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.     

Stress-assisted transformation in Ti-60 wt pct Ta alloys

An investigation of the influence of processing variables on mechanical properties and phase development for a Ti-60 wt pct Ta (Ti-28.5 at. pct Ta) alloy was conducted. The alloy was hot-rolled, subjected to heat-treatment temperatures above the β (bcc) transus (1 hour at 700 °C, 800 °C, or 900 °C), and water quenched. All heat treatment produced a combination of metastable β (bcc) and metastable α″ (orthorhombic martensite), with the amount of retained β essentially independent of heat treatment, ranging from 20 to 33 vol pct. Deformation of as-rolled and heat-treated tension specimens showed an anomalous leveling of the stress-strain curve in the stress-strain curves at low strains. X-ray diffraction (both simple 2ϑ diffractometry and texture analysis) on both deformed and undeformed material determined that the leveling of the stress-strain curve was a result of the β→α″ martensitic transformation. The stress required to initiate the transformation increased with prequench temperature. This was determined to be due to the presence of athermal ω. Grain growth kinetics have been determined in the course of this work.     

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.     

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.     

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.     

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.     

Substructure and dispersion hardening in aged,cold worked,and annealed Al-4 wt pct Cu alloy

Electron microscopy and X-ray line profile analyses have been employed to define the microstructures and substructures of pure aluminum and an overaged Al-4 wt pct Cu alloy after various thermomechanical treatments. Tensile tests were performed on the same materials, and the results have been interpreted in terms of structure. A given cold rolling reduction of the aged Al-4 wt pct Cu alloy produced a much higher dislocation density and a less cellular substructure than the same treatment produced in pure aluminum of comparable initial grain size. Annealing after cold work produced similar responses in both the pure metal and the alloy. For the aged alloy in the as-rolled, or rolled-and-annealed condition, dispersion strengthening and substructure strengthening were found to be linearly additive, and they accounted for virtually all the observed tensile yield strength. Substructure strengthening has been discussed in terms of the relation between dislocation density and the spacing and nature of the substructure boundaries.     

Elevated temperature elastic constants of Fe-30 wt pct Ni in austenitic and martensitic conditions

A pulsed laser technique was used to measure the polycrystalline elastic moduli of the martensite and austenite phases of an Fe-30.1 wt pct Ni alloy between 298 and 900 K. The moduli of an Fe-29.5 wt pct Ni-0.1 wt pct C alloy were similarly determined at 298 K. These data showed that the shear modulus and Young’s modulus of martensite were slightly decreased by carbon. The moduli of austenite behaved in the expected manner, changing monotonically with temperature above the Curie temperature. The moduli of the martensite changed monotonically with temperature from 298 K until the martensite-to-austenite transformation temperature was reached at 650 K. At this temperature abrupt changes in moduli were observed. The same austenite start temperatures were determined both from moduli measurements and from dilatometric measurements. No unusual decrease of the martensite elastic constants was observed as a precursor to the reverse martensite-to-austenite transformation. Formerly with Sandia Laboratories, Albuquerque, New Mexico     

The microstructures and properties of an al- 12 wt pct

A new process for producing rapidly solidified bulk alloys was developed based on the hammer- and-anvil concept. In the process, an A1-12 wt pct Si alloy slab was built up layer by layer and then hot worked to get a solid and integral sheet. The oxygen content of the layer-deposited alloy is less than the typical value of powder metallurgy (PM) alloys by one order, and the cooling rate can reach 10⁴ K/s, which is higher than that of the spray deposition process. In comparison with the ingot-processed Al-12 wt pct Si alloy, layer-deposited alloy exhibits su- perior mechanical properties. This is attributable to the fine and uniform silicon-particle distri- bution which not only brings on dispersion hardening effect but also raises the elongation and fracture strain. The mechanisms responsible for this enhancement were discussed in terms of particle size and effective volume fraction.     

The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation while encouraging matrix precipitation of the T₁ (Al₂CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However, no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating a very large processing window. Grain size and δ′ (Al₃Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys.     

An electron microscopic and diffraction study of Cu-9 wt pct Al alloy

An electron microscopic investigation of Cu-9 wt pct Al alloy low thermal treated at 250°C for 30 min was performed in the deformed and non-deformed condition. For this composition and ordering treatment the alloy exhibits the higher increase in strength. It was found that the images formed in the non-deformed alloy, reveal the presence of ordered domains of an average size of 80Å, being the amount of order dependent of whether or not quenched-in vacancies are present. It is probable that the domains nucleate preferentially at stacking faults in the deformed condition. A periodic antiphase structure was determined from computations and comparison with electron diffraction data. The superlattice cell is based on the LI₂ type, tetragonal face centered with a period approximately three times the lattice parameter of the matrix, having three variants of orientation within an ordered region.     

