Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1992"

Thermodynamic Assessment and Calculation of the Ti-Al System

Abstract: A thermodynamic description of the Ti-Al system has been developed. Three different analytical descriptions were used to describe the three different types of phases occurring in the Ti-Al system: the stoichiometric compounds, the disordered solution phases, and the ordered inter-metallic compounds which have homogeneity ranges. A least-squares technique was used to optimize the thermodynamic quantities of the analytical description using experimental data available in the literature. The calculated phase diagram, as well as the thermodynamic func-tions, agree well with the critically evaluated experimental data from the literature.

Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel

Abstract: The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100 °C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained aus- tenite stability.

An analytical model for the interaction between an insoluble particle and an advancing solid/liquid interface

Abstract: Most models that describe the interaction of an insoluble particle with an advancing solid-liquid interface are based on the assumption of steady state. However, as demonstrated by experimental work, the process does not reach steady state until the particle is pushed for a while by the interface. In this work, a dynamic mathematical model was developed. The dynamic model demonstrates that this interaction is essentially non-steady state and that steady state eventually occurs only when solidification is conducted at sub-critical velocities. The model was tested for three systems: aluminum-zirconia particles, succinonitrilepolystyrene particles, and biphenyl-glass particles. The calculated values for critical velocity of the pushing/engulfment transition were in same range with the experimental ones.

Analysis of low-temperature intermetallic growth in copper-tin diffusion couples

Abstract: A multiphase diffusion model was constructed and used to analyze the growth of the e- and η-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements of intermetallic layer growth were used to compute the interdiffusivities in thee andη phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the e phase(≈De) is well fit by an Arrhenius expression with D0 = 5.48 × 10−9 m2/s andQ = 61.9 kJ/mole, while that in the η phase (≈Dη) has D0= 1.84 × 10−9 m2/s andQ = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in theη phase has the consequence that theη phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in theη phase has the result that thee phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20 °C, the predicted e-phase thickness is ≈10 pct of that ofη, while at 200 °C, its thickness is 66 pct of that ofη. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within theη-phase layer after diffusion. It lies near the midpoint of theη-phase layer at higher temperature (220 °C) and, hence, appears to shift toward the Sn side of the couple. The results are compared to experimental observations on intermetallic growth at solder-Cu interfaces.

Automatic analysis of electron backscatter diffraction patterns

Abstract: The ability to measure lattice orientation in individual crystallites enables a more complete characterization of microstructure by combining lattice orientation with morphological features. Lattice orientation can be obtained by analyzing electron backscatter diffraction patterns (EBSPs). However, current computer-aided EBSP analysis techniques make it impractical to obtain the number of measurements needed for statistically reliable characterizations of microstructure. An effective fully automated technique for determining crystallographic orientation from EBSPs is described. Bands are identified by linear regions of correlation in the image intensity gradient direction. The most probable orientation is then found using the angles between the detected bands. The reliability of the technique was tested using a set of 1000 patterns obtained from annealed oxygen-free electrical grade (OFEC) copper. The orientation of each test pattern found using automatic indexing was checked against the corresponding orientation as determined by manual indexing. Ninety-nine percent of auto-indexed orientations were found to lie within 5 deg of the misorientation angle of the manual-indexed orientations. By simulating noise in the test patterns, it was found that image quality has a strong effect on the reliability of the technique. An image quality parameter is described which allows the reliability of the technique to be predicted for a pattern of given quality.

The effect of participate reinforcement on the sliding wear behavior of aluminum matrix composites

Abstract: The aim of the present investigation is to characterize the friction and wear behavior of aluminum matrix composites reinforced with particulates of SiC, TiC, TiB2, and B4C. Sliding wear tests were conducted at two loads (80 and 160 N) using a pin-on-disc apparatus and under dry conditions. The results of the investigation indicate that the coefficient of friction of the composites is about 30 pct lower than that of pure aluminum, while the wear rates of the com- posites are lower by a factor of about 3 and 100 at loads of 80 and 160 N, respectively. The type and size of the reinforcement have a negligible influence on the wear rate and the coefficient of friction of the composites. However, the volume fraction of the reinforcement has a marginal influence on the wear rate. Though the coefficients of friction and the wear rates of the com- posites were broadly similar, the Al-TiC composite alone exhibits a somewhat higher wear rate. The above results of the present investigation have been rationalized on the basis of the inverse rule of mixtures and the existing models for friction and wear.

Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

Abstract: The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.

The growth of Cu-Sn intermetallics at a pretinned copper-solder interface

Abstract: This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer The η phase (Cu6Sn5) forms when Cu is wet with eutectic solder at temperatures below 400 °C The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and e phase (Cu3Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich boundary layer forms in the solder The aging behavior of a thin, pretinned eutectic layer is qualitatively different At 170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to e phase The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk How ever, the development of the intermetallic largely ceases when the Sn is consumed; e phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases

Simple constitutive equations for steel at high temperature

Abstract: This work develops and investigates simple unified constitutive equations to model the mechanical behavior of plain carbon steel in the austenite temperature region for use in finite element stress analysis of processes such as continuous casting. Four different forms of constitutive relations are considered: constant structure, time-hardening, strain-hardening, and simultaneous time- and strain-hardening models. Each relation is judged on its ability to reproduce experimental data from both tensile and creep tests and its ability to exhibit reasonable behavior under complex loading conditions. Three of the equations appear suitable for small strain monotonic loading conditions for a wide range of low strain rates (10−3 to 10−6 s−1), high temperatures (950 °C to 1400 °C), and varying carbon contents (0.005 to 1.54 wt pct C).

Low-temperature oxidation of molybdenum disilicide

Abstract: Cyclic oxidation rates of 95 to 97 pct dense, powder-source molybdenum disilicide (MoSi2) in dry air, wet air, and oxygen have been measured between 400°C and 600°C. Dense MoSi2 does not disintegrate catastrophically (pest) in these atmospheres for exposure times up to 688 hours. Between 400°C and 500°C, Mo and Si oxidize simultaneously to form amorphous SiO2, monoclinic Mo9O26, and vapor-deposited MoO3 plates, and the oxidation rate of MoSi2 in air is influenced by its microstructure, composition, and surface defects. Rapid oxidation obeying a linear rate law occurs over a narrow temperature range near 500°C, where Mo vapor transport by (MoO2) n species is sufficiently rapid to produce large numbers of surface MoO3 plates but simultaneously is slow enough to allow nucleation and growth of solid Mo oxides in conjunction with SiO2. Addition of water vapor to the oxidant stream at 500°C retards nucleation and growth of solid Mo oxides by formation of MoO3·H2O (g), which has a high vapor pressure relative to those of (MoO3) n species. The transition from nonselective oxidation to high-temperature selective oxidation of Si to form a protective SiO2 layer occurs between 500°C and 550°C. Preoxidation of MoSi2 at 1200°C creates a SiO2 barrier layer which prevents further oxidation upon subsequent exposure at 500°C. The oxidation kinetics and microstructural observations support the model of MoSi2 pest in which oxidation in pores and cracks is required for disintegration. Based on these results, low-temperature oxidation phenomena are not expected to restrict the use of MoSi2 as a high-temperature material.

Kinetics of the growth of spinel, MgAl2O4, on alumina particulate in aluminum alloys containing magnesium

Abstract: The growth of spinel in melts of alumina-reinforced composites not only consumes the magnesium required for age hardening, but also acts to increase the viscosity and thus adversely affects the castability of the material. The kinetics of this growth have been studied in molten Al-1 pct Mg alloys in the temperature range of 948 to 1073 K. The results are shown to fit an equation of the form ln(1−α = k(t0 −t) describing deceleratory growth on fine particles, where α is a dimensionless reaction parameter. The rate constant,k, fits an Arrhenius equation giving an activation energy of 103 ± 7 kJ and a time constant of 50 s−1. The incubation time,t0, which is about 2000 seconds forT 1000 K.

Ferrite: Cementite crystallography in pearlite

Abstract: The interlamellar crystallography of pearlite in iron-carbon alloys is examined in Fe-0.8C and Fe-0.81C-12Mn steels with attention given to the Bagaryatsky, Pitsch-Petch, and Isaichev orientation relationships and their related atomic habit planes. A survey of the overall literature reveals confusion about the crystallographic uniqueness in these orientation relationships. The present article focuses on this uniqueness and on habit planes between the pearlitic ferrite and cementite, from which the crystallography of ferrous pearlite orientation relationships is proposed to emanate. Techniques to characterize and identify orientation relationships unambiguously are developed. Habit planes are shown to be consistently obtained, even during curvature, with all deviations accommodated by microscopic steps.

