Effect of thermomechanical processing on superplasticity in Ti–Al–Mo–Zr alloy

Abstract

Superplasticity in terms of total tensile elongation was studied in a titanium alloy of nominal composition Ti–6·5Al–3·3Mo–1·6Zr (wt-%) for three strain rates (1·04 × 10^?3, 2·1 × 10^?3, and 4·2 × 10^?3s^?1) and in the temperature range 1123–1223 K for microstructures obtained by different processing schedules. Fine equiaxed microstructure with a low aspect ratio of 1·15 was accomplished in this alloy by combining two types of deformation. While the first step consists of heavy deformations for refining and intermixing the phases, a second step, consisting of light homogeneous reductions in several stages, was necessary to remove the banding that developed during the first step. The resulting microstructure underwent enormous tensile elongation (1700–1725%), even under relatively high strain rates (1·04 × 10^?3 and 2·1 × 10^?3s^?1), making this alloy most suitable for commercial superplastic forming. The present investigation also revealed that the usual sheet rolling practice of heavy reductions to refine the microstructure leads to localised banding which could not be removed by annealing; therefore, the tensile elongation was limited to 770% only. The reason for this may be attributed to the resistance in grain boundary sliding and rotation encountered in microstructures with shear bands and grains with high aspect ratio. Strain enhanced grain growth was also greater in these microstructures.

MST/555     

Osseointegration behavior of novel Ti–Nb–Zr–Ta–Si alloy for dental implants: an in vivo study

This study aimed to evaluate the effects of Ti–Nb–Zr–Ta–Si alloy implants on mineral apposition rate and new BIC contact in rabbits. Twelve Ti–Nb–Zr–Ta–Si alloy implants were fabricated and placed into the right femur sites in six rabbits, and commercially pure titanium implants were used as controls in the left femur. Tetracycline and alizarin red were administered 3 weeks and 1 week before euthanization, respectively. At 4 weeks and 8 weeks after implantation, animals were euthanized, respectively. Surface characterization and implant-bone contact surface analysis were performed by using a scanning electron microscope and an energy dispersive X-ray detector. Mineral apposition rate was evaluated using a confocal laser scanning microscope. Toluidine blue staining was performed on undecalcified sections for histology and histomorphology evaluation. Scanning electron microscope and histomorphology observation revealed a direct contact between implants and bone of all groups. After a healing period of 4 weeks, Ti–Nb–Zr–Ta–Si alloy implants showed significantly higher mineral apposition rate compared to commercially pure titanium implants (P?<?0.05), whereas there was no significant difference between Ti–Nb–Zr–Ta–Si alloy implants and commercially pure titanium implants (P?>?0.05) at 8 weeks. No significant difference of bone-to-implant contact was observed between Ti–Nb–Zr–Ta–Si alloy implants and commercially pure titanium implants implants after a healing period of 4 weeks and 8 weeks. This study showed that Ti–Nb–Zr–Ta–Si alloy implants could establish a close direct contact comparedto commercially pure titanium implants implants, improved mineral matrix apposition rate, and may someday be an alternative as a material for dental implants.     

Effects of Nb content in Ti–Ni–Nb brazing alloys on the microstructure and mechanical properties of Ti–22Al–25Nb alloy brazed joints

Vacuum brazing was successfully used to join Ti–22Al–25Nb alloy using Ti–Ni–Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti–Ni–Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, ?3, and Ti2 Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached359 MPa when the joints were brazed with eutectic Ti40Ni40Nb20 alloy at 1180?C for 20 min, and it was321, 308 and 256 MPa at 500, 650 and 800?C, respectively. Cracks primarily initiated and propagated in ?3compounds, and partially traversed B2+O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2+O region, which was favorable for the mechanical properties of the brazed joint.     

