Kinetics and Equilibrium of Age-Induced Precipitation in Cu-4 At. Pct Ti Binary Alloy

Transformation kinetics and phase equilibrium of metastable and stable precipitates in age-hardenable Cu-4 at. pct Ti binary alloy have been investigated by monitoring the microstructural evolution during isothermal aging at temperatures between 693 K (420 °C) and 973 K (700 °C). The microstructure of the supersaturated solid solution evolves in four stages: compositional modulation due to spinodal decomposition, continuous precipitation of the needle-shaped metastable β′-Cu₄Ti with a tetragonal structure, discontinuous precipitation of cellular components containing stable β-Cu₄Ti lamellae with an orthorhombic structure, and eventually precipitation saturation at equilibrium. In specimens aged below 923 K (650 °C), the stable β-Cu₄Ti phase is produced only due to the cellular reaction, whereas it can be also directly obtained from the intergranular needle-shaped β′-Cu₄Ti precipitates in specimens aged at 973 K (700 °C). The precipitation kinetics and phase equilibrium observed for the specimens aged between 693 K (420 °C) and 973 K (700 °C) were characterized in accordance with a time–temperature–transformation (TTT) diagram and a Cu-Ti partial phase diagram, which were utilized to determine the alloy microstructure, strength, and electrical conductivity.

     

Microstructural Evolution during Laser Resolidification of Fe-18 At. Pct Ge Alloy

The technique of laser resolidification has been used to study the rapid solidification behavior of concentrated Fe-18 at. pct Ge alloy. The microstructural evolution has been studied as a function of scanning rate of laser beam. Scanning electron microscopy (SEM) reveals the formation of a two-layer (designated as “A” and “B”) microstructure in the remelted pool. The A layer shows a band consisting of a network of interconnected channels and walls, quite similar to cell walls. The B layer shows dendritic growth. Transmission electron microscopic observations reveal the formation of bcc α-FeGe in the B layer. Laser melting has been found to play an important role in formation of the A layer. Microstructural evolution in B has been analyzed using the competitive growth criterion, and formation of bcc α-FeGe has been rationalized in the remelted layers.     

High-temperature deformation behavior of the intermetallic Ti-47 At. Pct Al-3 At. Pct Cr alloy

A study of the high-temperature deformation behavior of a binary α ₂+γTi-47 at. pct Al-3 at. pct Cr alloy was undertaken. The alloy was produced by induction melting and exhibited a structure of coarse columnar grains oriented in the radial direction. After a solution treatment at 1653 K for 3600 seconds and aging at 1223 K for 28,800 seconds, a nearly lamellar structure was formed. Deformation behavior was investigated by compression-strain-rate-change tests at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹ and temperatures ranging from 1073 to 1373 K. This alloy shows at low temperature/high stress a stress exponent of about 5. The deformation behavior is explained in this regimen by a dislocation climb mechanism, which includes a threshold stress. Finally, at the lowest stress levels and highest temperatures of testing, a stress exponent of about 3 is observed, which suggests that deformation is controlled by the viscous glide of dislocations.     

Magnetic studies of phase equilibria in Ti-Ai (30 to 57 At. Pct) alloys

Room temperature magnetic susceptibility studies have been made on a series of Ti-Al alloys in the composition range 30 to 57 at. pct Al, quenched from anneals at 900, 1065, 1165, 1215, 1265, 1315, and 1365°C. As a result of this work an equilibrium partial phase diagram has been constructed, compared with literature data, and discussed with reference to the results of a comprehensive metallographic and microhardness study.     

