Phase transformations in Ti-6.8Mo-4.5Fe-1.5Al

Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature range from 300 °C to 750 °C utilizing hardness measurements, X-ray diffraction, optical microscopy, and electron microscopy. Artificial aging following solution treatment and water quenching initially involved growth of the athermal ω phase. This was followed by formation of the α phase, either in association with the ω phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly, isothermal decomposition of the metastable β phase resulted in the precipitation of ω phase exhibiting an ellipsoidal morphology. While precipitation of ω was immediate at 345 °C, an incubation period was observed upon aging at 390 °C. Isothermal aging above this temperature involved direct precipitation of the α phase, either homogeneously within the β matrix or heterogeneously at β grain boundaries. The extent of homogeneous vs heterogeneous α nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation and higher aging temperatures promote α heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization of the α phase.     

Improvements in the Superelasticity and Change in Deformation Mode of β-Type TiNb24Zr2 Alloys Caused by Aging Treatments

The superelasticity and deformation behaviors of β-type TiNb₂₄Zr₂ subjected to aging treatment were investigated in this study. As the aging time increased, the precipitation of isothermal ω phase was found to restrain the formations of twinning and stress-induced α″ martensite. The superelastic recovery of the sample first improves and then deteriorates with extended aging times. A maximum and stable superelastic recovery of 4.3 pct is obtained for an aging time of 7.2 ks.     

Improvement in Fatigue Strength of Biomedical β-type Ti–Nb–Ta–Zr Alloy While Maintaining Low Young’s Modulus Through Optimizing ω-Phase Precipitation

The improvement in fatigue strength, with maintenance of a low Young’s modulus, in a biomedical β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), by thermomechanical treatment was investigated. A short aging time at an ω-phase-forming temperature combined with severe cold rolling was employed. A fine ω phase is observed in TNTZ subjected to this thermomechanical treatment. Because the rolling texture of β phase is formed by cold rolling, such as the ω phase may be preferentially oriented to a direction that is effective for inhibiting the increase in Young’s modulus. The samples aged at 573 K (300 °C) for 3.6 ks and 10.8 ks after cold rolling exhibit a good balance between a high tensile strength and low Young’s modulus. In the case of the sample aged for 3.6 ks, the tensile strength is improved, although the fatigue strength is not improved significantly. Both the tensile strength and the fatigue strength of the sample aged for 10.8 ks are improved. This fatigue strength is the highest among the TNTZ samples used in the current and in previous studies with Young’s moduli less than 80 GPa.     

Isothermal Phase Transformation in Biomedical Co-29Cr-6Mo Alloy without Addition of Carbon or Nitrogen

The isothermal phase transformation behavior in a biomedical Co-29Cr-6Mo alloy without carbon or nitrogen was investigated during aging at temperatures between 973 K and 1273 K (700 °C and 1000 °C) for up to 90 ks. Transformation from the γ to the ε phase did not occur at 1273 K (1000 °C) as the γ phase was more stable than the ε phase, and the σ phase precipitated at the γ grain boundaries. At 1173 K (900 °C), a γ → ε ₁ phase transformation occurred by massive precipitation. Prolonged annealing at 1173 K (900 °C) led to a lamellar structure of ε ₂ and σ phases at ε ₁/ε ₁ boundaries by a discontinuous/cellular reaction, expressed by the reaction equation ε ₁ → ε ₂ + σ. After decreasing the aging temperature to 973 K (700 °C), transformation from the γ to the ε phase occurred mainly by isothermal martensitic transformation, but a lathlike massive ε ₁ phase and ε ₂/σ lamellar colonies were also observed at the original γ-grain boundaries. It is likely that not adding carbon results in the promotion of the massive transformation and the precipitation of the σ phase during isothermal aging in the Co-29Cr-6Mo alloy system, whose composition corresponds to the ASTM F75 standard for metallic materials for surgical implantation. The resultant isothermal transformation behavior of the present alloy is described on the basis of thermodynamic calculations using Thermo-Calc.     

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract

This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.     

Structural properties and stability of metastable phases in the Zr-Nb system: Part I. Systematics of quenching-and-aging experiments

An experimental study is presented of the effect of an isothermal treatment (“aging”) at 773 K upon the structural properties of three metastable phases formed by quenching Zr-Nb alloys from 1273 K, viz. hcp (α), bcc (β), and Ω. Using neutron-diffraction (ND) measurements and Rietveld analysis, the lattice parameters (LPs) and the constitution of Zr-Nb alloys with up to 18 at. pct Nb are determined. By combining these data with the LP vs at. pct Nb relations previously determined by us on quenched alloys, an analysis is performed of the composition changes and phase reactions occuring upon aging. The present results open up the possibility of using quenching-and-aging experiments to gain insight into the α+β metastable equilibrium in the Zr-rich side of the Zr-Nb system.     