The Role of grain boundary misorientation in intergranular cracking of Ni-16Cr-9Fe in 360 °C argon and high-Purity water

Abstract: The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining if low-angle boundaries (LABs) or coincident site lattice boundaries (CSLBs) are more crack resistant than general high-angle boundaries (GHABs) in argon and high-purity water. Cracking susceptibility of boundary types was determined using constant extension rate tensile tests (CERTs) in 360 °C argon and in deaerated, high-purity water. Annealed samples contained 12 to 20 pct CSLBs, while CSLB-enhanced samples contained 27 to 44 pct CSLBs; GHAB proportions varied accordingly. Cracked boundary fractions for CSLB-enhanced samples tested in either environment ranged from 0.01 to 0.08, while those for annealed samples ranged from 0.07 to 0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked in either environment. In annealed samples, the proportion of CSLBs that cracked in water was 6.7 pct compared to 1.5 pct in argon; the proportion of GHABs that cracked in water was 9.3 pct compared to 6.6 pct for argon. Thus, CSLBs are more crack resistant than GHABs in either environment, and both are more crack resistant in argon than in water. The higher amounts of cracking and the higher CSLB cracking susceptibility in high-purity water indicate the presence of an environmental effect on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary stresses and the propensity for crack initiation. The results indicate that control of grain boundary proportions can improve the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe.

Prediction of sintered density for bimodal powder mixtures

Abstract: Bimodal mixtures improve the green density of powder systems and are used in processes such as slip casting and powder injection molding. The packing density can be predicted with reasonable accuracy, but there is great uncertainty in the sintered density of a bimodal mixture. The large/small composition effect on packing density and sintered density is treated using the specific volume. For a given composition, the packing density is accurately predicted when four parameters are known: particle size ratio, packing density of the small powder, packing density of the large powder, and mixture homogeneity. Prediction of the sintered density is possible from knowledge of the densification of the large and small powders and mixture homogeneity. The model is applied to bimodal mixtures of molybdenum, stainless steel, iron, and alumina. Certain criteria must be satisfied by the constituent powders for a bimodal mixture to exhibit the highest sintered density. In many situations, the highest sintered density occurs at the 100 pct small powder composition.

Homogeneous nucleation kinetics of Al3Sc in a dilute Al-Sc alloy

Abstract: The homogeneous nucleation kinetics of the L12 A13Sc phase in the face-centered cubic (fcc) matrix of an A1-0.11 at. pct Sc alloy were measured at 561 and 616 K using the isothermal transformation technique and transmission electron microscopy (TEM). Applying classical homogeneous nucleation theory in conjunction with available thermodynamic data, an average interphase boundary energy of about 94 ±23 mJ/m2 was estimated from the nucleation rate data. This energy was found to vary weakly with temperature. The effects of heat-treatment technique (isothermal transformationvs quench and age) on nucleation kinetics indicate that quenched-in vacancies may influence nucleation kinetics primarily by increasing the interdiffusion of Sc in Al.

On the kinetics of mechanical alloying

Abstract: The kinetics of solid-state displacement reactions during mechanical alloying have been investigated. The effects of charge ratio and ball size on the progress of the reaction between CuO and Fe have been evaluated from measurements of ignition temperature, combustion time, and crystallite size. The reaction kinetics are shown to increase with charge ratio. This is rationalized in terms of the effect of charge ratio on the number of ball/particle collisions. Ball size influences reaction kinetics through both the particle collision frequency and collision energy.

Correlation of deformation mechanisms with the tensile and compressive behavior of NiAl and NiAl(Zr) intermetallic alloys

Abstract: To identify the mechanisms controlling strength and ductility in powder-extruded NiAl and NiAl + 0.05 at. pct Zr, tensile and compressive testing was performed from 300 to 1300 K for several grain sizes. Grain size refinement significantly increased yield stress in both alloys and, in some cases, slightly lowered the ductile-to-brittle transition temperature (DBTT), although no room-temperature tensile ductility was observed even in the finest grain size specimens. The small Zr addition increased the DBTT and changed the low-temperature fracture mode from intergranular in NiAl to a combination of intergranular and transgranular in the Zr-doped alloy. Scanning electron microscopy (SEM) of compression specimens deformed at room temperature revealed the presence of grain-boundary cracks in both alloys. These cracks were due to the incompatibility of strain in the poly crystalline material, owing to the lack of five independent slip systems. The tendency to form grain-boundary cracks, in addition to the low fracture stress of these alloys, contributed to the lack of tensile ductility at low temperatures. The operative slip system, both below and above the DBTT, was {110} 〈001〉 for both alloys. The change from brittle to ductile behavior with increasing temperatures was associated with the onset of diffusional processes.

