{112}<111>孪生的形核和长大及终止的ω点阵机制

ω LATTICE MECHANISM OF {112}<111> TWINNING NUCLEATION AND GROWTH AND TERMINATION

针对体心立方(bcc)结构金属及合金{112}<111>孪生的w点阵机制,利用点阵模型详解了bcc结构金属及合金{112}<111>孪晶形核、长大和终止全过程.模型揭示了孪晶可以通过ω→bcc转变过程形成孪晶核胚,再通过孪晶核胚生长或合并的方式长大,最终与特殊位向ω相作用受阻而停止.该机制说明了{112}<111>类型孪晶是一种相变孪晶.

{112}〈111〉-type twin is a common twinning structure in quenched carbon steel. As carbon content increases, the density of the twin becomes high in the quenched state. Researchers have suggested that understanding such twinning mechanism may help us to understand the martens itic transformation in steel. {112} 〈 111〉- type twin is also commonly observed in other body centered cubic （bcc） metals and alloys, especially deformed under the conditions of low temperatures and/or high strain rates. Yet, due to the intrinsic non-close-packed structure and the rapid speed of twinning process, the mechanisms of twinning nucleation, growth and termination have not been clearly understood although phenomenological mechanisms such as the classical sheafing mechanism, dislocation mechanism, or shuffling mechanism, etc., were proposed. Recently, after reviewing numerous investigations on {112} 〈 111〉-type twinning process both experimentally and theoretically in bcc metals and alloys, it was found that the twinning boundaries are always embedded with co phase, i.e., the displacement of the first layer of the twin is 1/12 〈111〉 for co instead of 1/6 〈111〉 for twin, thus, an co phase-related {112}〈111〉-type twinning mechanism （so-called co lattice mechanism） in our previous study is proposed. In order to better understand the co lattice mechanism, in this work, a detailed description of the whole process of nucleation, growth and termination of the {112} 〈 111 〉-type twinning was offered by using the atomic lattice model. The model shows that the twin could nucleate during ω→bcc transition process, and then grow up by extending or merging of twin embryos, and finally terminate during encountering the different co variants. Such two-dimensional atomic model can be extended to three-dimensional one, which can finally explain the formation mechanism of an internal twin in one bcc crystal. Moreover, the model suggests that the diffuse co lattice （ωdiff） between the ideal co lattice and bcc lattice （in the twin boundary） plays an important role in promoting the transition of ω←→bcc during twinning nucleation and growth processes. The results suggest that the { 112}〈111〉-type twins are phase transition twin or phase transformation product.