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.     

The effects of stress level and grain size on the ambient temperature creep deformation behavior of an alpha Ti-1.6 wt pct V alloy

Ambient (room) temperature studies have been carried out on an α-Ti-1.6 wt pct V alloy to determine the effects of stress level and grain size on ambient temperature creep behavior. Creep tests were performed at five different stress levels ranging from 75 to 95 pct of the yield stress value on specimens with an average grain size of 226 μm. It has been found that the alloy exhibits appreciable creep at stress levels far below the yield stress, with creep occurring at values as low as 75 pct of the yield stress. The extent of creep strain was found to decrease with a decrease in stress level. Creep tests were also performed on this alloy with different grain sizes ranging from 38 to 226 μm at a stress level of 90 pct of the yield stress. It was seen that the extent of creep strain decreased with a decrease in grain size. Fine slip and time-dependent twinning were found to be the creep deformation mechanisms. Based on the results of this investigation and earlier studies, it is suggested that time-dependent twinning is a major creep deformation mechanism in α-titanium alloys that contain small amounts of alloying elements. The time-dependent twinning phenomenon has been attributed to the diffusion of oxygen away from the twin-matrix interface, permitting the growth of twins.     

An electron microscopic and diffraction study of Cu-9 wt pct Al alloy

An electron microscopic investigation of Cu-9 wt pct Al alloy low thermal treated at 250°C for 30 min was performed in the deformed and non-deformed condition. For this composition and ordering treatment the alloy exhibits the higher increase in strength. It was found that the images formed in the non-deformed alloy, reveal the presence of ordered domains of an average size of 80?, being the amount of order dependent of whether or not quenched-in vacancies are present. It is probable that the domains nucleate preferentially at stacking faults in the deformed condition. A periodic antiphase structure was determined from computations and comparison with electron diffraction data. The superlattice cell is based on the LI₂ type, tetragonal face centered with a period approximately three times the lattice parameter of the matrix, having three variants of orientation within an ordered region.     

On the microstructure and mechanical behavior of melt-spun Fe-24 pct Ni-0.5 pct C Ribbons: Effect of austenite grain size

The role of carbon on the retention and decomposition of austenite in a melt-quenched Fe-24 wt pct Ni-0.5 wt pct C alloy made by the melt-spinning method has been investigated, using a combination of X-ray diffractometry, optical and TEM metallography, microhardness measurements, and tensile tests. It is found that the addition of 0.5 wt pct C to Fe-24 wt pct Ni alloy leads to retention of austenite to a temperature close to -196 °C, when the alloy is quenched from the melt. The austenite grain size varies from ∼0.2 μm to ∼2 μm on going from the wheel to the gas side. The cooling rate, accordingly, changes from 5 × 10⁷ to 4 × 10⁴ Ks^-1. The changes in the mechanical properties have been correlated with the accompanying changes in the ribbon microstructure. The Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt     

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.     

Influence of the precipitate-free zone width on the tensile properties of an Al-6 Wt pct Zn-1.2 Wt pct Mg alloy

The mechanical properties of an Al-6 wt pct Zn-1.2 wt pct Mg alloy with various width of precipitate-free zones have been investigated. The width of the precipitate-free zone (PFZ) has been changed by the quench interruption technique without any appreciable change in the size and distribution of precipitates. An important relationship has been observed between the width of the PFZ and the quench-interruption period;i.e., the width of the PFZ increases in proportion to the square root of the holding time at 200°C. From the analysis of stress-strain curves as well as the observation of dislocation arrangements in slightly deformed specimens, the plastic deformation has been found to occur preferentially in the PFZ. The initial stage of deformation is much affected by the change in the width of the PFZ, but in the later stage, the work-hardening rate seems to be almost independent of the PFZ width. Tensile tests show that the ultimate tensile strength and the 0.2 pct proof stress decrease very little with increasing width of the PFZ, while the uniform elongation is practically constant regardless of the reduction in the nonuniform elongation. The work-hardening rate at the initial stage of deformation is found to decrease in proportion to the reciprocal of the PFZ width. This relationship can be explained from the dislocation model for work hardening.     

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.     

Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy

The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder size <26 μm was 322 MPa, while for the coarser powder (45 to 106 μm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 μm) was also higher than that of the Al-20 wt pct Si-3 wt pct Fe (powder size: 60 to 120 μm) alloys. With decreasing powder size from 45 to 106 μm to <26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worn surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders.