The microstructural influence of a 2 wt pct Mo addition to a Ti-1.5 wt pct Ni alloy

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Elevated temperature deformation behavior of an Al-8.4 wt pct Fe-3.6 wt pct Ce alloy

The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS) aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable particles, such as γ^′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in strength of at least some of the second phase particles at temperatures above 723 K. Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

The precipitation behavior of a Zr-2.5 wt pct Nb alloy

The morphology, crystallography, and nature of precipitates in a quenched and aged Zr-2.5 wt pct Nb alloy has been studied by transmission electron microscopy. The needle-shaped matrix precipitates and equiaxed twin boundary nucleated precipitates produced by aging at 500 °C were the equilibrium Nb-rich β₂ phase. On aging at 600 °C, the matrix precipitation was a mixture of β₂ needles and coarse metastable Zr-rich β₁ particles, while only β₁ particles were found at twin boundaries. The growth direction of the needle-shaped particles, 6.6 deg to 8.2 deg from (1-100)_h, and their orientation relationship can be predicted by an invariant line strain model. The β₁ precipitates have the Burgers orientation relationship. The formation of metastable β₁ and stable β₂ particles is considered from the free energy approach of Menon, Banerjee, and Krishnan.     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

Tracer diffusion of63Ni in Fe-17 wt pct Cr-12 wt pct Ni

The tracer diffusion of⁶³Ni in Fe-17 Cr-12 Ni by both volume and grain boundary transport has been studied from 600° to 1250°C. The use of an RF sputtering technique for serial sectioning allowed the determination of very small volume diffusion coefficients at the lower temperatures. Volume diffusion of nickel in this alloy was observed to be much slower than in pure iron or austenitic stainless steel at comparable temperatures. The volume diffusion coefficient is described byD _v =8.8 exp (−60,000/RT) cm²/s and grain boundary diffusion is described by σD _gb =3.7×10⁻⁹ exp (−32,000/RT) cm³/s. R. A. PERKINS, formerly Presidential Intern, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak, Ridge, Tenn. 37830, is     

Molybdenum accumulation at ferrite: Austenite interfaces during isothermal transformation of an Fe-0.24 pct C-0.93 pct Mo alloy

A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (α:γ) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650°C, 630°C, and 610°C. These concentrations were quite small at 650°C, which is just below the bay temperature of the time-temperature-transformation (TTT) curve for the initiation of ferrite formation. There were larger concentrations at 630°C, a temperature at which transformation stasis (incomplete transformation) occurred. Concentrations at 610°C were intermediate between the values observed at 650°C and 630°C. The average accumulation at the latter temperatures increased appreciably as a function of transformation time. After each heat treatment, there was considerable variation in Mo accumulation from one α:γ interface to another and, to a lesser extent, from one region to another along the same interface. These higher Mo concentrations were deduced to have developed largely through volume diffusion of Mo, mainly through ferrite, to interfaces whose ledgewise growth had been interrupted by growth stasis. (Mo₂C precipitation at α:γ boundaries occurred only at the end of growth stasis.) It appears that only a very small amount of Mo segregation is needed, probably at specific interfacial sites, in order to produce growth cessation. Growth kinetics anomalies of this kind continue to provide the best evidence available for the operation of a coupled-solute drag effect. This article is based on a presentation given in the symposium “The Effect of Alloying Elements on the Gamma to Alpha Transformation in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel Research and the ASM-TMS Phase Transformations Committee.     

Electron diffraction study of aging processes in Fe-1.83 wt pct C martensite at room temperature

Aging processes occurring in an Fe-1.83 wt pct C martensite at room temperature have been studied by electron diffraction, assisted by transmission electron microscopy observation. Fourier analysis of diffuse scattering from the martensite suggested that a spinodal decomposition occurred therein. The spinodally modulated microstructure was composed of a dominant wave with a wavelength of about 1 nm and nondominant waves with longer wavelengths and was observed not to grow even after aging for several months. The reason for the nongrowth may be that the coarsening was stopped by strong elastic interactions between carbon-enriched and carbon-depleted regions. Furthermore, the tetragonality of the martensite was observed to remain constant during aging, suggesting that carbon redistribution during spinodal decomposition occurred within one set of the octahedral interstitial sitesO _z only. formerly Visiting Researcher, The Institute of Scientific and Industrial Research, Osaka University     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

The quench sensitivity of cast Al-7 wt pct Si-0.4 wt pct Mg alloy

The effect of quenching condition on the mechanical properties of an A356 (Al-7 wt pct Si-0.4 wt pct Mg) casting alloy has been studied using a combination of mechanical testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). As the quench rate decreases from 250 °C/s to 0.5 °C/s, the ultimate tensile strength (UTS) and yield strength decrease by approximately 27 and 33 pct, respectively. The ductility also decreases with decreasing quench rate. It appears that with the peak-aged condition, both the UTS and yield strength are a logarithmic function of the quench rate,i.e., UTS orσ _y =A logR +B. The termA is a measure of quench sensitivity. For both UTS and yield strength of the peak-aged A356 alloy,A is approximately 32 to 33 MPa/log (°C/s). The peak-aged A356 alloy is more quench sensitive than the aluminum alloy 6063. For 6063,A is approximately 10 MPa/log (°C/s). The higher quench sensitivity of A356 is probably due to the high level of excess Si. A lower quench rate results in a lower level of solute supersaturation in the α-Al matrix and a decreased amount of excess Si in the matrix after quenching. Both of these mechanisms play important roles in causing the decrease in the strength of the peak-aged A356 with decreasing the quench rate.     

