Wenjun Ye

Microstructure and tensile properties of low cost titanium alloys at different cooling rate

Titanium and titanium alloys have several advantages, but the cost of titanium alloys is very expensive compared with the traditional metal materials. This article introduces two new low-cost titanium alloys Ti-2.1Cr-1.3Fe (TCF alloy) and Ti-3Al-2.1Cr-1.3Fe (TACF alloy). In this study, we used Cr-Fe master alloy as one of the raw materials to develop the two new alloys. We introduce the microstructure and tensile properties of the two new alloys from β solution treated with different cooling methods. Optical microscopy (OM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM) were employed to analyze the phase constitution, and scanning electron microscopy (SEM) was used to observe the fracture surfaces. The results indicate that the microstructures consist of β grain boundary and α′ martensite after water quenching (WQ), β matrix and α phase after air cooling (AC) and furnace cooling (FC), respectively. Also, the microstructure is the typical basketweave structures after FC. Of course, athermal ω is also observed by TEM after WQ. The strength increases with decreasing cooling rates and the plasticity is reversed. Because of the athermal ω, the strength and ductility are highest and lowest when the cooling method is WQ. The strength of TACF alloy is higher than the TCF alloy, but the plasticity is lower. The fracture surfaces are almost entirely covered with dimples under the cooling methods of AC and FC. Also, we observe an intergranular fracture area that is generated by athermal ω, although some dimples are observed after WQ.

Martensitic transformation of Ti-18Nb(at.%) alloy with zirconium

The addition of 3%∼9% Zr on the martensitic transformation of Ti-18Nb(at.%) alloy was investigated. The results of microstructure and X-ray diffraction (XRD) analysis show that the phase constitution of as-quenched Ti-18Nb-9Zr(at.%) alloy consists of the retained β matrix and α″ martensite, while that of the other three alloys is single α″ martensite. No trace of athermal ω phase was found in any of the as-quenched alloys. Unlike the effect of Nb addition on the martensitic transformation start temperature Ms of Ti-18Nb(at.%) alloy, Ms decreased nonlinearly as increasing the Zr addition from 3% to 9% and Ms decreased much more sharply as increasing the Zr addition. The Ms of as-quenched Ti-18Nb-9Zr alloy was around room temperature. The effect of Zr addition on the β phase stabilizing in the Ti-18Nb(at.%) alloy was briefly discussed.

Tensile and fracture properties of Ti-62A alloy plate with different microstructures

Ti-62A alloy plates with three different types of microstructure, fully equiaxed, bimodal, and Widmanstätten, were obtained by various heat treatments to investigate the effects of microstructure on the tensile and fracture properties at room temperature. The results reveal that Widmanstätten microstructure exhibits good damage tolerance behavior considering strength, fracture toughness, and fatigue crack growth behavior, while the bimodal microstructure shows good comprehensive properties considering the plasticity synthetically. Optical microscopy (OM) and scanning electron microscopy (SEM) microstructure analyses on fracture and fatigue crack path demonstrate that the dependence of mechanical properties and fatigue crack growth behavior on microstructural feature are attributed to the α lamellae width and the α colony size.

Residual Stress Distribution and Mechanical Properties of TA15/BTi-6431S Titanium Alloy Welding Joints by Ultrasonic Impact Treatment

TA15 (Ti-6.5Al-2Zr-1Mo-1V) and BTi-6431S (Ti-6.5Al-3Sn-3Zr-3Mo-3Nb-1W-0.2Si) titanium alloy plates were welded through gas tungsten arc welding (TIG) and different ultrasonic impact treatment (UIT) were conducted on the weldment. The effects of ultrasonic impact treatment (UIT) on the microstructure and residual stress distribution and mechanical properties for the welding joint were investigated through optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and tensile tests. After TIG welding, the structure of welding joint is composed of fusion zone (FZ), heat-affected zone (HAZ) and base metal. The FZ is widmannstatten structure with coarse β grains and a large number of acicular α due to the fast cooling rate. The microstructure in the HAZ shows a gradual change because of the presence of temperature gradients during welding. The residual stress after TIG is mainly tensile stress and the maximum longitudinal stress appears in the centerline of welding joint. The UIT process shows dramatic influence on residual stress distribution. After employing UIT twice, the residual stress near the welding joint shows a uniform distribution and the maximum tensile stress changes to compressive stress. However, the tensile properties at room temperature almost remain unchanged after UIT.

Investigation on Microstructure and Properties of Ti-Al-Cr-Fe-V-Zr Alloy

Ti-1023 alloy has been widely used in aerospace field as a typical titanium alloy with high strength and toughness. The relatively high content of Fe causes beta fleck which will make uneven microstructure and less plasticity. The new Ti-Al-Fe-V (Cr, Zr) alloys have been designed, based on Molybdenum equivalency and Bo-Md molecular orbital method, to aim at developing a new type of titanium alloy with high strength and fracture toughness. After primary design computation, Ti-Al-Fe-V (Cr, Zr) alloy was optimized finally. A large scale ingot was made by vacuum arc re-melting (VAR) for further property evaluation. Resultantly, it shows that athermal ω phase forms in Ti-Al-Cr-Fe-V-Zr alloy when solution treated at a temperature above the β-transus. Micro-hardness of alloy conducted to different aging conditions decreases with the aging temperature and time increasing respectively. Moreover, the new alloy has got a more sluggish age hardening response relating to the aging time. Additionally, after modulated aging treatment, the alloy can obtain high strength levels with acceptable ductility. When the alloy solution treated at 770 °C for 1h, followed by aging at 500 °C for 2h, the tensile strength of the alloy can achieve 1268 MPa, with an elongation of 11.5%, at the same time, the reduction of area has surpassed 30%. As a result, the newly designed alloy can achieve a good combination of tensile strength and plasticity through appropriate heat treatment.

Effect of Boron Addition on Microstructure and Property of Low Cost Beta Titanium Alloy

A study on effect of boron addition on microstructure and property of low cost beta Ti-Al-Cr-Fe-B alloys are undertaken in the present. The Ti-Al-Cr-Fe alloys were developed as low cost beta Ti alloys for automotive springs, based on Molybdenum equivalency. Low priced Cr-Fe master alloys were introduced as beta stabilizing alloying elements for lowering cost, and elastic modulus. The boron addition is introduced in to refine the ingot microstructure and thus simplify hot working, which should lead to a cost reduction. On the other hand, the Ti-B compositions are able to restrict beta grain growth during heat treatments. So it can increase strength and microstructural stability. The mechanical tests show that the boron modified alloy has good tensile properties in solution condition. Moreover, the alloy can obtain well-balanced high strength levels with acceptable ductility after modulated aging treatment.

A Study on Microstructures and Hardening Behaviors of Ti-12.1Mo-1Fe Alloy

Ti-Mo-Fe alloy was developed as low cost beta Ti alloys for automotive springs, and designed based on Molybdenum equivalency and Bo-Md molecular orbital method. Low priced Mo-Fe master alloys were introduced as alloying elements for the cost and elastic modulus reduction. A laboratory scale ingot was melted by ISM (Induction Skull Melting). Then, microstructure characterization and hardening behavior of a new designed Ti-12.1Mo-1Fe alloy during solution and aging treatment were investigated in the present study by microscopy, X-ray diffraction and hardness test. The results showed that ω phase played a more important role than a phase in hardening. The hardening due to ω phase can lead to high hardness about 470 Hv. However, the coarse a phase resulted in hardness below 300 Hv. On the other hand, the alloy exhibited fast aging response due to high diffusion rate of Fe element in the Ti matrix.