β Phase decomposition in Ti–5Al–5Mo–5V–3Cr

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ～500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ～90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ～1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties.     

Shape memory behavior of Ti–Ta and its potential as a high-temperature shape memory alloy

In this study, the effect of Ta content on shape memory behavior of Ti–Ta alloys was investigated. The shape memory effect was confirmed in Ti–(30–40)Ta alloys. The martensitic transformation start temperature (M_s) decreased by 30 K per 1 at.% Ta. The amount of ω phase formed during aging decreased with increasing Ta. A stable high-temperature shape memory effect was confirmed for Ti–32Ta (M_s = 440 K) during thermal cycling between 173 and 513 K. On the other hand, the high-temperature shape memory effect of Ti–22Nb, which has a similar M_s to Ti–32Ta, exhibited poor stability due to the large amount of ω phase formed during thermal cycling. It is suggested that Ti–Ta is an attractive candidate for the development of novel high-temperature shape memory alloys.     

Anomalous transformation-induced deformation in 〈1 1 0〉 textured Gum Metal

Tensile tests on single crystals of Gum Metal (Ti–36Nb–2Ta–3Zr–0.3O (wt.%)) showed, anomalously, that while a stress-induced β(bcc) → α″(orthorhombic) transformation occurred in a crystal pulled in the 〈1 1 0〉 direction, (1) no transformation was observed in crystals pulled in the 〈1 0 0〉 or 〈1 1 1〉 directions and (2) little or no transformation occurred in severely worked rods, which are polycrystals with very strong 〈1 1 0〉 texture. Analysis of the energetics of the β → α″ transformation offers straightforward explanations: (1) an α″ precipitate has zero elastic energy if it forms as a thin plate with the habit {1 1 1.5}; a 〈1 1 0〉 tensile load significantly decreases the energy of this plate; loading along 〈1 0 0〉 or 〈1 1 1〉 is less effective; (2) while worked rods have a strong 〈1 1 0〉 axial texture, the perpendicular planes are severely distorted, increasing the elastic energy of α″ and inhibiting the transformation.     

The impact of peak shock stress on the microstructure and shear behavior of 1018 steel

As-received and shock-prestrained 1018 steel specimens were subjected to forced shear experiments in a split-Hopkinson pressure bar (SHPB) at room temperature and a strain rate of 3800 s⁻¹ to determine the influence of shock-prestraining on the shear behavior of ferrite. Shock-loading was performed below (12.5 GPa) and above (14 GPa) the pressure-induced epsilon phase transition occurring at 13 GPa. Using electron microscopy and electron backscatter diffraction, twinning and microbanding were observed only in the shock-prestrained specimens. Quasi-static compression tests showed an increase in yield and compressive strengths with increased peak shock stress. SHPB tests produced shear localization in all specimens, with shear banding occurring only in the shock-prestrained specimens. Transmission electron microscopy revealed that, at the shear band edge, elongated cells dominate the microstructure, with more shock-induced twins remaining intact in the 12.5 GPa specimen than in the 14 GPa specimen.     

ω-Assisted nucleation and growth of α precipitates in the Ti–5Al–5Mo–5V–3Cr–0.5Fe β titanium alloy

This paper discusses the structural and compositional changes at the nanometer scale associated with the nucleation and growth of α precipitates in the β titanium alloy Ti-5553 (Ti–5Al–5Mo–5 V–3Cr–0.5Fe) with ω precipitates acting as heterogeneous nucleation sites. The microstructural evolution in this alloy, during β-solutionizing, quenching and aging type heat-treatments, has been investigated by combining results from scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy, high-resolution TEM and three-dimensional atom probe (3DAP) tomography. Athermal ω precipitates form in this alloy on quenching from above the β transus temperature. On isothermal annealing at low temperatures, these ω precipitates coarsen to form chemically ordered ω precipitates, accompanied by the nucleation of the stable α phase. Annealing at higher temperatures leads to dissolution of ω and further growth of α precipitates accompanied by clustering of different α variants in self-accommodating morphologies. 3DAP results indicate that annealing at lower temperatures (～350 °C) leads to initial nucleation of α precipitates with a non-equilibrium composition, nearly identical to that of the β matrix. Subsequent aging at higher temperatures (～600 °C) leads to more pronounced partitioning of alloying elements between the two phases. These results indicate that the structural body-centered cubic to hexagonal close-packed transformation and the compositional partitioning of alloying elements occur in sequential steps, resulting in a mixed-mode displacive-diffusional transformation, similar to the bainite transformation in steels.     

