Search

The dynamic recrystallization (DRX) mechanism and microstructure evolution of the asforged Ti-43Al-9V-Y-0.2Y alloy were investigated by plane strain compression tests under different deformation conditions. The results show that the strain rate has little influence on the microstructure components of present alloy, whereas a large amount of -phase undergoes γ α2-phase transformation with the total reduction from 40 to 60% at a constant strain rate. Additionally, DRX and deformation twinning are the primary dynamic softening mechanisms of present alloy. The extent of DRX increases with the decreasing strain rate and increasing total reduction. Meanwhile, twin boundaries (TBs) are observed in γ-phase of all the samples, which provides coordinated deformation for non-uniform deformation of present alloys. The fraction of TBs reduces from 9.51 to 0.657% with the increase of strain rate, and rises from 0.657 to 1.05% with the increase of total reduction, testifying that TBs are connected with DRX nucleation. At the same time, the trend of pseudo twin boundaries (PTBs) content is similar to that of TBs, which is also related to the strain rate and total reduction. Besides, many fine DRX grains located around the TBs and PTBs further explain that the TBs and PTBs promote DRX nucleation. The fiber intensity peak value decreases, and the fiber distribution disperses in the pole figure of a2-phase after DRX. [ABSTRACT FROM AUTHOR]

In this work, we present a density-functional theory (DFT) investigation of the phase stability, electrochemical stability and phase transformation mechanisms of the layered and over-lithiated Mn oxides. This study includes the thermodynamic stability of Li and oxygen vacancies, to examine the electrochemical activation mechanisms of these cathode materials. The DFT calculations provide phase diagrams of the Li–Mn–O system in both physical and chemical potential spaces, including the crystals containing vacancies as independent phases. The results show the ranges of electrochemical activity for both layered LiMnO2 and over-lithiated Li2MnO3. By using a thermodynamic model analysis, we found that the required temperature for oxygen evolution and Li vacancy formation is too high to be compatible with any practical synthesis temperature. Using solid-state transition calculations, we have identified the key steps in the phase transition mechanism of the layered LiMnO2 into the spinel phase. The calculated effects of pH on the Li–Mn–O phase stability elucidated the mechanism of Mn2+ formation from the spinel phase under acidic conditions. [ABSTRACT FROM AUTHOR]

Effects of trace TiB2 on solidification microstructure of Ti-6Al-4V alloy were investigated. The result shows that the microstructure of the ingot was refined by the added TiB2. The grain size and α lath size were reduced gradually with the increase in TiB2. The grain size is reduced by about an order of magnitude with an addition of 0·96 wt-%TiB2, and the dendrite appears in the ingot. Such a tendency is similar to the change of the microstructure with boron addition. Ti-6Al-4V-B phase diagram and growth restriction factor Q are applied to analyse the influence mechanism of trace TiB2 on the microstructure. [ABSTRACT FROM AUTHOR]

For the first stage, a metastable β titanium alloy, Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe reinforced with trace amounts of TiB whiskers and TiC particles was fabricated by vacuum arc melting process and hot forging followed by heat treatment at 780 °C/740 °C, then by aging at 500 °C, 550 °C, 570 °C and 600 °C. For the second stage, the unreinforced titanium alloy was also fabricated by the same process. The microstructural characteristics were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Traces of TiB whiskers and TiC particles (2.2 vol.%) with a volume ratio of 2:3 synthesized in situ exerted a hybrid reinforcing effect on the β titanium alloy. The reinforcements were uniformly distributed in the matrix and the elastic modulus was improved about 25 GPa. Ultimate tensile strength and yield strength achieves about 1625 MPa and 1500MPa respectively, with ductility at 7% when the aging temperature is 500 °C. The ductility of (TiB + TiC)/(Ti-3.5Al-5Mo-4 V-2Cr-2Sn-2Zr-1Fe) matrix composite could be enhanced by increasing the aging temperatures. After 780 °C followed by aging at 570 °C, excellent strength and plasticity properties were obtained (ultimate tensile strength of matrix alloy is 1350 MPa with elongation of 18% and ultimate tensile strength of composite is 1500 MPa with elongation of 13%). [ABSTRACT FROM AUTHOR]

Hot deformation behavior of high Nb containing TiAl based alloy with a nominal composition of Ti-44Al-8Nb-0.2W-0.2B-0.1Y at 1225 °C and 1275 °C is investigated. The deformation mechanism of α phase is dynamic recovery at 1275 °C, whereas γ phase is mainly dynamic recrystallized. It is revealed that α phase is softer and easier to be deformed when the alloy is deformed at 1275 °C. The reasons of the different deformation behavior between 1225 °C and 1275 °C, as well as the different deformation behavior between α phase and γ phase are analyzed. A multistep deformation process at gradually lowered temperature is proposed to eliminate the adverse influence of remnant lamellar structure on mechanical properties. [ABSTRACT FROM AUTHOR]