Yue Fei

Microstructure and mechanical properties of new metastable β type titanium alloy

Abstract A new metastable β type titanium alloy called TB-13 with the combination of excellent strength and ductility was developed successfully. In order to develop a perspective on this new alloy, the influence of several commonly used heat treatments on the microstructure and properties was studied. In solution-treated and quenched samples, a low-temperature aging at 480 °C results in the precipitation of finer α phase. The precipitation of coarser α phase plate at higher aging temperature (560 °C) leads to the increase of tensile ductility but reduction of strength. During low-temperature aging at 300 °C, quite homogeneous distribution of fine isothermal ω phase particles was found. The isothermal α phase provides nucleation sites for α phase during two-step aging process and makes α phase extremely fine and disperse uniformly in β matrix. Thus, TB-13 alloy is strengthened and its mechanical properties are improved.

Characterization of Hot Deformation Behavior of Ti-4.5Al-3V-2Mo-2Fe Titanium Alloy

The hot deformation behavior of Ti-4.5Al-3V-2Mo-2Fe (SP-700) titanium alloy in the temperature range of 650°C~950°C and constant strain rate of 0.01, 0.1, 1 and 10s-1 has been investigated by hot compressive testing on the Gleeble-1500D thermal simulation test machine. The experimental results indicated that the hot deformation behavior of SP-700 alloy was sensitive to the deformation temperature and strain rate. The peak flow stress decreased with the increase of temperature and the decrease of strain rate. The flow curves characteristic under different deformation parameters show significant different. Analysis of the flow stress dependence on strain rate and temperature gives a stress exponent of n as 4.8235 and a deformation activation energy of Q as 410kJ/mol. Based on the dynamic materials model, the processing map is generated, which shows that the most peak efficiency domain appears at the temperature of 725°C~775°C and the strain rate of 0.001 s-1~0.003s-1 with a peak efficiency of 45% at about 750°C/0.01s-1.

Effect of Hot Forging Strain Rate on the Microstructure and Mechanical Properties of TC4-DT Titanium Alloy

The microstructures, tensile properties and fatigue property of TC4-DT titanium alloy hot die forged under different strain rates were investigated. The results show that the microstructures and mechanical properties of TC4-DT titanium alloy die forging using different strain rates can meet the requirement of technical standard. At the strain rates of 1mm/s and 10mm/s, the microstructure was not sensitive to the strain rate, and the alloys showed uniform, fine and fuzzy crystal grains, and no metallurgical defects. However, more uniform tensile properties in different orientations were obtained by the low strain rates (1mm/s), while the high strain rate (10mm/s) could lead to slightly increase in strength but obviously decline in ductility. High fatigue strength could be obtained by the low strain rate, but the fracture toughness and fatigue crack growth rate were not sensitive to the strain rates.

Hot Compressive Deformation Behavior of TC4-DT Titanium Alloy

Hot compressive experiments of TC4-DT titanium alloy were performed on Gleeble 3500 hot simulator. The influence of hot deformation parameters on high temperature deformation behaviors were investigated, including deformation temperature (938°C~1038°C), deformation degree and strain rate (0.01s-1~10s-1). The results indicated that the peak (σp) and steady-state flow stress (σs) of TC4-DT alloy decreased with the increase of deformation temperature under the same strain rate, especially under a high strain rate. The flow stress increased sharply then decreased and kept invariant finally with the increase of deformation degree. The flow stress increased with the strain rate increasing and exhibited different characteristics in different strain rate range. The optimum conditions obtained based on this investigation of TC4-DT alloy as follows: temperature was 938°C~1008°C, stain rate was 0.01s-1~0.1s-1.

Influence of Cooling Rate on Phase Transformation of a α+β Titanium Alloy

Effect rules of cooling rate on phase transformation and microstructure of a α+β titanium alloy were studied by thermal dilatometer method. Specimens under different cooling rates from 0.05°C/s to 2.5°C/s were analyzed by using X-ray diffraction (XRD), optical micrograph (OM) and Vickers micro-hardness. The results showed that the starting temperature of β to α phase transformation gradually decreased with the increase of cooling rate. The finishing temperature of β to α phase transformation firstly decreased and then increased with the increase of cooling rate. The typical lamellar structures were observed under the cooling rates from 0.05°C/s to 0.2°C/s. The layer thickness of α phase became thinner and the precipitation content of α phase reduced under the cooling rate of 0.5°C/s. The α phase was fine acicular and the precipitation content of α phase reduced obviously under the cooling rate of 2.5°C. The effect rule of cooling rate on micro-hardness was that the value of micro-hardness firstly decreased and then increased with the cooling rate increasing.

