应力作用下的相变

Phase Transformations Under Stress

水静压抑制Fe -C和钢中体积膨胀型相变 ,如铁素体、珠光体、贝氏体和马氏体相变。单向应力促发铁素体和珠光体相变 ,拉应力的效果尤为显著。 0 .38C -Cr-Mo钢中 ,铁素体、珠光体和贝氏体相变在应力下的动力学 ,可由Johnson -Mehl -Avrami方程中加入应力因子经修正来描述。由于铁素体和珠光体的化学驱动力小 ,应力所作膨胀功使形核率 (J )增高和孕育期τ缩短。而贝氏体相变中 ,可能由于应力下碳自奥氏体贫化 ,或减少相界面能 ,从而使τ增高和τ缩短 ,待试验予以证明。水静压对Ms温度影响的定量描述因不同材料而异。列出Patel-Cohen所建立的dMs/dσ方程 ,及本文作者提出的应力影响Ms的方程。应力诱发马氏体会改变其晶体学及形态 ,提出以形核率的数值来判定其形态。奥氏体的力学稳定化在马氏体相变中主要由于奥氏体强化所导致 。

Hydrostatic pressure depresses the dilatational transformations, such as ferrite, pearlite,bainite and martensite transformations in Fe-C and steel. Uniaxial stress enhances ferrite and pearlite transformations and the effect of tensile stress is more effective than compressive one. Kinetics of ferrite, pearlite and bainite transformations in a 0.38C-Cr-Mo steel can be described by a modified Johnson-Mehl-Avrami equation in which the stress factor is involved. For ferrite and pearlite transformations, since the chemical driving force is small, the work done by applied stress may increase the nucleation rate J * and shorten the incubation time τ. However, for bainitic transformation, the depletion of carbon from austenite and decrease of interfacial energy may contribute to increase J * and reduce τ that needs to be proved by experiment. Equations expressing the effect of hydrostatic pressure on Ms vary with different materials. Equation of dMs/dσ established by Patel and Cohen and that expressing the effect of stress on M s suggested by the present author are introduced. Stress induced martensite may change their crystallographic characteristic and morphologies and it is suggested that the value of nucleation rate can determine the martensitic morphology. The mechanical stabilization of austenite occurring in martensitic transformation is mainly caused by the strengthening of austenite, however, that occurring in bainite formation may mainly be due to hindrance of growth by crystallographic interference.