The compctition between the alpha and omega phases in aged Ti-Nb alloys

The compctition between the stable α phase and metastable ω phase to precipitate in a metastableβ phase matrix was investigated in the present study. Four binary Ti-Ni alloys with compositions between 20 and 35 at. pct Nb were air cooled to room temperature from 1000 °C and then aged at temperatures between 300 and 500 °C. For aging temperatures of 400 °C and lower it was found that the quench before aging enabled ω phase precipitates to grow to the exclusion of α precipitates. When specimens were directly aged at 400 °C only α precipitates were observed. Precipitates which could not be identified using SAD were observed in specimens of the 30 and 35 at. pct Nb alloys. All precipitation reactions became more sluggish as the niobium content of an alloy was increased. The results reported here form the second part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison     

Aging behavior of an Al-Li-Cu-Mg-Zr alloy

The aging processes in an Al-Li-Cu-Mg-Zr alloy have been examined by means of electrical resistivity measurements and high-resolution transmission electron microscopy coupled with an image-processing system. The specimens, quenched in iced brine after solution treatment, were reheated at a constant rate of 1 K/min up to 773 K. Six reactions were clearly separated in the temperature derivative of the resistivity/temperature curve, i.e., there was a slight increase at temperatures around 333 K, a large decrease at around 368 K, a significant decrease at around 448 K, a large increase at around 538 K, a remarkable decrease at around 568 K, and a final broad increase at around 623 K. Each reaction observed by the electrical resistivity measurement was examined metallographically. In the as-quenched specimen, spherical undissolved β′ (AlZr₃ L1₂ structure) particles dispersed, but the matrix was already ordered congruently into an L1₂ structure. The first reaction at around 333 K is probably due to the increase of the degree in the congruent ordering, but the second one, at around 368 K, is thought to rise from the rearrangements of antiphase domain boundaries (APDBs) such as the partition of Li atoms between an APDB and the matrix, the APDBs lying parallel to the {100} and {110} planes. Reheating to temperatures around 448 K induces the phase separation, with well-defined interfaces into Li-rich, ordered δ′ (L1₂) and Li-poor, less-ordered regions, and the Ostwald ripening of the ordered regions follows. The reactions at 538, 568, and 633 K were identified as the dissolution of δ′ particles into the matrix, the precipitation of δ (AlLi B32) and S′ compounds into the matrix, respectively.     

Phase Diagrams of the Zr–Si–RE (RE=La and Er) Ternary Systems at 773?K (500?°C)

The isothermal sections of the phase diagram of the Zr–Si–RE (RE=La and Er) systems at 773 K (500 °C) have been investigated using X-ray power diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM) with the aid of metallographic analysis. The existences of 10 binary compounds, namely ZrSi₂, α-ZrSi, α-Zr₅Si₄, Zr₃Si₂, Zr₂Si, RESi₂, RESi_2–x , RESi, RE₅Si₄, and RE₅Si₃ have been confirmed in the Zr–Si–RE (RE=La and Er) systems, respectively. As for the reported binary compound RE₃Si₂, only La₃Si₂ has been observed in the Zr–Si–La system, whereas Er₃Si₂ was not found. No binary compound was found in the Zr–RE binary systems, and no ternary compound was found in the current ternary systems. None of the phases in Zr–Si–La system reveals a remarkable solid solution at 773 K (500 °C). However, the maximum solid solubility of Zr in Er, Er₅Si₃, Er₅Si₄, ErSi, ErSi_1.67, and ErSi₂ is determined to be approximately 12.0 at. pct, 2.4 at. pct, 3.0 at. pct, 3.3 at. pct, 2.2 at. pct, and 1.8 at. pct, respectively. The maximum solid solubility of Er in ErSi₂ is approximately 1.8 at. pct. No remarkable solid solubility of the elements in any of the other phases has been observed.     