The deformation of Gum Metal through in situ compression of nanopillars

The name “Gum Metal” has been given to a set of β-Ti alloys that achieve exceptional elastic elongation and, with appropriate preparation, appear to deform by a dislocation-free mechanism triggered by elastic instability at the limit of strength. We have studied the compressive deformation of these materials with in situ nanocompression in a quantitative stage in a transmission electron microscope. The samples studied are cylindrical nanopillars 80–250 nm in diameter. The deformation pattern is monitored in real time using bright-field microscopy, dark-field microscopy or electron diffraction. Interesting results include the following: (i) nanopillars approach, and in several examples appear to reach, ideal strength; (ii) in contrast to conventional crystalline materials, there is no substantial “size effect” in pillar strength; (iii) the deformation mode is fine-scale with respect to the sample dimension, even in pillars of 100 nm size; (iv) shear bands (“giant faults”) do form in some tests, but only after yield and plastic deformation; and (v) a martensitic transformation to the base-centered orthorhombic α′′ phase is sometimes observed, but is an incidental feature of the deformation rather than a significant cause of it.     

Influences of chromium and hafnium additions on the microstructures of β-nial coatings on superalloy substrates

In this study, Hf-doped and Cr/Hf-modified NiAl coatings were deposited onto René N5^® substrates via direct current magnetron sputtering. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) showed the as-deposited coatings to be single phase with B2 crystal structures. Post-deposition annealing at 1000 °C in an Ar + 5%H₂ atmosphere resulted in the formation of nanometer-sized precipitates along grain boundaries and within grain interiors. TEM analyses showed most of the precipitates to be β′-Ni₂AlHf. Three-dimensional atom probe tomography (3D-APT) also revealed the presence of α-Cr precipitates within the NiAlCrHf coatings after annealing. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl–Hf coatings.     

Strain compensation by twinning in Au thin films: Experiment and model

Epitaxial Au thin films with thicknesses of 40–160 nm were grown on (1 0 0)-oriented single-crystal NaCl substrates at 300 °C by magnetron sputtering. Microstructural analyses revealed that all films possess orthogonal twin networks along the 〈0 1 1〉 directions. The experimentally observed relationship of an increase in twin density with decreasing film thickness is explained by kinematical and thermodynamical modeling. The developed twin model predicts a nanometer-sized width of the twins, in agreement with the experiment.     

Phase equilibria between the B2, L12, and fcc phases in the Ir–Ni–Al system

The isotherms of the Ir–Ni–Al in the composition range up to 50 at% Al are presented at 1573 K. The phase constitution and microstructure of the Ir–Ni–Al alloys were examined using X-ray diffractometry (XRD), scanning electron microscopy (SEM) with an electron probe microanalyzer (EPMA), and transmission electron microscopy (TEM) after heat treatment at 1573 K for 168 h. The B2-NiAl and B2-IrAl phases connected with each other at 1573 K. The highest solubility limit of Ir into Ni₃Al was about 3.5 at% in the tested alloys. Then, a wide fcc + B2 and a narrow fcc + L1₂ and B2 + L1₂ two-phase region appeared in the isothermal section. In part of the B2 phase, a martensitic transformation from the B2 to the L1₀ phase was observed.     

The crystal structure of high temperature phase Ta2O5

The high temperature phase of Ta₂O₅ and its variants were maintained in a pure Ta₂O₅ specimen using the conventional solid-state reaction method and the advanced laser irradiation technique. The structure of the high temperature phase is determined to be tetragonal with lattice parameters of a = 3.86 Å and c = 36.18 Å and space group of I4₁/amd. Compared to the previously reported crystal structure, the number of the total basis atoms was reduced to be 6 from 11. The modified crystal structure interprets well the recorded high-resolution electron microscopy images and the selected area electron diffraction patterns. Based on the basic tetragonal structure, the crystal lattice structures of the orthorhombic and monoclinic variants were determined and the corresponding space groups were analyzed according to the variation of the symmetrical elements. In addition, crystal atomic structures of these two variants were proposed based on the tetragonal structure.     

Ordered ω phases in a 4Zr–4Nb-containing TiAl-based alloy

A considerable amount of B2 phase with a cellular morphology is retained in a 4Zr–4Nb-containing TiAl-based alloy. Heterogeneous precipitation of ordered ω from B2 is found to occur readily after HIPping: B2 → ω with the B8₂-structure in cell regions and B2 → ω with the D8₈-structure in cell-wall regions. Congregated D8₈-ω domains and particles form as a network surrounding the well-developed B8₂-ω cells. The heterogeneous formation of different ω variants is caused by a heterogeneous distribution of Zr + Nb elements across B2, which plays an important role in stabilizing vacancies and promotes the formation of D8₈-ω. Fine D8₈-ω particles are also observed to precipitate from the B8₂-ω cell matrix after ageing at 700 °C for 1000 h, showing a transformation path of β → B2 → B8₂-ω → D8₈-ω for the aged cells. The heterogeneous formation of a D8₈-ω network and B8₂-ω cells is found to be detrimental to ductility and fatigue strength. A very brittle fine-grained TiAl alloy is produced as a result.     

Synthesis and characterization of bismuth alkaline titanate powders

In this work, samples of bismuth alkaline titanate, (K_0.5Na_0.5)_(2?x/2)Bi_(x/6)TiO₃, (x = 0.05–0.75) have been prepared by conventional ceramic technique and molten salts. Metal oxides or carbonates powders were used as starting raw materials. The crystalline phase of the synthesized powders was identified by the X-ray diffraction (XRD) and particle morphology was characterized by scanning electron microscopy (SEM). Solid state reaction method was unsuccessful to obtain pellets. From XRD results, a rhombohedral structure was detected and the parameter lattice were estimated to be a = 5.5478 Å and α = 59.48°. These parameters were used to refine the structure by Rietveld analysis. SEM results showed several morphologies. Apparently, bismuth is promoting the grain growth whose sizes vary from 30 nm to 180 nm It is expected that these materials can be utilized in practical applications as substitutes for lead zirconatetitanate (PZT)-based ceramics.     

Investigation of early stage deformation mechanisms in a metastable β titanium alloy showing combined twinning-induced plasticity and transformation-induced plasticity effects

As expected from the alloy design procedure, combined twinning-induced plasticity and transformation-induced plasticity effects are activated in a metastable β Ti–12 wt.% Mo alloy. In situ synchrotron X-ray diffraction, electron backscatter diffraction and transmission electron microscopy observations were carried out to investigate the deformation mechanisms and microstructure evolution sequence. In the early deformation stage, primary strain/stress-induced phase transformations (β → ω and β → α″) and primary mechanical twinning ({3 3 2}〈1 1 3〉 and {1 1 2}〈1 1 1〉) are activated simultaneously. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {3 3 2}〈1 1 3〉 twinning and the subsequent secondary mechanisms dominate the early-stage deformation process. The evolution of the deformation microstructure results in a high strain-hardening rate (～2 GPa), bringing about high tensile strength (～1 GPa) and large uniform elongation (>0.38).     

Plasma nitridation of a novel Ni–10.8 wt.%Cr nanocomposite

A Ni–10.8Cr nanocomposite (by wt.%), consisting of nanocrystalline Ni matrix (mean grain size: 60 nm) and dispersed Cr nanoparticles (mean particle size: 42 nm), has been synthesized by nanocomposite electrodeposition. The unique structure causes the nanocomposite to form a double-layered nitrided zone during plasma nitridation at 560 °C for 10 h. The outer layer (∼50 μm thick) precipitates nanometer-sized CrN (<100 nm), which increased in size but decreased in number with increasing nitridation depth (following Böhm–Kahlweit’s mode). The inner layer (∼5 μm thick) exhibits larger-coarsened nitride precipitates (100–200 nm) which almost link together. The greatly enhanced nitriding kinetics in the nanocomposite compared to a compositionally similar but microstructurally different Ni–10Cr alloy (mean grain size: 30 μm) is mainly associated with the fact that the numerous grain boundaries dramatically increase the nitrogen permeability, according to the treatment using a classical Wagner’s approach. The nanohardness profile in relation to the microstructure of the nitrided zone in the nanocomposite has also been investigated.     

