Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Thermal stability and mechanical properties of nanostructured Ti–24Nb–4Zr–7.9Sn alloy

Thermal stability of the nanostructured grains of cold-rolled Ti–24Nb–4Zr–7.9Sn alloy and corresponding variations in mechanical properties were investigated. The activation energy for grain growth was found distinct below and above the ( + β)/β transus of 950 K, with values of 47 and 206 kJ/mol, respectively. Due to the pinning effect of the precipitates at β grain boundaries, grains sizes can be maintained at less than 100 nm during prolonged annealing at temperatures up to 773 K, and are less than 1 μm for annealing temperature up to 923 K and time up to 2 h. Annealing above the β transus resulted in coarse grains with sizes of tens of micrometers in less than 2 h. Tensile and hardness tests showed rapid strengthening with the increase of annealing time below 773 K, which was attributed to both the rapid formation of nano-sized precipitates and the slow growth rate of β grains. By adjusting the grain size of the cold-rolled material the high strength/low Young's modulus match desirable for implant applications can be improved over the hot-rolled bars with coarse grains.     

Formation of hydroxyapatite on low Young's modulus Ti–30Nb–1Fe–1Hf alloy via anodic oxidation and hydrothermal treatment

Hydroxyapatite (HA) on the Ti–30Nb–1Fe–1Hf alloy has been fabricated via anodic oxidation followed with the hydrothermal treatment. The anodic oxide film (AOF) containing Ca and P was formed by anodic oxidation in a solution consisting of β-glycerophosphate disodium pentahydrate(β-GP), and calcium acetate monohydrate(CA). The AOF was formed by a 2-stage growth: (1) a thin oxide film that intimately contacted the substrate formed prior to sparking; (2) after sparking, the thickness of AOF increased rapidly, accompanying with the formation of numerous craters in the AOF. When anodizing to 300 V, the AOF had a glassy amorphous structure. Increasing anodizing potential increased the Ca/P ratio and contents of Ca and P, but decreased the adhesion strength between the AOF and the substrate. After 6 h of hydrothermal treatment at 250 °C, a great number of crystalline HA precipitated on the surface of AOF anodized to 300 V. The shapes and population density of HA crystals can be controlled by modifying the anodizing potential and the solution pH of hydrothermal treatment. Increasing the pH of the solution in hydrothermal treatment enhanced the precipitation of HA crystals. Numerous needle-like HA crystals that nearly covered the surface of AOF were obtained when hydrothermally treated in the pH 13 solution.     

Surface hardening of biomedical Ti–29Nb–13Ta–4.6Zr and Ti–6Al–4V ELI by gas nitriding

The microstructure and hardness near the surface of a biomedical titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical Ti–6Al–4V ELI (Ti64). After gas nitriding, the microstructure near the specimen surface was observed by optical microscopy, X-ray diffraction (XRD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). In both alloys, two types of titanium nitrides (TiN and Ti₂N) are formed and the phase is precipitated by gas nitriding. Furthermore, the oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO₂ is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.     

Anisotropy of Young's modulus and tensile properties in cold rolled α′ martensite Ti–V–Sn alloys

Young's modulus and tensile properties of cold rolled Ti–8 mass% V and (Ti–8 mass% V)–4 mass% Sn alloy plates consisting of α′ martensite were investigated as a function of tensile axis orientation in this work. A single phase of α′ (hcp) martensite is obtained in Ti–8 mass% V and (Ti–8 mass% V)–4 mass% Sn alloys by quenching after solution treatment. By 86% cold rolling, acicular α′ martensite microstructures change into extremely refined dislocation cell-like structure with an average size of 60 nm, accompanied with the development of cold rolling texture in which the basal plane normal is tilted from the plate normal direction (ND) toward transverse direction (TD) at angles of ±49° for Ti–8% V alloy and ±46° for (Ti–8 mass% V)–4 mass% Sn alloy. No apparent anisotropy of Young's modulus (E) is observed for as-quenched Ti–8% V (E = 76–83 GPa) and (Ti–8% V)-4%Sn (E = 69–79 GPa). In contrast, Young's modulus increases with increasing angle from the rolling direction (RD) to TD for cold rolled Ti–8% V (E = 72–94 GPa) and (Ti–8% V)–4%Sn (E = 63–85 GPa). The observed anisotropy of Young's modulus can be reasonably explained in terms of the cold rolling α′ texture.0.2% proof stress and tensile strength are independent of tensile orientation for cold rolled Ti–8% V and (Ti–8% V)–4%Sn alloys. In contrast, larger elongation to fracture is obtained in specimens deviated by 30°, 45° and 60° from RD than by 0°, 75° and 90°. Scanning electron microscopy (SEM) fractographs reveal that quasi-cleavage-like fracture plane appears in 0° specimen of cold rolled Ti–8% V which shows brittle fracture and other specimens of cold rolled Ti–8% V and (Ti–8% V)–4%Sn alloys are fractured accompanied with necking and dimple formation. It is suggested from these results that brittle fracture is related to the activation of limited number of slip system and Sn addition leads to the activation of multiple slip systems.     

