Morphology and Crystallography of α Precipitates in β Ti–Mo Binary Alloys

Multiscale Architecture and Superior High-Temperature Performance of Discontinuously Reinforced Titanium Matrix Composites

Discontinuously reinforced titanium matrix composites (DRTMCs), as one of the most important metal matrix composites (MMCs), are expected to exhibit high strength, elastic modulus, high-temperature endurability, wear resistance, isotropic property, and formability. Recent innovative research shows that tailoring the reinforcement network distribution totally differently from the conventional homogeneous distribution can not only improve the strengthening effect but also resolve the dilemma of DRTMCs with poor tensile ductility. Based on the network architecture, multiscale architecture, for example, two-scale network and laminate-network microstructure can further inspire superior strength, creep, and oxidation resistance at elevated temperatures. Herein, the most recent developments, which include the design, fabrication, microstructure, high-temperature performance, strengthening mechanisms, and future research opportunities for DRTMCs with multiscale architecture, are captured. In this regard, the service temperature can be increased by 200 °C, and the creep rupture time by 59-fold compared with those of conventional titanium alloys, which can meet the urgent demands of lightweight nickel-based structural materials and potentially replace nickel base superalloys at 600-800 °C to reduce weight by 45%. In fact, multiscale architecture design strategy will also favorably open a new era in the research of extensive metallic materials for improved performances.

Effect of the High-Pressure Torsion (HPT) and Subsequent Isothermal Annealing on the Phase Transformation in Biomedical Ti15Mo Alloy

Ti15Mo metastable beta Ti alloy was solution treated and subsequently deformed by high-pressure torsion (HPT). HPT-deformed and benchmark non-deformed solution-treated materials were annealed at 400 °C and 500 °C in order to investigate the effect of UFG microstructure on the α-phase precipitation. Phase evolution was examined using laboratory X-ray diffraction (XRD) and by high-energy synchrotron X-ray diffraction (HEXRD), which provided more accurate measurements. Microstructure was observed by scanning electron microscopy (SEM) and microhardness was measured for all conditions. HPT deformation was found to significantly enhance the α phase precipitation due the introduction of lattice defects such as dislocations or grain boundaries, which act as preferential nucleation sites. Moreover, in HPT-deformed material, α precipitates are small and equiaxed, contrary to the α lamellae in the non-deformed material. ω phase formation is suppressed due to massive α precipitation and consequent element partitioning. Despite that, HPT-deformed material after ageing exhibits the high microhardness exceeding 450 HV.

Transformation Pathway upon Heating of Metastable β Titanium Alloy Ti-15Mo Investigated by Neutron Diffraction

A transformation pathway during thermal treatment of metastable β Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases α, β, and ω was investigated during linear heating with two heating rates of 1.9 ∘C/min and 5 ∘C/min and during aging at 450 ∘C. The results showed that with a sufficient heating rate (5 ∘C/min in this case), the ω phase dissolves before the α phase forms. On the other hand, for the slower heating rate of 1.9 ∘C/min, a small temperature interval of the coexistence of the α and ω phases was detected. Volume fractions and lattice parameters of all phases were also determined.

On the role of composition and processing parameters on the microstructure evolution of Ti-xMo alloys

Laser Engineered Net Shaping (LENS™) was used to produce a compositionally graded Ti-xMo (0 ≤ x ≤ 12 wt %) specimen and nine Ti-15Mo (fixed composition) specimens at different energy densities to understand the composition–processing–microstructure relationships operating using additive manufacturing. The gradient was used to evaluate the effect of composition on the prior-beta grain size. The specimens deposited using different energy densities were used to assess the processing parameters influence the microstructure evolutions. The gradient specimen did not show beta grain size reduction with the Mo content. The analysis from the perspective of the two grain refinement mechanisms based on a model known as the Easton & St. John, which was originally developed for aluminum and magnesium alloys shows the lower bound in prior-beta grain refinement with the Ti–Mo system. The low growth restriction factor for the Ti-Mo system of Q = 6,5C₀ explains the unsuccessful refinement from the solute-based mechanism. The energy density and the grain size are proportional according to the results of the nine fixed composition specimens at different energy densities. More energy absorption from the material represents bigger molten pools, which in turn relates to lower cooling rates.

First-principles study of electronic structures and stability of body-centered cubic Ti-Mo alloys by special quasirandom structures

The electronic structures and structural properties of body-centered cubic Ti-Mo alloys were studied by first-principles calculations. The special quasirandom structures (SQS) model was adopted to emulate the solid solution state of the alloys. The valence band electronic structures of Ti-Mo and Ti-Mo-Fe alloys were measured by hard x-ray photoelectron spectroscopy. The structural parameters and valence band photoelectron spectra were calculated using first-principles calculations. The results obtained with the SQS models showed better agreement with the experimental results than those obtained using the conventional ordered structure models. This indicates that the SQS model is effective for predicting the various properties of solid solution alloys by means of first-principles calculations.

Crystalline characteristics of alpha precipitates in Ti-15V-3Sn-3Al-3Cr alloy

The experiment was designed to analyse the orientation relationship between alpha precipitates and beta matrix and to determine the habit plane of alpha phase in Ti-15-3 alloy using transmission electron microscopy. The orientation relationship was turned out to be 110(alpha)||111(beta) and (001)(alpha)||{110}(beta) obtained from diffraction patterns, which corresponded to Burgers orientation relationship. Based on the patterns and the crystal structure, it was determined that there were 12 possible orientation relationships between alpha precipitates with beta matrix and alpha precipitates had 12 variants. Meanwhile, the transformation matrixes of 12 orientation relationships were established. Diffraction patterns of 001(beta), 110(beta) and 311(beta) zone axes were also calibrated by these matrixes, which verified the correctness of proposed orientation relationship. Diffraction spots of lamelliform alpha precipitates spread along the [111](beta) and [1 1 1 ](beta) directions in diffraction patterns of [1 1 0](beta) zone axis, but they do not spread in diffraction patterns of [111](beta) zone axis, which indicates that the habit plane of variant 1, 2 and 3 is (111)(beta). Moreover, the habit planes of variant 4-6, variant 7-9 and variant 10-12 are (1 1 1)(beta), (1 1 1)(beta) and (1 1 1 )(beta) respectively. To sum up, the habit plane of alpha precipitates is {111}(beta).