The structure and mechanical properties of metallic nanocrystals

Abstract: Metallic nanocrystals are ultrafine-grained polycrystalline solids with grain sizes in the range of 1 to 10 nm in at least one dimension. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and thus, they possess novel, and often improved, properties over those of conventional polycrystalline or glassy materials. In comparison to more conventional materials, nanocrystalline materials show a reduced density; increased thermal expansion, specific heat, and strength; a supermodulus effect; and extremely high diffusion rates. Traditionally brittle materials can be made ductile by nano-structure processing. At present, there is considerabe confusion on the nature of the micro-structure and mechanical properties of the nanocrystalline materials, especially of the equiaxed (three-dimensional, 3-D) type. The present article reviews the current understanding of nanocrystals and evaluates the data available on structure and mechanical properties of nanocrystalline metals.

The role of microstructural instability on creep behavior of a martensitic 9Cr-2W steel

Abstract: The microstructural instability during creep and its effect on creep behavior were investigated for a martensitic 9Cr-2W steel. The steel was developed as a low radioactive steel suitable for fusion reactor structure. Creep testing was carried out at 873 K for up to 15,100 ks (4200 hours). The creep curve consisted of transition creep, where creep rate decreased with time, and acceleration creep, where creep rate increased with time. During creep, microstructural instability, such as the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains, was observed to occur, which resulted in softening but no hardening. The transition creep was a consequence of the movement and annihilation of excess dislocations, resulting in the decrease in dislocation density and the increase in martensite lath size with time. The acceleration creep was a consequence of a gradual loss of creep strength due to the microstructural instability which occurred from the initial stage of creep.

Development of Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed martensitic transformations: Part I. shape memory behavior

Abstract: As part of a study on the newly developed Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed (fcc-hcp) martensitic transformations, transfor-mation behavior is characterized utilizing a combination of electrical resistance, dilatometry, and magnetic susceptibility measurements. The characteristics of thermally induced and strain-induced e martensitic transformations under the influence of antiferromagnetism are discussed based on the experimental results. The variations of shape memory properties with prestraining temperature are interpreted in terms of the transformation characteristics. It is shown that the e martensite can be readily strain-induced under the stabilization effect of the antiferromagnetism which strongly suppresses the thermally induced transformation. The strain-induced transfor-mation of e martensite is more preferred as a predominant deformation mechanism at low tem-peratures under a combined influence of the antiferromagnetism and other physical factors, whereas the irreversible deformation mode is more likely with prestrain at relatively high tem-peratures. The transformation characteristics can be significantly changed by alloying and mechanical /thermal treatments. This offers a possibility of developing new practical Fe-based shape memory alloys with a wide range of mechanical and physical properties.

The Al-Cu-Fe phase diagram: 0 to 25 At. pct Fe and 50

Abstract: Isothermal sections of the Al-Cu-Fe equilibrium phase diagram at temperatures from 680 °C to 800 °C were determined in the region with 50 to 75 at. pct Al and 0 to 25 at. pct Fe using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) techniques. This re- gion includes the face-centered icosahedral phase (Ψ-Al6Cu2Fe) which has unprecedented struc- tural perfection and no apparent phason strain. The icosahedral phase has equilibrium phase fields with four distinct phases at 700 °C and 720 °C (β-Al(Fe, Cu), λ-Al13Fe4, ω-Al7Cu2Fe, and liquid) and three phases at 680 °C(β, ω, and λ) and 800 °C (β, λ, and liquid). The B2 ordered β phase has considerably greater solubility for Cu than previously reported, extending from AlFe to ∼Al50Fe5Cu45. The equilibrium range of composition for the icosahedral phase at these temperatures was determined, and a liquidus projection is proposed.