Microstructure and Texture Formation During Near Conventional Forging of an Intermetallic Ti–45Al–5Nb Alloy

Texture formation was studied in an intermetallic Ti‐45at%Al‐5at%Nb alloy after uniaxial compression and near conventional forging. Depending on the deformation conditions the texture of the γ‐TiAl phase is formed by pure deformation components, components related to dynamic recrystallization, or transformation components. This changing corresponds with microstructural observations. The α₂‐Ti₃Al and the α‐Ti(Al) phase show a similar texture as it is known for Ti and Ti‐base alloys after compressive deformation at elevated temperatures. In contrast to the γ texture, no significant change of the α/α₂ texture was observed in the temperature range between 800 °C and just below the α‐transus temperature (T_α = 1295 °C).     

The effect of microstructure and processing variables on the yield to ultimate tensile strength ratio in a Nb–Ti and a Nb–Ti–Mo line pipe steel

In a hot rolled Nb–Ti and a Nb–Ti–0.09%Mo micro-alloyed steel, the ratio of yield strength to tensile strength (YS/UTS) was found to be a function of the microstructure and cooling rate in those tests where no coiling simulation and no prior deformation. The coarse bainite or acicular ferrite, which was formed at high cooling rates, raised the YS/UTS ratio under these process conditions. With coiling simulation, the ratio was not sensitive to the cooling rate or the microstructure as coiling allows the recovery of dislocations, thereby decreasing the difference in dislocation density that had arisen between a low and a high cooling rate. Deformation with a 33% reduction below the nil-recrystallisation temperature (T_nr) prior to the transformation, led to a high YS/UTS ratio that ranged from 0.81 to 0.86. The prior deformation, therefore, had a stronger effect on the YS/UTS ratio than microstructural changes through cooling rate variations.     

Hot Deformation Behavior,Dynamic Recrystallization,and Texture Evolution of Ti–22Al–25Nb Alloy

The hot deformation behavior, dynamic recrystallization, and texture evolution of Ti–22Al–25Nb alloy in the temperature range of 950–1050 °C and strain rate range of 0.001–1 s^?1 is investigated by plane‐strain compression testing on the Gleeble‐3500 thermo‐mechanical simulator. The results show that the flow stress decreases with the increase of temperature and decrease of strain rate. Besides, the flow curves appear a serrate oscillation at a strain rate of 0.1 s^?1 for all the temperature ranges, which may result from instability such as flow localization or micro‐cracking. The flow behavior can be expressed by the conventional hyperbolic sine constitutive equation and the calculated deformation activation energy Q in the (α₂ + B2) and B2 regions are 631.367 and 304.812 kJ mol^?1, respectively. The microstructure evolution is strongly dependent on the deformation parameters, and dynamic recrystallization (DRX) is the dominant softening mechanism in the (α₂ + B2) region, including discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX). In addition, the η_bcc‐fiber of {110} <001> is the dominant texture component in deformed Ti–22Al–25Nb alloy. It is observed that the weakening of the deformation texture is accompanied by the occurrence of DRX, which can be attributed to the large misorientation between DRX grains and neighboring B2 matrix induced by the rotation of DRX grains toward the preferred slip systems.

     

Effect of Recrystallization on Tensile Behavior,Texture, and Anisotropy of Ti‐3Al‐2.5 V Cold Pilgered Tubes

The recrystallized volume fraction of Ti 3Al 2.5 V seamless tubes is measured using electron BackScatter diffraction (EBSD) after annealing under various conditions. Standard tensile tests and contractile strain ratio (CSR) measurements are carried out in order to analyze the effect of recrystallization on the tensile behavior and the anisotropy of the tubes. The tensile tests show anomalous yield‐point phenomena, which become stronger when the recrystallized fraction is increased. CSR value changes through recrystallization, from 0.8 in the cold worked stress relieved (CWSR) state to 1.1 in the fully recrystallized structure. Orientation distribution functions (ODFs) calculated from X‐ray data reveal a decay in the intensity of the crystallographic texture as recrystallization advances. This can explain the tendency toward isotropy when complete recrystallization is achieved.     

Effects of stress ratio and temperature on fatigue crack growth in a Ti–6Al–4V alloy

Fatigue thresholds and fatigue crack growth (FCG) rates in corner notched specimens of a forged Ti–6Al–4V aero-engine disk material were investigated at room temperature and 350 °C. The threshold stress intensity range, ΔK_th, was determined by a method involving a step change in stress ratio (the ‘jump in’ method). It was found that for three high stress ratios (R=0.7–0.9), where crack closure effects are widely accepted to be negligible, there were similar ΔK_th values at room temperature and 350 °C under the same R. For a given temperature, ΔK_th was observed to decrease from 3.1 to 2.1 MPam with R increasing from 0.7 to 0.9. The fatigue crack growth rate was influenced by increasing temperature. For high stress ratios, FCG rate at 350 °C was higher than that at room temperature under the same ΔK. For a low stress ratio (R=0.01), higher temperature led to higher FCG rates in the near-threshold regime, but showed almost no effect at higher ΔK. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a K_max effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the K_max and fatigue threshold, is proposed to describe FCG rates in the near-threshold and Paris regimes for both temperatures. The predictions compare favourably with experimental data.