Age-Induced Precipitating and Strengthening Behaviors in a Cu–Ni–Al Alloy

Microstructural response and variations in strength and electrical conductivity of a Cu−20 at. pct Ni–6.7 at. pct Al alloy during isothermal aging at temperatures from 723 K to 1023 K (450 °C to 750 °C) were investigated to discuss the age-induced precipitation behavior and strengthening mechanism. At all aging temperatures, fine spherical γ′-Ni₃Al particles were found to nucleate coherently with parent Cu grains by continuous precipitation and then grew gradually by Ostwald ripening. Domains with a high density of twins developed at grain boundaries during aging below 873 K (600 °C) followed by cellular components composed of fiber-shaped γ′-Ni₃Al and Cu solid solution phases at the domain boundaries later. Both the domains and cellular components were suppressed at aging above 923 K (650 °C). The age-induced strengthening principally resulted from fine dispersion of γ′-Ni₃Al coherent particles in the grains. The precipitation strengthening by the fine γ′-Ni₃Al coherent particles exhibited a maximum at an aging temperature of 873 K (600 °C), resulting in excellent mechanical properties such as a high hardness of 340 ± 7 HV and an ultimate tensile strength of 980 ± 14 MPa, which are comparable to those of other commercial age-hardened Cu–Be, Cu–Ni–Si, and Cu–Ti alloys.

     

Analysis of Particle and Crystallite Size in Tungsten Nanopowder Synthesis

Tungsten nanopowders were synthesized by a low-temperature technique and then heat treated in a gaseous reductive atmosphere in order to study the phase evolution, crystallite size, and particle size of the powders as the heat treatment temperature was modified. Synthesis of the powders was carried out in aqueous media using NaBH₄ as a reducing agent using careful control of the pH of the solutions. The XRD patterns of the as-synthesized powders showed an amorphous phase. After washing, energy dispersive spectroscopy showed that the powders had peaks for oxygen and tungsten. In order to promote crystallization and eliminate the oxygen, the powders were heat treated at 773 K, 923 K, and 1073 K (500 °C, 650 °C, and 800 °C) in a H₂/CH₄ reducing atmosphere for 2 hours. XRD after heat treatment showed α-W peaks for the powders treated at 1073 K and 923 K (800 °C and 650 °C) and a mixture of β-W and α-W for the powders treated at 773 K (500 °C). The crystallite sizes determined from X-ray peak broadening were 12, 16, and 20 nm, whereas the average particle sizes from dynamic light scattering were 260, 450, and 750 nm, for heat treatment temperatures of 773 K, 923 K, and 1073 K (500 °C, 650 °C, and 800 °C), respectively. The average crystallite size and particle sizes increased proportionally with the treatment temperature, in contrast to what has been found for some ceramics, in which as the heat treatment temperature is increased, the crystallite size increases, but the particle size stays constant.     

Grain-size effect on shape-memory behavior of Ti35.0Ni49.7Zr15.4 thin films

The grain-size effect on shape-memory behavior of fine-grained Ti_35.0Ni_49.7Zr_15.4 thin films was investigated by transmission electron microscopy. The films, with various grain sizes ranging from about 150 to about 400 nm, were prepared by heat treatment of amorphous films at the three different annealing temperatures of 773, 873, and 973 K, for three different annealing times of 5 minutes, 1 hour, and 10 hours. The film annealed at 773 K for 5 minutes showed a nearly single phase of (Ti,Zr)Ni, while the films annealed at high annealing temperatures and/or long annealing times showed λ ₁ precipitates. For a high annealing temperature of 973 K, the critical yield stress (σ _c) was dominantly dependent on the grain size of the matrix, obeying the Hall-Petch equation. On the other hand, for a low annealing temperature of 773 K, σ _c was dominantly dependent on the amount of λ ₁ precipitates. The M _S temperature decreases almost linearly with increasing σ _c. The films showed sufficient fracture toughness, probably due to the nanometer scale of the grain size and the agglomerated shape of λ ₁ particles.     

Aging behavior of an Al−Li−Cu−Mg−Zr alloy

The aging processes in an Al-Li-Cu-Mg-Zr alloy have been examined by means of electrical resistivity measurements and high-resolution transmission electron microscopy coupled with an image-processing system. The specimens quenched in iced brine after solution treatment, were reheated at a constant rate of 1 K/min up to 773 K. Six reactions were clearly separated in the temperature derivative of the resistivity/temperature curve, i.e., there was a slight increase at temperatures around 333 K, a large decrease at around 368 K, a significant decrease at around 448 K, a large increase at around 538 K, a remarkable decrease at around 568 K, and a final broad increase at around 623 K. Each reaction observed by the electrical resistivity measurement was examined metallographically. In the as-quenched specimen, spherical undissolved β′ (AlZr₃ L1₂ structure) particles dispersed, but the matrix was already ordered congruently into an L1₂ structure. The first reaction at around 333 K is probably due to the increase of the degree in the congruent ordering, but the second one, at around 368 K, is thought to rise from the rearrangements of antiphase domain, boundaries (APDBs) such as the partition of Li atoms between an APDB and the matrix, the APDBs lying parallel to the {100} and {110} planes. Reheating to temperatures around 448 K induces the phase separation, with well-defined interfaces into Li-rich, ordered δ′ (L1₂) and Li-poor, less-ordered regions, and the Ost-wald ripening of the ordered regions follows. The reactions at 538, 568, and 633 K were identified as the dissolution of δ′ particles into the matrix, the precipitation of δ (AlLi B32) and S′ (Al₂CuMg orthorhombic) particles, and the dissolution of both δ and S compounds into the matrix, respectively. SADAYOSHIITO, formerly Graduate Student, Department of Materials Science and Engineering, Ehime University     