Preliminary evaluation of tensile and stress-rupture behavior of W + 24 At. Pct Re + 0.4 At. Pct HfC wire

The tensile properties of hafnium carbide-dispersed tungsten-rhenium alloy wire, W + 24 at. pct Re + 0.4 at. pct HfC (W24ReHfC), were studied from liquid nitrogen temperature (LN₂) to 1750 K and its stress-rupture behavior determined from 1144 to 1500 K. These results are compared to previous data on W + 4 at. pct Re + 0.4 at. pct HfC (W4ReHfC) and W + 0.4 at. pct HfC (WHfC) wire.[5] The room-temperature (RT) tensile strength of the W24ReHfC wire was about 3250 MPa and higher than that of the W4ReHfC (3160 MPa) and WHfC (2250 MPa) wires. The RT ductility of the W24ReHfC wire was quite high with a 50 pct reduction of area, whereas the W4ReHfC wire and the WHfC wire had RT ductilities of 28 and 2 pct, respectively. At temperatures of 1144 to 1366 K, the W24ReHfC wire had tensile strengths favorably comparable to the W4ReHfC and WHfC wires. However, above 1366 K, the W4ReHfC wire had both a greater tensile strength and stress-rupture strength than the W24ReHfC wire. The main contributions to the strengthening of the W24ReHfC wire were the fine and elongated fibrous grain microstructures and the dispersion of the HfC particles in the W-Re matrix. These properties suggested that the W24ReHfC wires hold promise as potential fiber reinforcements in composites from RT to about 1350 K.     

Kinetics of chlorination of Co and Co-10 At. Pct Pt alloy by reaction with HCI gas

The reaction of alloys with HCI gas is generally more complicated than that of pure metals, and is, typically, a dealloying process. In this work the rates of a model dealloying process, Co(Co-Pt) + 2HC1 → CoCl₂(g) + H₂ were measured in the temperature range 973 to 1273 K. The only reaction product for the HC1/H₂ gas mixtures used in the experiments was gaseous CoCl₂. Weight loss during the reaction was continuously monitored with a recording balance. Experimental variables included gas composition, gas flow rate, and temperature for Co-10 at. pct Pt and for pure Co samples. The rate of the reaction was constant with time and very nearly as high for the alloy as for the pure metal, in spite of the fact that Pt was inert for the conditions. A surface instability occurred for the alloy yielding an open porous structure of ever increasing surface area. Transport of CoCl₂(g) in the gas boundary layer was important for determining the rate of the chlorination reaction for both the alloy and pure metal. Formerly Research Associate, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802.     

Precipitation in a Cu-30 Pct Ni-1 Pct Nb alloy

Decomposition of a Cu-30 pct Ni-1 pct Nb alloy on aging in the range of 866 K (600°C) to 1073 K (800°C) was investigated. The initial decomposition, concomitant with age hardening, occurred through the precipitation of body centered tetragonal metastable Ni₃Nb-γ” precipitates on the 100 matrix planes. Equilibrium orthorhombicβ phase formed either through a grain boundary cellular reaction at low temperature (≤973 K (700°C)) or as Widmanstaettenplatelets on the 1ll planes at higher temperatures (≥1073 K (800°C)) with the following crystallographic relationship: (0l0)_β//111_γ [100]β//[1•11]_γ. Based on the observations, a schematic transformation sequence is presented.     

Room-temperature deformation and stress- induced phase Transformation of Laves Phases in Fe-10 At. Pct Zr Alloy

The microstructure within particles of the Laves phase Fe₂Zr in a two-phase Fe-10 at. pct Zr alloy after compression was investigated using X-ray diffraction and electron microscopy. Stress- induced phase transformation between C36 and C15 structures was found to be a major defor- mation mode for the Laves phase. Twinning and stacking faults were also found within C15 regions. Phase transformation models based on partial dislocations are discussed.     

