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

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Displacive transformations in Au-18 wt pct Cu-6 wt pct Al

A gold alloy with 18 wt pct Cu and 6 wt pct Al undergoes a reversible displacive phase transformation between an incompletely ordered L2₁ parent phase and a tetragonal product. The characteristics of these transformations were studied using acoustic emission, dilatometry, X-ray diffraction, and metallography. The morphology of the transformation products, the structure of the parent phase, and the generation of significant acoustic emission during the transformations indicate that they are at least quasi-martensitic, if not martensitic, and that this system is an example of a β-phase shape-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the sample. The martensite start (M _s ) temperature is between 30 °C and 20 °C. The system exhibits hysteresis and will revert to the parent phase when reheated, with an austenite start (A _s ) temperature between 55 °C and 80 °C. However, freshly cast or solution-annealed and quenched samples of the alloy do not transform to the tetragonal phase. Aging of such material at temperatures between 30 °C and 200 °C is required before they will manifest the displacive transformation. The “martensite” phase is considerably more resistant to aging-induced stabilization than that of most other SMAs.     

Evolution of microstructure and tensile strength of rapidly solidified Al-4.7 pct Zn-2.5 pct Mg-0.25 pct Zr-X wt pct Mn alloys

Analytical transmission electron microscopy and thermal analysis of as-extruded Al-4.7 pct Zn-2.5 pct Mg-0.2 pct Zr-X wt pct Mn alloys, with Mn contents ranging from 0.5 to 2.5 wt pct, were carried out to elucidate the microstructural change and accompanying mechanical properties during subsequent heat treatments. The as-extruded alloy was fabricated from rapidly solidified powder and consisted of a fine, metastable manganese dispersoid and the ternary eutectic T phase (Al₂Mg₃Zn₃). Solution heat treatment resulted in the formation of the stable Al₆Mn phase and complete dissolution of the T phase. Formation of stable Al₆Mn was made by two routes: by phase transition from metastable Mn dispersoids which already existed, and from the supersaturated solid solution by homogeneous nucleation. The density of the Al₆Mn phase increased with the addition of manganese, while the shape and average size remained unchanged. A significant increase in the hardness was observed to coincide with the formation of the Al₆Mn phase. Similarly, the tensile strength increased further after the aging treatment, and the increment was constant over the content of Mn in the alloy, which was explained by the contribution from the same amount of precipitates, MgZn₂. Results of thermal analysis indicated that the dissolution of the T phase started near 180 °C and that formation of Al₆Mn occurred at about 400 °C, suggesting that further enhancement of strength is possible with the modification of the heat-treatment schedule.     

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     

Microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 Wt pct Cu-2.0 wt pct Mn alloys

The microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 pct Cu-2.0 pct Mn alloy specimens solidified at various cooling rates were investigated. The equilibrium phases Al₆Mn and θ-Al₂Cu, which are observed in the conventionally solidified alloy in the as-cast condition, were not detected in rapidly solidified (melt-spun) material. Instead, the ternary compound Al₂₀Cu₂Mn₃ was present in addition to the α phase, which was present in all cases. The morphological and kinetic nature of corrosion was investigated metallographically and through potentiostatic techniques in 3.5 wt pct NaCl aqueous solution. Corrosion of the as-cast material was described by two anodic reactions: corrosion of the intermetallic phases and pitting of the α-Al solid solution. The corrosion rate increased with cooling rate from that for the furnace-cooled alloy to that for the copper mold-cast alloy and, subsequently, decreased in the rapidly solidified alloy. In the heat-treated material, corrosion could be described by two anodic reactions: corrosion of Al₂₀Cu₂Mn₃ precipitate particles and pitting of the α-Al matrix. S.M. Skolianos, formerly Graduate Student, Department of Metallurgy, University of Connecticut     

The low strain tensile behavior of U-7.5 wt pct Nb-2.5 wt pct Zr

The stress-strain response of polycrystalline, γ-quenched U-7.5 wt pct Nb-2.5 wt pct Zr alloy was studied as a function of strain rate and compared to equilibrium stress-strain tests performed by allowing the strain to reach a maximum value at incrementally increasing stresses. Equilibrium stress-strain tests were also performed on prestressed samples. Sheet tensile specimens were held at various states of strain in an X-ray diffractometer to determine crystal structural changes during deformation. Prestressed tensile bars were sectioned and examined metallographically and with the X-ray diffractometer. Two linear regions were observed in the equilibrium stress-strain tests: a low stress region with a slope of 5.3 to 5.5 x 10⁶ psi, and a region above 40,000 psi with a slope of 3.3 x 10⁶ psi. Finite strain rates tended to increase both slopes. The diffractometer experiments yielded plots of lattice parameter vs strain which showed a shift from a bcc structure of the γ^s phase, to a bct structure of the γ₀ phase between 1 and 3 pct deformation. It is postulated that this is a thermoelastic martensite transformation. A semiempirical equation was developed which describes the equilibrium stress-strain behavior of this alloy in terms of a stress induced phase transformation.     

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.     