Effects of superimposed electric field and porosity on the hydrostatic pressure-induced rhombohedral to orthorhombic martensitic phase transformation in PZT 95/5 ceramics

The hydrostatic pressure-induced martensitic transformation from the ferroelectric rhombohedral to antiferroelectric orthorhombic phase in PZT 95/5 ceramics has been studied using neutron diffraction. The transition to the orthorhombic phase initiates at a pressure of 260 MPa and is almost complete at 290 MPa. This stress range is much narrower than that observed in uniaxial loading, which starts at 200 MPa and is incomplete even at 400 MPa. The narrower stress range observed under hydrostatic loading is attributed to a lack of internal stress developed during the transformation. By contrast, the work required to start the transformation is approximately the same under both types of loading. The transformation progresses more gradually with increasing pressure when a static electric field is applied to a specimen in a pre-poled state. Tests carried out on porous specimens, having a relative density of approximately 90%, demonstrated that the transformation occurred over a narrow pressure range but with a lower transformation pressure of approximately 220 MPa.     

Stress–strain relationship between ferrite and martensite in a dual-phase steel studied by in situ neutron diffraction and crystal plasticity theories

The stress–strain relationship between ferrite and martensite phases in the commercial dual-phase DP980 steel was studied using in situ neutron diffraction and the crystal plasticity finite element method (CPFEM). The phase identification method based on the image quality of electron backscatter diffraction and a filtering process was used to obtain information concerning individual crystallographic orientations for ferrite and martensite phases in DP980 steel. The (2 0 0) and (2 1 1) lattice strains of ferrite and martensite phases were measured along the loading and transverse directions as a function of macroscopic stress using in situ neutron diffraction. A CPFEM based on representative volume elements (RVE) was applied to determine the microscopic hardening parameters for each phase by fitting the measured macroscopic stress and measured (2 0 0) and (2 1 1) lattice strains. The microscopic hardening parameters for each phase successfully captured the influence of the crystallographic orientation of the ferrite phase on the localization of shear strain and the behavior of ductile failure in RVE of the unit cell during uniaxial tension.     

High temperature compressive deformation and fracture characteristics of the activated sintered W–Ni compacts

In order to determine the working conditions for the Ni-activated sintered W compact, a wide range of compression test was carried out under controlled temperature conditions between 900 and 1100 °C and a constant true strain rate of 1 × 10⁻³–1 × 10⁰/s. A moderate true strain up to 0.6 was obtained without a fracture at the testing temperature of 900 °C irrespectively of the strain rate for the specimen of an appropriate porosity and controlled grain size, being comparable to that of the pure W specimen. The relationship between the brittle features and the segregated Ni phase was able to be discussed on the basis of the microstructural analysis on the fracture surface and the true stress–strain curve analysis.     

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at ?80 °C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ → ε → α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (～10¹⁵ m^?2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.     

Ti50Cu23Ni20Sn7 bulk metallic glasses prepared by mechanical alloying and spark-plasma sintering

A powder metallurgy technology was developed to prepare Ti₅₀Cu₂₃Ni₂₀Sn₇ bulk metallic glasses (BMGs). Firstly, amorphous powder was prepared by mechanical alloying (MA) method successfully after being milled for 30 h. Phase transformation of the as-milled powder was characterized by X-ray diffraction (XRD). Morphology of the as-milled amorphous powder was observed by scanning electron microscopy (SEM). Onset temperature of glass transformation and onset temperature of crystallization (T_x and T_g) of the as-milled amorphous powder were evaluated by differential scanning calorimeter (DSC). Secondly, the as-milled amorphous powder was then consolidated by spark-plasma sintering (SPS) method into a specimen with the shape of cylindrical stick, with a diameter and height of about 20 and 10 mm, respectively. The SPS experiment was conducted under a pressure of 500 MPa at a heating rate of 40 K/min, sintering and holding for 1 min at the temperature of 763 K. It was confirmed that the as-milled powder is of fully amorphous however the consolidated specimen shows to be an amorphous matrix with partial crystallization. Compressing strength, Young's modulus, micro-hardness, friction and density of the consolidated specimen are about 975 MPa, 121 GPa, 13 GPa, 0.12 and 6599 kg/m³, respectively. Fractograph of the specimen appears to be shear fracture and very few defects can be seen from the picture of SEM.     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.