Prevention of macrosegregation in squeeze casting of an Al-4.5 wt pct Cu alloy

The squeeze casting of an Al-4.5 wt pct Cu alloy was carried out to investigate the conditions for the formation and the prevention of macrosegregation. The effects of the process parameters, applied pressure, die temperature, pouring temperature, delay time, and humidity on the formation of macrosegregation were investigated in correlation with the evolution of macrostructure and shrinkage defects. Two critical applied pressures were defined: one is the critical applied pressure, P _SC , under which shrinkage defects form, and the other is the critical applied pressure, P _MS , above which macrosegregates form in the squeeze castings. A quantitative diagram describing the optimum process conditions was proposed for obtaining sound squeeze castings. It was found that the pouring temperature, the die temperature, the delay time, and the humidity are closely related to the two critical applied pressures P _SC and P _MS , in different manners. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained when the applied pressure is in the range of P _SC <P<P _MS .     

Relationship between austenite dislocation density introduced during thermal cycling and M s temperature in an Fe-17 wt pct Mn alloy

The reason why thermal cycling decreases the martensite start (M _s ) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitic transformation. The M _s temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strain energy (ΔG _sh ) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling. The ΔG _sh value increased from 19.3 to 28.8 MJ/m³ due to the increase in austenite dislocation density from 1.5 × 10¹² to 3.8 × 10¹³/m² with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the increase in austenite hardness with the number of thermal cycles.     

A structural investigation of multiple aging of Al-7 wt pct Zn-2.3 wt pct Mg

A transmission electron microscopy study of the structural changes which attend aging at 180°C with and without pre-aging at 100°C was conducted on a high purity aluminum alloy containing 6.8 pct Zn and 2.3 pct Mg. The refinement in precipitate dispersion accompanying multiple aging is caused by the operation of aging sequences which differ from those occurring in material given the single age at 180°C only. The high nucleation rate which occurs during the low temperature pre-aging treatment is responsible for the observed precipitate refinement. The results of this investigation appear to agree favorably with the Pashleyet al. model of multiple aging.     

Discontinuous precipitation at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals

The misorientation dependence of discontinuous precipitation (DP) at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals has been systematically investigated in the temperature range 523 to 723 K. A good correlation is found between both the incubation period, τ, to initiate DP and cell growth rate, ν, and the energy of boundaries. The maximum of τ and the minimum of ν occur where the cusps of the boundary energy exist. The formation and growth of DP are easier at higher-energy boundaries. The DP cells at [001] twist boundaries nucleate and grow less than those at [001] symmetric tilt boundaries in the same-alloy bicrystals. A kinetic analysis of DP using the models of Turnbull and Petermann and Hornbogen has yielded grain-boundary diffusion data. Although the activation energy, Q _b , of boundary diffusion changes with the models, the values of Q _b are smaller than the activation energy for volume diffusion of Be in Cu. The diffusivity in a boundary shows a close correlation with the energy of the boundary. A lower-energy boundary has a lower diffusivity with a larger activation energy and a larger pre-exponential factor.     

Effect of stress during aging on the precipitation of θ′ in Al-4 Wt pct Cu

Experiments have been made on Al-4 wt pct Cu single crystals to determine the influence of stress, applied during aging, on precipitation. Without stress, θ platelets precipitate on all (100) planes. Transmission electron microscopy showed a marked effect of aging stress on the microstructure. A compressive stress of approximately 48 MPa (～ 7,000 psi) parallel to [001] favored precipitation on (010) and (100) and inhibited precipitation on (001). When a tensile stress of similar magnitude was applied parallel to [001], precipitation was favored on (001) and inhibited on (010) and (100).     

Simultaneous creep and oxidation of an Fe-1 wt pct Ai alloy at 973 to 1073 K

Studies of the simultaneous creep and oxidation of an Fe-lwt pct Al alloy at constant stresses of 16 to 26 MN/m² in the temperature range 973 to 1073 K have shown that the steady state creep obeys a power law. The stress exponentn was found to be 6.9 for creep in an argon atmosphere (Po₂ = 10^?3 mbar). Values of the apparent activation energy for creep were in the range 225 to 351 kJ/mol and appeared to be dependent on the P_{0 2} of the test environment. Oxidationstrengthening (?e decreasing) occurred at 998 K but only at P_{0 2} = 10^?3 mbar and was primarily due to intergranular oxidation and the mechanical constraint of a strongly adherent scale. Oxidationweakening (?e increasing), however, occurred at 973 to 1073 K in environments of the lowest P_{0 2} (10^?9 and 10^?5 mbar) and in the highest P_{0 2} (162 to 1013 mbar). The factors contributing to weakening are believed to be oxidation-induced vacancies, weakly adherent scales and the loss of solute strengthening aluminum through selective oxidation.     

Mushy zone morphology during directional solidification of Pb-5.8 wt pct Sb alloy

The Pb-5.8 wt pct Sb alloy was directionally solidified with a positive thermal gradient of 140 K cm⁻¹ at a growth speed ranging from 0.8 to 30 μm s⁻¹, and then it was quenched to retain the mushy zone morphology. The morphology of the mushy zone along its entire length has been characterized by using a serial sectioning and three-dimensional image reconstruction technique. Variation in the cellular/dendritic shape factor, hydraulic radius of the interdendritic region, and fraction solid along the mushy zone length has been studied. A comparison with predictions from theoretical models indicates that convection remarkably reduces the primary dendrite spacing while its influence on the dendrite tip radius is not as significant.