The effect of SMAT-induced grain refinement and dislocations on the corrosion behavior of Ti–25Nb–3Mo–3Zr–2Sn alloy

A nanocrystalline layer, which consists of pure β phase with high density of dislocations on Ti–25Nb–3Mo–3Zr–2Sn alloy, was fabricated by surface mechanical attrition treatment (SMAT). The corrosion behavior of the as-SMATed sample, together with the solution-treated coarse-grained and 200 °C annealed SMATed samples, was investigated by potentiodynamic polarization and electrochemical impedance spectroscope (EIS) techniques in physiological saline and simulated body fluid (SBF) solutions. The results demonstrate that the corrosion resistance of the studied alloy in both of the solutions considerably increased as the grain size decreased from microscale to nanoscale, which is ascribed to the dilution of segregated alloying elements at grain boundaries and the formation of more stable and much thicker passive protection films on the nanograined samples. Although the SMAT-induced grain refinement and dislocations both have positive effects on the corrosion behavior of the studied alloy, our post annealing experimental results indicate that the improved corrosion resistance is mainly due to the grain refinement.     

Effect of deformation on corrosion behavior of Ti–23Nb–0.7Ta–2Zr–O alloy

The influence of deformation on the corrosion behavior of a newly developed multifunctional beta titanium alloy Ti–23Nb–0.7Ta–2Zr–O (mol%) in Ringer's solution at 310 K was evaluated using an electron backscatter diffraction technique and electrochemical measurements. The results showed that the effect of deformation on the corrosion resistance of the beta titanium alloy is complicated. Small levels of plastic deformation are detrimental to the corrosion resistance, whereas large deformations tend to eliminate this detrimental effect.     

Effect of heat treatment with the mould on the super-elastic property of Ti–Ni alloy castings for dental application

Tensile property of Ti–50.85Ni (mol %) alloy castings was investigated quantitatively in relation to the thermal behavior accompanied with phase transformation to evaluate the effect of heat treatment after casting with the mould in air. The heat treatment temperature was 713 or 773 K, and the period was 0.9, 1.8, or 3.6 ks. Apparent proof stress of the castings decreased with increasing period of heat treatment, and the decrease was larger with the treatment at 773 K. Residual strain also decreased by the heat treatment, however, it was low with the treatment for relatively short period, i.e. 713 K-0.9 and 1.8 ks, and 773 K-0.9 ks treatments. From the thermal behavior measured by differential scanning calorimetry (DSC), the ascent in the transformation temperatures and the increase in the thermal peak height appeared to influence the changes in the tensile property. These changes by heat treatment were believed to be effective to utilize more flexibility, less stress and less permanent deformation in dental castings.     

Influence of cold rolling on the precipitation in an Al–Mn–Zr alloy

The aim of the experiments was to study the influence of the rolling reduction (39, 87, and 96%) on the size and distribution of secondary particles and recrystallization behaviour of Al–Mn sheets with Zr addition prepared by twin-roll casting (TRC) in the industrial conditions. Samples, cold rolled on a laboratory mill, were subjected to a two-step precipitation annealing. Their microstructure (grain structure, substructure, particle analysis) as well as mechanical and physical properties (microhardness, electrical conductivity, resisitivity) were then characterized. The material cold worked with 39% reduction shows a moderate decrease of microhardness due to recovery, while sheets cold worked with 87 and 96% reduction soften more due to recrystallization. Regardless of the cold rolling reduction, the highest density of precipitates of α-Al(Mn,Fe)Si phase form during heating to 450 °C in the temperature range from 350 °C to 450 °C. Quantitative particle analysis of the sheets after the whole annealing cycle indicate that the alloy deformed 96% shows the highest density of particles in the size range from 50 to 140 nm. In addition, many spherical particles up to 15 nm were observed by transmission electron microscopy, some of them were identified by electron diffraction as Al₃Zr phase with L1₂ crystal structure.     

