to crystallographic orientation. On the basis of the description of the results in the theory, the origin of a number of anomalies of a general nature in measurements by both neutron and X-ray diffraction can be understood. The theory is used to calculate which crystallographic reflections are least sensitive to intergranular effects under uniaxial tension.

Microstructural and Mechanical Behavior of a Duplex Stainless Steel under Hot Working Conditions [pp. 2975-2986]
A. IZA-MENDIA, A. PIÑOL-JUEZ, J.J. URCOLA, and I. GUTIÉRREZ
In the hot deformation of the duplex stainless steels, the complexity of the microstructure evolution and mechanical response is increased as compared with those of single-phase ferritic or austenitic stainless steels. In the present work, plane strain compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior of a duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of interface, and the relative mechanical properties of both phases. The law of mixtures has been used to explain the different flow curves obtained when changing the phase distribution and/or the deformation mode. On deforming as-cast microstructures, the deformation partitions vary heterogeneously between both phases and some austenite areas act as hard nondeforming particles. Cracks have been observed to occur at the interface of such regions, from relatively low strains, for which the initial Kurdjumov-Sachs orientation relationship between ferrite and austenite is still present.

Noncontact Ultrasonic Spectroscopy on Deforming Polycrystalline Copper: Dislocation Damping and Acoustoelasticity [pp. 2987-2993]
HIROTSUGU OGI, NAGAYOSHI SUZUKI, and MASAHIKO HIRAO
Electromagnetic acoustic resonance (EMAR) is developed for the continuous measurement of the bulk-wave attenuation and phase velocities in a metal during a deformation process. The EMAR enables one to perform the noncontact measurement with extremely high sensitivity, in which the electromagnetic acoustic transducer (EMAT) generates and detects the bulk waves without any coupling material. The attenuation and velocity responses to the uniaxial stress were continuously recorded for 99.99 wt pct pure polycrystalline copper annealed at 200°C for 1 hour before loading. We separated the velocity change due to the acoustoelastic effect from the contribution of the dislocation movement responding to the ultrasonic waves, and determined the pure third-order elastic constants. The shear wave showed much larger sensitivity to the dislocation mobility than the longitudinal wave. The discontinuous change in the incremental rate of the shear wave attenuation was observed in the elastic region, which was interpreted as the onset of the microscopic yielding.

Thermomechanical Fatigue Behavior of the High-Temperature Titanium Alloy IMI 834 [pp. 2995-3004]
P. POTOTZKY, H.J. MAIER, and H.-J. CHRIST
The isothermal and thermomechanical fatigue (TMF) behavior of the titanium alloy IMI 834 was studied between 350°C and 650°C in air and vacuum, respectively. Transmission electron microscopy (TEM) observations revealed that the microstructure established in the TMF tests was governed by the maximum temperature within the cycle. However, if the maximum temperature does not exceed 600°C, planar dislocation slip prevails and similar microstructures are formed regardless of the test temperature and the testing mode (TMF and isothermal, respectively). As a result, the stress-strain response in TMF tests can be assessed from the corresponding isothermal data. Wavy dislocation slip was found to determine the stress-strain behavior if the maximum test temperature exceeded 600°C. Moreover, in TMF tests with a maximum test temperature of 650°C, the dislocation arrangement formed in the high-temperature part of the hysteresis loop was found to be stable throughout the cycle and to affect significantly the stress-strain response at the low temperatures. Although in-phase (IP) and out-of-phase (OP) TMF tests led to an almost identical microstructure, OP loading was always found to be most detrimental. The interaction between the embrittled subsurface layer, caused by oxygen uptake, and the high tensile stresses developing in the low-temperature part of the hysteresis loop in OP tests eases crack initiation and initial crack propagation and results in reduced fatigue life.

Dependence of Fracture Toughness of Austempered Ductile Iron on Austempering Temperature [pp. 3005-3016]
P. PRASAD RAO and SUSIL K. PUTATUNDA
Ductile cast iron samples were austenitized at 927°C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260°C, 288°C, 316°C, 343°C, 371°C, and 399°C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K²_IC against _y (XC)^1/2 resulted in a straight line, as predicted by the model.