Combined Strengthening-Toughening Technology of High Property Titanium Alloys for Aviation Uses in China

Combined strengthening-toughening technologies of several high property titanium alloys, such as TC4-DT, TC6, TC18, TC21 for aviation uses, have been studied via purification, quasi-b heat treatment, quasi-b forging and grain refinement. The effects of microstructure parameters of lamellar structure, basket-weave structure, refined grain structure etc. on the comprehensive mechanical properties of titanium alloys have been analyzed. The results have shown that, to acquire highly comprehensive static mechanical properties and excellent damage tolerance properties, quasi-b treatment and purification processing should be used for medium strength titanium alloys to get high ductility lamellar structure, while quasi-b forging processing be utilized for high strength titanium alloys to obtain high ductility basket-weave structure. Grain refinement processing is very necessary for both the strength levels of titanium alloys.

β Grain Growth Kinetics of a New Metastable β Titanium Alloy

Ti-Mo-Nb-Cr-Al-Fe-Si alloy is a new metastable β titanium alloy with excellent combination of strength and ductility. The β grain-growth exponent and the activation energies for β grain growth for the investigated alloy at specified temperature were computed by the kinetic equations and the Arrhenius-type equation. The rate of β grain growth decreases with elongating solution treated time and increases with the increasing solution-treated temperature. The β grain-growth exponents, n, are 0.461, 0.464 and 0.469 at 1113, 1133 and 1153K, respectively. The β grain growth activation energy is determined to be 274 KJ/mol.

Research on Effects of Uneven Macrostructure to Mechanical Properties in TC21 Forging

The microstructure, tensile properties, fracture toughness, fatigue crack growth rate and fatigue limit of different macrostructure regions in TC21 open-die forging were investigated. The results show that the microstructure is also uneven corresponding to non-uniform macrostructure. The ultimate tensile strength (UTS) and yield strength (YS) of coarse-grained zone are that UTS1070MPa, YS960MPa, which can meet the requirement of technical standard, whereas the strength of fine-grained zone drops more than 100MPa. The fracture toughness of this forging is that KIC90MPa·m1/2, being equivalent to the normal one. But when K=11MPa·m1/2, the fatigue crack growth rate is that da/dN=2.488758×10-5mm/cycle, higher than the normal TC21 open-die forging for an order of magnitude. The fatigue limit of fine-grained zone drops about 40MPa compared with coarse-grained zone. Uneven macrostructure and microstructure lead to a decline of tensile and fatigue property in TC21 forging.

Microstructure and Property of a New Metastable β Titanium Alloy

The evolution regularities of microstructure and property of a new metastable β titanium alloy with different solution treatment and aging treatment were studied using optical microscope (OM), scanning electron microscopy (SEM) and tensile test. The results show that, the volume fraction of primary α phase decreases and globularization of α phase occurs with the increasing aging temperature from 540 to 580 and solution treated temperature from 800 to 820. When the solution treated temperature is 820, the acicular secondary α phase precipitates along β grain boundary. The strength of the investigated alloy increases and the ductility decreases with the solution treated temperature increasing. While the strength of the investigated alloy increases and the ductility decreases with the aging temperature decreasing.

Experimental Study on Heat Yreatment Processing of a New Low Cost Titanium Alloy Used in Aviation Field

The effect of common solution treatment, two-step solution treatment and aging, solution treatment and aging (STA) on the microstructure and mechanical properties of a new low cost titanium alloy used in aviation field were investigated by optical microscope (OM), scanning electron microscopy (SEM) and tensile test. The results show that a typical equiaxed structure can be obtained by common solution treatment leading to a good combination of strength and plasticity. Besides, solution heat treating in the β region and subsequently ageing at a low temperature results in a significant increase in mechanical strength and a little decrease in plasticity. When the solution temperature is at α+β two-phase region (895), the low cost titanium alloy acquires the best combination of strength and ductility.