An investigation of the mechanical behaviour of fine tubes fabricated from a Ti–25Nb–3Mo–3Zr–2Sn alloy

This study investigates the mechanical properties and the deformation mechanisms active in Ti–25Nb–3Mo–3Zr–2Sn fine tubes. Ti–25Nb–3Mo–3Zr–2Sn alloy is a recently developed metastable β titanium alloy intended for biomedical applications. Tensile tests were carried out on the fine tubes. The modulus of the Ti–25Nb–3Mo–3Zr–2Sn fine tubes increased with reductions in the diameter for tubes in the cold rolled and annealed conditions. In comparison with cold rolled tubes, the annealed tubes exhibit increased strain hardening behaviour and superior ductility. Mechanical twins, stress-induced martensitic transformation and the textures of the β and α″ phases were investigated. The results show that the fine tubes exhibit different moduli which are related to the evolution of β and α″ phase textures during processing. Cold rolling facilitates the transformation from β to the α″ phase and mechanical {332}〈113〉 twinning. For the annealed tubes, mechanical twinning as well as primary and secondary martensitic transformations was activated at specific levels of tensile strain. Twins developed with increasing levels of strain, and secondary martensitic transformations occurred within the twinned β regions. Annealed fine tubes exhibit multistage strain hardening behaviour and superior ductility due to the synergetic effects of twinning induced plasticity and transformation induced plasticity during tensile deformation.     

A New Ti–15Zr–4Nb–4Ta alloy for medical applications

V ions exhibit cytotoxicity in a culture medium from concentrations of ≧0.2 mg/L. Ti, Zr, Nb and Ta are biocompatible elements. A new Ti–15Zr–4Nb–4Ta alloy for medical implants is being developed. Its microstructure, mechanical properties, corrosion resistance and corrosion fatigue properties in a physiological saline solution, biocompatibility with cultured cells, new bone tissue response through rat tibia implantation and surface modification are discussed. Medical applications will be also addressed.     

Surface modification of Ti–Nb–Zr–Sn alloy by thermal and hydrothermal treatments

Surface modifications by thermal and hydrothermal treatments in solution with calcium ions were investigated with the aim of improving bioactivity and wear resistance of a Ti–Nb–Zr–Sn alloy. The results showed that the first step of thermal treatment at 600 °C significantly increases the surface hardness and energy by forming oxides of Ti and Nb. The second step of hydrothermal treatment in a boiled supersaturated Ca(OH)₂ solution induces a bioactive layer containing CaTiO₃, CaCO₃, Ca(OH)₂ and TiO₂. Using this treatment, a complete Ca–P layer can be formed within 3 days of soaking in simulated body fluid (SBF). The origin of such fast apatite formation was analyzed by comparison with single step thermal or hydrothermal treatment and with thermal plus hydrothermal treatment without calcium ions. The results suggest that the increase of surface energy by thermal treatment and the incorporation of calcium ions by the hydrothermal treatment in calcium ion solution play important roles in the formation of bioactive apatite.     

Mechanical properties and corrosion resistance of Ti–6Al–7Nb alloy dental castings

With the aim of applying a novel titanium alloy, Ti–6Al–7Nb, to a dental casting material, a comprehensive research work was carried out on its characteristics, such as castability, mechanical properties and corrosion resistance in the present study. As a result, Ti–6Al–7Nb alloy exhibited sufficient castability by a dental casting method for titanium alloys and enough mechanical properties for dental application. It is also showed excellent corrosion resistance through an immersion test in 1.0% lactic acid and an anodic polarization test in 0.9% NaCl solution. From these results, it is concluded that this Ti–6Al–7Nb alloy is applicable as a dental material in place of Ti–6Al–4V alloy, which includes cytotoxic vanadium.     

Maximisation of the ratio of microhardness to the Young's modulus of Ti–12Mo–13Nb alloy through microstructure changes

Alloys for orthopaedic and dentistry applications require high mechanical strength and a low Young's modulus to avoid stress shielding. Metastable β titanium alloys appear to fulfil these requirements. This study investigated the correlation of phases precipitated in a Ti–12Mo–13Nb alloy with changes in hardness and the Young's modulus. The alloy was produced by arc melting under an argon atmosphere, after which, it was heat treated and cold forged. Two different routes of heat treatment were employed. Phase transformations were studied by employing X-ray diffraction and transmission electron microscopy. Property characterisation was based on Vickers microhardness tests and Young's modulus measurements. The highest ratio of microhardness to the Young's modulus was obtained using thermomechanical treatment, which consists of heating at 1000 °C for 24 h, water quenching, cold forging to reduce 80% of the area, and ageing at 500 °C for 24 h, where the final microstructure consisted of an α phase dispersed in a β matrix. The α phase appeared in two different forms: as fine lamellas (with 240 ± 100 nm length) and massive particles of 200–500 nm size.     