Compositional Variations between Different Generations of <Emphasis Type="Italic">γ</Emphasis>′ Precipitates Forming during Continuous Cooling of a Commercial Nickel-Base Superalloy

The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.     

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.     

The Coexistence of Two Different Pearlites,Lamellae of (Ferrite + M<Subscript>3</Subscript>C), and Lamellae of (Ferrite + M<Subscript>23</Subscript>C<Subscript>6</Subscript>) in a Mn-Al Steel

Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel. The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M₃C and M₂₃C₆ carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously at temperatures below 923 K (650 °C). One is lamellae of ferrite and M₃C carbide, and the other is lamellae of ferrite and M₂₃C₆ carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M₂₃C₆. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately.     

Experimental Investigation and Thermodynamic Calculations of the Bi-Ge-Sb Phase Diagram

A phase diagram of the Bi-Ge-Sb ternary system was investigated experimentally by differential thermal analysis (DTA), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray powder diffraction (XRD) methods and theoretically by the CALPHAD method. The liquidus projection; invariant equilibria; and three vertical sections, Sb-Bi_0.5Ge_0.5, Ge-Bi_0.5Sb_0.5, and Bi-Ge_0.5Sb_0.5, as well as isothermal sections at 773 K and 373 K (500 °C and 100 °C), were predicted using optimized thermodynamic parameters for constitutive binary systems from the literature. In addition, phase transition temperatures of the selected samples with compositions along calculated isopleths were experimentally determined using DTA. Predicted isothermal sections at 773 K and 373 K (500 °C and 100 °C) were compared with the results of the SEM-EDS and XRD analysis from this work. In both cases, good agreement between the extrapolated phase diagram and experimental results was obtained. Alloys from the three studied vertical sections were additionally analyzed using the Brinell hardness test.     

Recrystallization-thermal etching of local plastic strain in heat resistant alloys

A recrystallization-thermal etching technique used with heat resistant alloys (Type 304, 2.25Cr-lMo and HastelloyX) has been developed, by which the measurement of the plastic zone and plastic strain around notch or crack at elevated temperatures can by quantitatively made. The optimum annealing conditions are in a vacuum in the range of 2.7 × 10⁻³ to 1.3 × l0⁻⁴ Pa for 86.4 ks at 1223 K (Type 304), at 1098 K (2.25Cr-lMo) and at 1248, 1303 and 1343 K (HastelloyX). Detectable critical strain is 0.02 for Type 304, 0.2 for 2.25Cr-lMo and 0.2 ∼ 0.03 for HastelloyX. The plastic zones in notched tensile specimens loaded to the ultimate tensile strength at temperatures from 298 to 1273 K were successfully observed with the technique. Local plastic strain near the crack was also measured. The effect of the work hardening characteristic on the strain gradient is shown.     

Phase transformations and phase equilibria in the Fe-N system at temperatures below 573 K

The phase transformations of homogeneous Fe-N alloys of nitrogen contents from 10 to 26 at. pct were investigated by means of X-ray diffraction analysis upon aging in the temperature range from 373 to 473 K. It was found that precipitation of α″-Fe₁₆N₂ below 443 K does not only occur upon aging of supersaturated α (ferrite) and α′ (martensite), but also upon transformation of γ′-Fe₄N_1-z and ɛ-Fe₂N_1-x (<20 at. pct N). No α″ was observed to develop upon aging of γ(austenite). Therefore, it is proposed that γ′ is a stable phase at temperatures down to (at least) 373 K. Phase formation upon annealing at low temperatures is apparently governed by the (difficult) nucleation and (slow) growth of new Fe-N phases: α″ forms as a precursor for α because of slow nitrogen diffusion, and nitrogen-enriched ɛ develops as a precursor for γ′ because of a nucleation barrier.