Investigation of effect of stress on formation of omega-phase crystals in beta-Ti alloys using in-situ HREM method

在β-钛合金中，欧米伽相变的发生有三种方式，即温度高于β-转变温度的淬火、时效和应力。欧米伽相晶体使β-钛合金变得更硬、更脆。近年来，人们发现欧米伽相晶体通过较长时间的时效或较高的温度时效成为α相变的形核点。含有细小α相颗粒的β-钛合金、(β+α)双相钛合金具有更高的强度和更好的韧性。本研究的目的是开发一种新的方法，使β-钛合金中密度较高的omega相晶体成为α相粒子的形核场所，并阐明omega相变的机制。用高分辨电子显微镜方法在原子水平上解释了β到omega的转变行为。所得结果如下：(1)。淬火后的欧米茄相晶体不稳定，在加热过程中几乎全部消失。时效的omega相晶体是由剩余的淬火的omega相晶体作为形核中心生长出来的。时效过程中的拉应力和静液加压能有效地促进高密度时效欧米相的生长。人们认识到，形成欧米茄相晶体的临界静水压力在0.5-0.7 Gpa之间。

In beta-Ti alloys, omega-phase transformations are occurred due to three ways, that is, quenching from temperature above beta-transus, aging and stressing. omega-phase crystals make beta-Ti alloys harder and more brittle. Recently, it was found that omega-phase crystals become nucleation sites of alpha-transformation by means of longer-time aging or higher temperature aging. beta-Ti alloys including fine alpha-phase particles, (beta+alpha) duplex Ti alloys are stronger and more ductile. The purpose of this research is to develop new methods to make higher density of omega-phase crystals in beta-Ti alloys as nucleation-siters of alpha-phase particles and to clarify the omega-phase transformation mechanism. A high resolution erectron microscope method was used to clarify the beta to omega transformation behaviors at atomic level. The results obtained are as follows :(1). As-quenched omega-phase crystals were unstable, and almost of them disappeared during the heating.(2). Aged omega-phase crystals grew from the remained as-quenched omega-phase crystals as nucleation sites.(3). A tensile stress during the aging and hydrostatic pressurizing were effective to promote high density aged omega-phase crystals.(4). It was recognized that the critical hydrostatic pressure to make omega-phase crystals was between 0.5 and 0.7 GPa.