Effects of Test Temperature on Internal Fatigue Crack Generation Associated with Nonmetallic Particles in Austenitic Steels [pp. 3017-3028]
OSAMU UMEZAWA and KOTOBU NAGAI
Internal crack generation associated with nonmetallic inclusions or precipitates has been investigated on high-cycle fatigue at 4 K, 77 K, and 293 k of 25Mn-5Cr high-manganese austenitic steel and nitrogen-strengthened 25Cr-13Ni austenitic stainless steel. In both steels, the internal crack initiation typically occurred at 4 K or in long-life range over 10⁶ cycles at 77 K. Particles such as inclusions and precipitates were responsible for the internal crack-generation behavior, and the origins were identified as mainly Al₂O₃ inclusions in 25Mn-5Cr steel and AlN precipitates in 25Cr-13Ni steel, respectively. We discuss the crack-generation stage I mechanism and the relationship between stress range and size of crack-initiation site. The generation of fatigue cracks associated with the nonmetallic particles in the specimen interior involved a stage I crack. A threshold condition assumption was proposed, that the crack propagation occurred at any stress level when the local stress intensity factor range reached over a constant at or around the initiation crack associated with defects.

Use of the Nanoindentation Technique for Studying Microstructure/Crack Interactions in the Fatigue of 4340 Steel [pp. 3029-3036]
FAN YANG, ASHOK SAXENA, and LAURA RIESTER
The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.

WELDING & JOINING

SOLIDIFICATION

MATERIALS PROCESSING

Improving the Weldability and Service Performance of Nickel- and Iron-Based Superalloys by Grain Boundary Engineering [pp. 3069-3079]
E.M. LEHOCKEY, G. PALUMBO, and P. LIN
The principal limitation of today's Ni- and Fe-based superalloys continues to be their susceptibility to intergranular degradation arising from creep, hot corrosion, and fatigue. Many precipitation-strengthened superalloys are also difficult to weld, owing to the formation of heat-affected zone (HAZ) cracks during postweld heat treatments (PWHTs). The present work highlights significant improvements in high-temperature intergranular degradation susceptibility and weldability arising from increasing the relative proportion of crystallographically "special" low- CSL grain boundaries in the microstructure. Susceptibility to intergranular degradation phenomena is reduced by between 30 and 90 pct and is accompanied by decreases in the extent and length of PWHT cracking of up to 50-fold, with virtually no compromise in mechanical (tensile) properties upon which the functionality of these specialty materials depends. Collectively, the data presented suggest that "engineering" the crystallographic structure of grain boundaries offers the possibility to extend superalloy lifetimes and reliability, while minimizing the need for specialized welding techniques which can negatively impact manufacturing costs and throughput.

On the Relation between the Number-Weighted and Volume-Weighted Grain Volume Distribution Parameters [pp. 3081-3086]
HAIBO YU, GUOQUAN LIU and XIAOYAN SONG
Quantitative relations are shown to exist between the number-weighted and the volume-weighted grain volume distribution parameters, provided the theoretical grain volume distribution function (GVDF) is given. These relations have been derived when the GVDF is described by a gamma or a lognormal distribution function. By comparing the experimental data of austenite grains in a low-carbon steel to those reported in literature, it is shown that the GVDF may be described more appropriately by a gamma than by a lognormal distribution function. Furthermore, a new method for calculating the GVDF from measurements made on random plane sections has been discussed and justified.

COMPOSITE MATERIALS

In Al-3 wt pct Mg/Al₂O₃ (or SiC) composites fabricated by the pressureless infiltration method, the infiltration behavior of molten metal, the mechanical properties, and the interfacial reactions were investigated. The spontaneous infiltration of the molten Al-3 wt pct Mg alloy into the powder bed occurred at a relatively low temperature (700°C for 1 hour under a nitrogen atmosphere). Spontaneous infiltration of the molten metal is related to the formation of Mg₃N₂ by the reaction of Mg and nitrogen. The tensile strength and 0.2 pct offset yield strength and elongation tend to decrease with increasing infiltration temperature and time, because of an increased interfacial reaction. In Al-3Mg/Al₂O₃ composites, MgAl₂O₄ was observed at interfaces between Al₂O₃ and the matrix, as well as at oxide films of the Al powder surface. In addition, MgO was observed at interfaces between Al₂O₃ and the matrix. On the other hand, Al₄C₃ was formed at interfaces between SiC and the matrix in Al-3Mg/SiC composites. In addition, MgAl₂O₄ was observed as a reaction product at the interfaces between oxide films of SiC and the matrix, as well as at oxide films of the Al powder surface. Since the Si released as a result of the interfacial reaction is combined with Mg, age hardening can occur by the precipitation of Mg₂Si via T6 treatment.

Table of Contents and Abstracts, Metallurgical and Materials Transactions B, December 1998 [pp. 3096-3097]

Combined Index to Volumes 29A and 29B [follows page 3097]