Effect of CeLa addition on the mechanical properties of Al–Cu–Mn–Mg–Fe alloy

The rapid development of new energy automobiles leads to an increasing demand for high-strength lithium battery shell alloy. The microstructures, electrical conductivity and mechanical properties of CeLa-containing Al–Cu–Mn–Mg–Fe alloys were investigated with scanning electron microscopy (SEM), X-ray diffraction, Eddy Current conductivity tester, tensile testing and Erichsen cup testing. Experiment results indicate that Al₆(Mn, Fe) particles could be refined by CeLa alloying and AlCuCeLa phase nucleates and grew up at the surface of Al₆(Mn, Fe) particle. Major texture of the CeLa-containing alloys was different from that of the CeLa-free alloy. The electrical conductivity decreased with increase of the CeLa content. CeLa addition could greatly enhance the tensile strength of the alloy at temperatures ranging from –40°C to 300°C.     

Effects of trace Er addition on the microstructure and mechanical properties of Mg–Zn–Zr alloy

The effects of trace Er addition on the microstructure in Mg–9Zn–0.6Zr alloy during casting, homogenization, pre-heating, and hot extrusion processes were examined. The mechanical properties of alloys with and without Er were compared. The results showed that Er exhibited a lower solubility in solid magnesium and formed thermally stable Er- and Zn-bearing compounds. The Er-bearing alloy exhibited a considerably improved deformability, as well as a fine and uniform microstructure. Moreover, dynamic precipitation of fine MgZn₂ particles with a modified spherical morphology occurred during hot extrusion, resulting in a tensile yield strength of 313 MPa and a high elongation to failure value of 22%. Further aging of the Er-bearing alloy led to an increment of another 30 MPa in yield strength. In addition, Er markedly increased the thermal stability of the alloy structure.     

Electrochemical behaviour of Ti–25Nb–3Mo–3Zr–2Sn biomedical alloy in different simulated physiological environments

The electrochemical corrosion behaviour of biomedical Ti–25Nb–3Mo–3Zr-2Sn (TLM) alloy was investigated in various simulated body fluids at 37±0·5°C utilising potentiodynamic polarisation and current–time curves. The Ti–6Al–4V (TC4) alloy was also investigated to make a comparison. The different simulated body fluids comprised of 0·9%NaCl saline, Hank’s and Ringer’s solution were employed. The effect of heat treatment on the electrochemical behaviour of the TLM alloy was also considered. It was discovered that all the test specimens were passivated once immersed into the simulated body fluids. It was also found that the TLM alloy has poorer corrosion resistance in Hank’s solution, due to the chemical composition of the Hank’s. After different heat treated, the TLM alloy had different phases and microstructure, and the corrosion behaviour of the TLM alloy was different. In this study, after the heat treatment of 760°C/1 h/AC+550°C/6 h/AC, the TLM alloy had better corrosion resistance. Owing to the corrosion resistance of the TLM alloy was influenced by numerous factors, such as microstructure and the chemical composition of electrolyte, the corrosion behaviour of the TLM alloy is complex. By comparing with the corrosion behaviour of the TC4 alloy, the TLM alloy has poorer corrosion resistant than the TC4 alloy under the same conditions. But the current–time curves of the TLM alloy were more stable than these of the TC4 alloy with further experiments, because of the more passivation film on the surface of the TLM alloy.     