Electrical conductivity in directionally solidified lead-9 and -20 wt pct copper alloys

Composites consisting of aligned copper dendrites in a lead matrix have been produced by directional solidification processing for potential application as grids in lead-acid batteries. To promote a uniform composite of aligned copper dendrites in a protective lead matrix, two alloy compositions, Pb-9 and -20 wt pct Cu, have been directionally solidified through a temperature gradient,G _l of 4.5 Kmm^-1 at constant growth velocities which ranged from 1 to 100 μm s^-1. With slow growth rates (≲10 μm s^-1 ), the copper dendrites were generally columnar and continuous along the sample length; at higher velocities (≳60 μm s^-1), they assumed an intricate and equiaxed morphology. In accordance with copper content and growth rate, the electrical conductivity of the directionally solidified composites was found to be as much as a 2.5 times that of pure lead. The results are compared with that predicted by a model based on a geometrical dendrite. Formerly Doctoral Student, Department of Materials Science and Engineering, Vanderbilt University.Seoul Korea. This paper is based on work leading to the successful completion of his Ph.D. degree at Vanderbilt University.     

The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg2Si-0.4 wt pct Mg-0.5 wt pct Ag Alloy

The influence of addition of 0.4 wt pct Mg on the precipitation sequence in the balanced Al-1.0 wt pct Mg₂Si bearing 0.5 wt pct Ag has been investigated during the continuous heating of the quenched alloy from the solid solution state. Differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy techniques have been used. The DSC experiments showed that all processes occurred are thermally activated. The activation energies of the precipitation processes have been determined and hence the kinetics of these precipitates have been determined. The obtained results have shown that the existence of excess Mg inhibits the formation of the early stage clusters of solute-vacancy clusters. These clusters can be assisted by the binding energies between solute Si, Mg, and Ag atoms and the excess vacancies. On the other hand, excess Mg accelerates the precipitation of random, β′-phase and β-phase precipitates.     

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.     

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.     

The early stages of GP zone formation in naturally aged Ai-4 wt pct cu alloys

The early stages of the room temperature decomposition of supersaturated solid solutions of Al-4 wt pct Cu alloys have been investigated. Diffuse satellites have been found to flank fundamental reflections in electron diffraction patterns prior to the formation of the platelike GP zones. These are herein interpreted as evidence for the occurrence of spinodal decomposition at room temperature. The development of GP zones from the modulated microstructure is documented by the bright field and weak beam dark field techniques. This continuous formation of GP zones is then interpreted in terms of Cahn’s later stage spinodal theory.     

The cyclic stress-strain response of polycrystalline,pseudoelastic Cu-14.5 wt pct Al-3 wt pct Ni alloy

Limited results on the fatigue of pseudo-elastic material indicate that, as a class, these materials should have truly outstanding fatigue properties. To check this, the mechanisms of cyclic deformation and fracture have been studied in Cu−Al−Ni chosen because its transformation behavior is well understood. Since this alloy is notoriously brittle, pulsating compression fatigue tests were carried out in polycrystalline material. The details of the stress-induced martensite behavior were studied byin situ video observations. The alloy was found to undergo cyclic hardening and failure eventually occurred by multiple nucleation of cracks at grain boundaries, by a mechanism similar in principle to that which occurs in regular metals cycled at high plastic strains. The Coffin-Manson law was obeyed.     

Aging induced shape instability in an elastically bent uranium + 7.5 wt pct niobium + 2.5 wt pct zirconium alloy

The uranium +7.5 wt pct niobium +2.5 wt pct zirconium alloy when quenched from 1073 K was found to exist at room temperature as a metastable phase which was a slight tetragonal distortion of the elevated temperature body-centered-cubic (bcc) phase. Flat, asquenched specimens have been elastically deformed in four-point bending to maximum outer fiber stresses below the stress required for plastic deformation to occur but into a range of stress where pseudoelastic behavior has been observed. Aging of these elastically bent specimens in an oil bath at 423 K, while constrained by the bending jig, resulted in a permanent deflection and shape change. Further isothermal aging, after removal from the bending apparatus, caused increasing deflection and continued shape instability in spite of the absence of the applied load. X-ray examination of samples cut from a bent and aged specimen revealed important preferred orientation and lattice parameter differences between the tension and compression regions and the high and low stress parts of the specimen. These observations are described and compared to previous findings on quenched samples of this alloy that had been either deformed separately or aged separately. A rationalization of the shape instability is presented. Elastic twin nucleation and growth, preferred orientations, solute segregation and the interplay of all these seem to be involved.     

Aging induced shape instability in an elastically bent uranium + 7.5 wt pct niobium + 2.5 wt pct zirconium alloy

The uranium +7.5 wt pct niobium +2.5 wt pct zirconium alloy when quenched from 1073 K was found to exist at room temperature as a metastable phase which was a slight tetragonal distortion of the elevated temperature body-centered-cubic (bcc) phase. Flat, asquenched specimens have been elastically deformed in four-point bending to maximum outer fiber stresses below the stress required for plastic deformation to occur but into a range of stress where pseudoelastic behavior has been observed. Aging of these elastically bent specimens in an oil bath at 423 K, while constrained by the bending jig, resulted in a permanent deflection and shape change. Further isothermal aging, after removal from the bending apparatus, caused increasing deflection and continued shape instability in spite of the absence of the applied load. X-ray examination of samples cut from a bent and aged specimen revealed important preferred orientation and lattice parameter differences between the tension and compression regions and the high and low stress parts of the specimen. These observations are described and compared to previous findings on quenched samples of this alloy that had been either deformed separately or aged separately. A rationalization of the shape instability is presented. Elastic twin nucleation and growth, preferred orientations, solute segregation and the interplay of all these seem to be involved.     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.