The rolling texture of Pu-3.4 At. Pct Ga

The rolling texture of a Pu-3.4 at. pct Ga alloy has been measured as a function of reduction. Both (111) and (200) pole figures were plotted from reflection data for specimens rolled 70, 80, 90, and 95 pct at room temperature. The texture develops smoothly and at all reductions bears a good resemblance to that of copper. At 95 pct reduction there is a strong <111> pole located 20 deg to either side of the rolling plane normal in the rolling direction, and the central pattern generally resembles an ellipse whose major axis lies in the cross direction. However, the spread of (111) pole intensity leads to interpretation of the pole figures in terms of a spread of orientations. Paired axes of rotation are used to describe this spread. In the early states of reduction the distribution of grain orientations can best be described by a rotation about the two strong <111> poles located to either side of the rolling plane normal in the rolling direction. As deformation proceeds two distinct orientations develop, related by rotations about these <111> poles, but the pole figure does not show all the associated <111> poles as expected. This can be explained by a spread of orientations, about a set of <110> poles, that decreases with deformation. At 95 pct reduction the missing <111> poles in the pole figure appear, though they are not as sharp as the others.     

Understanding the Shape-Memory Behavior in Ti-(~49 At. Pct) Ni Alloy by Nanoindentation Measurement

The influence of aging treatment on the work-hardening behavior of near-equiatomic NiTi alloy has been studied at the microstructural scale by conducting the instrumented indentation measurement. The maximum shape recovery is achieved at the peak aged condition. The improvement in shape recovery has been attributed to the delayed onset of plasticity. A comparison has been made between the recoverable strain obtained from the tensile experiments and the recovery index parameter determined from the nanoindentation measurements.     

Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.     

Variations of zinc content in solid solution during reversion treatment in an Al-4.4 At. Pct Zn Alloy

In a previous work 1 it has been shown that the thermoelectric power (TEP), measured at 20 °C, is insensitive to GP zones existing in Al-Zn alloys. This result has been verified for large precipitates sizes (ø = 40 Å). The TEP seems to depend only on the zinc content remaining in solid solution in the matrix. Therefore this TEP property allows to follow, especially, the reversion of GP zones. This has been already verified in a qualitative way. 2 Thus the purpose of the present work is to follow quantitatively the evolution of the zinc content in solid solution by TEP measurements and to compare these results to those obtained by the measurement of the integral intensity of X-Ray small angle scattering (XSAS). Moreover it will be shown that the temperature at which TEP measurements are carried out, has no influence on zinc content determination.     

Product microstructures and properties induced by hot working a Pb-1.85 Pct Sb alloy

The concurrent effects of temperature and deformation on microstructure and mechanical properties of a 1.85 pct Sb lead alloy were examined by extrusion over the temperature range 0.7T _m to 0.9T _m, at deformation rates between 0.65 and 12 s^-1. Alloys deformed in the solid-solution regime or at the solvus temperature exhibited microstructures consisting of a mixture of elongated deformed grains and equiaxed recrystallized grains; some discontinuous precipitation was also evident. In the two-phase regime the structure was mainly fibrous; in this, the high resistance to recrystallization and the suppression of the discontinuous precipitation reaction could be attributed to the presence of fine precipitates in the dynamically formed substructure. A regime for providing optimum strength and stability was defined in the strain rate-temperature field investigated.     

Microstructure and mechanical properties of Ti-40 Wt Pct Ta (Ti-15 At. Pct Ta)

Of the β-isomorphous Ti-X alloy systems, Ti-Ta is one of the least studied. In the current work, the microstructure and mechanical properties of Ti-40 wt pct Ta (Ti-15 at. pct Ta) are investigated. Annealing at 810 °C produces a two-phase microstructure consisting of Ti-richa idiomorphs in a continuous Ta-rich β matrix; this suggests the β-transus temperature is higher than indicated by the most recently published phase diagram. Water quenching from 810 °C causes the β phase to partially transform to orthorhombic martensite (α), while furnace cooling yields secondarya The primary α formed isothermally remains unchanged in both cases. Subsequent aging causes transformation of the martensite to type 1a plus residual β, with a corresponding increase in strength and decrease in ductility. The maximum ductility (20 pct elongation) occurs in the water-quenched condition in which metastable β is retained. Analysis of the true stresstrue strain data suggests that transformation-induced plasticity may contribute to the enhanced ductility of the water-quenched material.     