Effects of minor Zr and Sr on as-cast microstructure and mechanical properties of Mg–3Ce–1.2Mn–1Zn (wt%) magnesium alloy

The effects of minor Zr and Sr on the as-cast microstructure and mechanical properties of the Mg–3Ce–1.2Mn–1Zn (wt%) alloy were investigated. The results indicate that adding minor Zr and/or Sr to the Mg–3Ce–1.2Mn–1Zn alloy does not cause an obvious change in the morphology and distribution of the Mg₁₂Ce phase. However, the grains of the Zr- and/or Sr-containing alloys are effectively refined. Among the Zr- and/or Sr-containing alloys, the grains of the alloy with the addition of 0.5 wt%Zr + 0.1 wt%Sr are the finest. Furthermore, adding minor Zr and/or Sr to the Mg–3Ce–1.2Mn–1Zn alloy can improve the tensile properties. Among the Zr- and/or Sr-containing alloys, the alloy with the addition of 0.5 wt%Zr + 0.1 wt%Sr obtains the optimum tensile properties. In addition, adding minor Zr and/or Sr to the Mg–3Ce–1.2Mn–1Zn alloy also can improve the creep properties, and the creep properties of the three alloys with the additions of 0.5 wt%Zr + 0.1 wt%Sr, 0.5 wt%Zr, and 0.1 wt%Sr are similar.     

Effects of germanium on quench sensitivity in Al–Zn–Mg–Zr alloy

Effects of the trace element germanium (Ge) on the quench sensitivity of an Al–Zn–Mg–Zr alloy were investigated in the present work. The results showed that the Ge-bearing alloy exhibited lower quench sensitivity as compared to the Ge-free alloy. This phenomenon could be reasonably interpreted in terms of the stability of supersaturated solid solution of alloys after quenching from an elevated temperature. The apparent vacancy formation energies for the Ge-free and Ge-bearing alloys were determined to be 0.49 and 0.58 eV respectively. This suggested that the addition of a small amount of Ge was able to trap excess vacancies, leading to a decrease in the amount of coarse dispersoids and resultant low quench sensitivity in Ge-bearing alloys. Therefore, Ge could be used in alloy productions, which require a slow cooling rate to reduce the residual stresses and distortions.     

Effect of manganese on microstructure of an isothermally transformed Fe–Nb–C alloy

Abstract

The effect of manganese on the microstructure developed by isothermal transformation of an Fe-Nb-C alloy was examined over a wide range of temperatures. A well defined discontinuity was found at about 750°C in the time-temperature-transformation curve of an Fe–0·033Nb–0·07C–1·07Mn (wt-%) alloy. The discontinuity appears close to the temperature at which the interphase precipitation of NbC begins to occur, this temperature being about 50 K lower than that for a corresponding Fe–Nb–C alloy. Carbide-free ferrite is principally formed between 850 and 780°C in the Mn-containing alloy, a wider temperature range than that for the alloy without manganese. The amount of NbC dispersed in ferrite is decreased by the addition of manganese. The changes that develop in the microstructure after the addition of manganese are explained in terms of the reduction of ferrite growth rate and the decrease in supersaturation of NbC in ferrite.

MST/45     

Evaluation of the bond strength of a low-fusing porcelain to cast Ti–24Nb–4Zr–7.9Sn alloy

The purpose of this study was to compare the bonding characteristics of titanium porcelain Duceratin bonded to Ti–24Nb–4Zr–7.9Sn (TNZS) alloy and commercial pure titanium (cp Ti). The bond strengths between porcelain and TNZS were tested by a three-point flexural device. The same tests for the cp Ti were used as for the control. Coefficient of thermal expansion (CTE) of TNZS was evaluated with a push-rod dilatometer. Interfacial characterization was carried out by X-ray energy-dispersive spectrometry (EDS) analysis operating in line scan mode. Additionally, microstructure characterizations of TNZS and cp Ti after debonding fracture were analyzed by scanning electron microscope (SEM) and EDS. The porcelain bond strength of TNZS alloy was 31.51 MPa, showing a significant increase relatively to that of cp Ti (23.89 MPa) (P < 0.05). Mean CTE values of TNZS alloy was 9.51 × 10^? 6/°C exceeding the porcelain by 0.81 × 10^? 6/°C, attesting to a better mechanical performance. Interfacial characterization showed the mutual diffusion of Ti, Si, O and Sn along the TNZS–ceramic interface. Both SEM and EDS results revealed that fracture modes of TNZS specimens exhibited a mixed mode of cohesive and adhesive failures. The results demonstrated that TNZS could be a good alternative for the metal–ceramic restoration in the future.