Effects of sic content on fatigue crack growth in

Abstract: An experimental study has been conducted with the purpose of examining the fatigue crack growth characteristics of cast aluminum alloy matrix composites reinforced with different vol- ume fractions of silicon carbide particles. Particular attention has been paid to developing com- posite microstructures with similar matrix aging condition, precipitation, matrix strength, reinforcement particle size distribution, and interfacial characteristics but with different con- trolled amounts of reinforcement particles. Fatigue crack growth experiments have been con- ducted using constant stress amplitude methods with a fixed load ratio as well as constant Kmax control involving a varying load ratio. The development of crack closure and the microscopic path of the crack through the composite microstructure are monitored optically and using the electron microscope in an attempt to examine the mechanisms of fatigue fracture. The results indicate that an increase in SiC content results in the suppression of striation formation in the ductile matrix. Although ductile matrix failure involving the formation of striations in the low SiC content composite or of void growth in the high SiC content composite is evident, the results also show that fracture of the reinforcement particles plays a significant role in dictating the rates of fatigue crack growth. Detailed quantitative analyses of the extent of particle fracture as a function of the reinforcement content have been performed to elucidate the mechanistic origins of fatigue resistance. The propensity of particle fracture increases with particle size and with the imposed value of stress intensity factor range. While discontinuously reinforced metal- matrix composites with predominantly matrix cracking are known to exhibit superior fatigue crack growth resistance as compared to the unreinforced matrix alloy, the tendency for particle fracture in the present set of experiments appears to engender fatigue fracture characteristics in the composite which are inferior to those seen in the unreinforced matrix material. Particle fracture also results in noticeable differences in the microscopic fracture path and causes a reduction in crack closure in the composites as compared to that in the matrix alloy. The results of this work are discussed in light of other related studies available in the literature in an attempt to develop a mechanistic perspective on fatigue crack growth resistance in metal-matrix composites.

An experimental and numerical study of cyclic deformation in metal-matrix composites

Abstract: The cyclic stress-strain characteristics of discontinuously reinforced metal-matrix composites are studied both experimentally and numerically The model systems used for investigation are aluminum alloys reinforced with SiC particulates and whiskers Finite element analyses of the fatigue deformation of the composite are performed within the context of axisymmetric unit cell formulations Two constitutive relations are used to characterize the matrix of the composite: the fully dense Mises model of an isotropically hardening elastic-viscoplastic solid and the Gurson model of a progressively cavitating elastic-viscoplastic solid (to simulate ductile matrix failure by the nucleation and growth of voids) The brittle reinforcement phase is modeled as elastic, and the interface between the ductile matrix and the reinforcement is taken to be perfectly bonded The analyses provide insights into the effects of reinforcement shape and concentration on (1) constrained matrix deformation under cyclic loading conditions, (2) cyclic hardening and saturation, (3) the onset and progression of plastic flow and cavitation within the matrix, and (4) cyclic ductility The numerical predictions of flow strength, strain hardening, evolution of matrix field quantities, and ductility under cyclic loading conditions are compared with those predicted for monotonic tensile deformation and with experimental observations

Grain refinement by dynamic recrystallization during the simulated warm-rolling of interstitial free steels

Abstract: Multipass torsion tests were used to investigate the warm-rolling behavior of three interstitial free (IF) steels containing Ti and/or Nb. All the tests were carried out at a strain rate of 2 s−l, and the samples were water-quenched immediately after particular passes. Whereas the finishing passes were always executed in the single-phase ferrite region, the roughing passes were applied either in the austenite phase (hot/warm-rolling) or in the ferrite region (warm/warm-rolling). In the former case, the temperature of the first roughing pass was 1260 °C while it was 850 °C for the warm/warm method. It is shown that dynamic recrystallization occurs to a degree that depends on the composition of the steel and the finishing temperature. Although the finish-rolling loads for warm-rolling (in the ferrite region) are no higher than for conventional strip-rolling in the austenite region, the ultrafine ferrite grain sizes of 1 to 2 μm that are produced by warm-rolling are an order of magnitude finer than those resulting from conventional processing.

Formation of magnesium aluminate (spinel) in cast SiC particulate-reinforced Al(A356) metal matrix composites

Abstract: Transmission (TEM) and scanning electron microscopy (SEM) are employed to study the SiC/Al-alloy interface in a cast SiCp/Al(A356) metal matrix composite (MMC). Magnesium aluminate (spinel), MgAl2O4, was found at the interface as a reaction product after material processing. Comparisons of the crystal structure, structure factor, and interface reaction ther-modynamics between MgAl2O4 and MgO have been carried out. The results from these com-parisons confirm the experimental observation;i.e., the favored interface phase is magnesium aluminate (spinel). Based on the thermodynamic analysis, the presence of oxygen in various forms in the system during processing, such as SiO2, A12O3, and MgO, is believed to be the source which supplies the oxygen for the formation of MgAl2O4.

