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

在β - ti合金中，ω -相变是通过三种方式发生的，即β -trans以上温度淬火、时效和应力。-相晶体使- ti合金更硬更脆。近年来，人们发现通过长时间时效或高温时效，ω相晶体成为α相变的成核点。含有细小α相颗粒的β -Ti合金，（β + α）双相钛合金强度更强，延展性更好。本研究的目的是开发在β - ti合金中制备密度更高的ω相晶体作为α相颗粒的成核助剂的新方法，并阐明ω相相变机理。采用高分辨率电子显微镜方法在原子水平上阐明了β -到-的转变行为。所得结果如下：(1)。淬火后的ω相晶体不稳定，在加热过程中几乎全部消失。老化的ω -相晶体从淬火后的ω -相晶体中生长出来，成为成核位点。时效过程中的拉应力和静水压对促进高密度时效ω相晶体是有效的。人们认识到制备ω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.