Characterization of recrystallization and grain growth in Ti-7.4 At. Pct Al (cph) and Ti-15.2 At. Pct Mo (bcc) alloys

Quantitative microscopy, texture and grain growth kinetic studies were made on swaged and recrystallized Ti-7.4 at. pct Al and Ti-15.2 at. pct Mo alloys. The quantitative microscopy studies indicated that the grain size distribution in both alloys is a constant for a given grain size, independent of annealing time and temperature and follows a log normal distribution. Moreover, there exists a range of grain sizes in space; the relative quantities of each size in the range varies with average grain size. Also, the grain shape factor decreases with increase in annealing time (grain size) at a constant temperature and with decrease in temperature for a constant grain size. The values of the shape factor for a given grain size and temperature were approximately the same for the two alloys. The quantitative microscopy features were essentially the same as those observed by Okazaki and Conrad for unalloyed titanium. The texture of the as-swaged Ti-7.4 at. pct Al wire specimens and the changes in this texture during grain growth were in accord with those previously reported for deformed and recrystallized titanium. The Ti-15.2 at. pct Mo alloy retained the deformation texture even after recrystallization. At ^1/3 time law was found to hold for the grain growth over most of the grain sizevs time curve. The values of the activation energy for grain boundary migration were 25.2 Kcal/mole for the Ti-7.4 at. pct Al alloy and 29 Kcal/mole for the Ti-15.2 at. pct Mo alloy. These are similar to those for diffusion of Al and Mo in titanium, indicating that the diffusion of these substitutional elements controls the rate of boundary migration in these alloys. This paper is based on a presentation made at a symposium on “Recovery, Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Transformation to Ni5Al3 in a 63.0 At. Pct Ni-Al alloy

Microstructures in a 63 at. pct Ni-Al alloy, produced by a powder metallurgy process, have been investigated in detail in as-quenched and aged (823 to 923 K) conditions. The parent L1₀ martensite plus B₂ NiAl microstructure in the as-quenched state transformed nearly completely to the orthorhombic Ni₅Al₃ phase upon aging at 823 K for 720 hours. The volume fraction of Ni₅Al₃ formed as a function of aging time at 823 K was observed to obey cellular reaction kinetic behavior. The specimens aged at 823 K for short times indicated that nucleation of the Ni₅Al₃ phase occurred preferentially at grain boundaries. Transmission electron microscopy (TEM) observations of short-time aged specimens revealed a complex microstructure consisting of shortrange ordered domains of Ni₂Al in a matrix of 7R martensite, in addition to new variants of 3R martensite and Ni₅Al₃. Aging at 873 and 923 K for 720 hours produced a stable two-phase microstructure consisting of NiAl and Ni₅Al₃. A quantitative phase analysis was carried out to calculate the (NiAl + Ni₅Al₃)/Ni₅Al₃ phase boundary locations. The measured lattice parameters of Ni₅Al₃ formed at 823, 873, and 923 K indicated an increase in tetragonality of the phase with increasing nickel content. Formerly with Case Western Reserve University Formerly with Case Western Reserve University     

Solid-State Oxidation Kinetics of a Lanthanum-Modified Al + 6% Li Alloy

This article describes the results of thermogravimetric analysis of the oxidation kinetics of Al + 6% Li alloy modified by lanthanum in amount up to 0.5%. It is demonstrated that the addition of lanthanum to the alloy or an increase in temperature provides an increase in the oxidation rate of the initial alloy in the solid state. The apparent oxidation activation energy of the initial (basic) alloy (35.2 kJ/mol) decreases to 17.3 kJ/mol for the alloy with 0.5% La. The oxidation curves of Al + 6% Li + La alloys in the solid state are described by equations evidencing a hyperbolic mechanism of this process.