Microstructure and creep properties of dispersion-strengthened aluminum alloys

Abstract: The mechanical properties of dispersion-strengthened aluminum alloys, with various dispersoid types, volume fractions, and grain structures, were investigated in conjunction with systematic microstructural examinations. New theoretical concepts, based on thermally activated dislocation detachment from dispersoid particles, were used to analyze the creep behavior. A particularly strong dispersoid-dislocation interaction was identified as reason for the excellent creep properties of carbide dispersion-strengthened aluminum. Oxide particles (Al2O3,MgO) seem to exert a weaker interaction force and are therefore less efficient strengtheners. Although fine crystalline in the as-extruded condition, all alloys are remarkably resistant against diffusional creep. It is demonstrated that this behavior can be consistently understood by extending the concept developed for the interaction between bulk dislocations and dispersoids to grain boundary dislocations.

An Assessment of Studies on Homogeneous Diffusional Nucleation Kinetics in Binary Metallic Alloys

Abstract: A critical review is presented of all studies on homogeneous nucleation kinetics in crystalline binary metallic alloys located in the literature. Emphasis was first placed upon examining the data on the number of precipitates per unit volume of matrix phase,N v , recorded as a function of isothermal reaction or aging time. With the exception of the results of a few studies on Cu-rich Cu-Co alloys, all of these data were extensively “contaminated” by significant overlapping of the diffusion fields of adjacent precipitates and especially by concurrent coarsening. The use of the “nucleation window” concept was advocated as a means of finding a range of alloy compositions and reaction temperatures in a particular alloy system within which sufficient data onN v vs time can be collected to evaluate the steady-state nucleation rate,J s * without significant intervention by either disturbing effect. Transmission electron microscopy (TEM) was identified as a particularly valuable experimental tool for measuringN v . However, smallangle neutron scattering (SANS) is also proving useful for this purpose, and the combination of SANS with FIM-AP (field ion microscope-atom probe) has uncovered information of crucial importance to understanding the transformation sequence in Cu-Co alloys. Wagner and co-workers[52,62,63,64-78-79] have demonstrated the presence of precursor Co segregations large in extent but small in amplitude, of which the most successful lead to the formation of identifiable precipitates (within which segregation is very much larger in amplitude but considerably smaller in extent). The Wagneret al. work suggests that the supersaturations at which they formed were insufficient to permit the fluctuations which did not eventually fulfill exactly the specifications for critical nuclei to evolve into precipitates. While classical, the Cahn-Hilliard continuum nonclassical [su2] and Cook-deFontaine discrete lattice point nonclassical nucleation theories[25,26,27] yield nearly identical results in the temperature-Co concentration range experimentally studied, theJ* s values thus calculated are a few orders of magnitude smaller than the experimentally measured rates when the concentration of vacancies present at the reaction temperature is (reasonably) assumed operative. On the basis of theoretical and computer simulation studies by Binderet al. [84,87,89,90] and Kleinet al.,[91–94] the observed precursor concentration fluctuations are indicative of relatively long-range interactions among adjacent atoms in Cu-Co alloys, whereas the solution thermodynamics so far applied to this system is based upon the use of short interaction distances. This is suggested to be the principal source of the discrepancy between measured and calculated nucleation kinetics in Cu-Co alloys. Suggestions are offered for future research intended to clarify some of the complexities which have recently become apparent in studies of homogeneous nucleation kinetics in binary metallic alloys.

An analysis of the isothermal hot compression test

Abstract: The nominally isothermal, uniaxial hot compression test has been analyzed with special reference to the effects of temperature nonuniformities and friction on sample deformation and flow stress estimates. A simple one-dimensional analysis was used to establish the influence of initial temperature nonuniformities, strain rate, and the temperature dependence of the flow stress on flow localization tendencies. Noticeable strain concentrations were predicted to occur only at high strain rates (∼10 s−1) in materials such as titanium alloys, but not in steels, for typical values of the initial temperature nonuniformity. More extensive numerical (finite element method) simulations of the compression test with various values of the friction shear factor corroborated the conclusions of the flow localization analysis. In addition, it was established that initial temperature nonuniformities, as well as friction, have an almost negligible effect on flow stress data deduced from measurements of average pressurevs true height strain, at least for reductions of the order of 50 pct. The analysis results were supported by observations of the deformation behavior of a near-gamma titanium aluminide and a low-alloy steel.