纳米材料的相变

PHASE TRANSFORMATIONS IN NANOCRYSTALLINE MATERIALS

纳米纯金属常呈现与大块金属不同的晶体结构 ,如Co和Fe在室温呈fcc结构。不同制备方法所得的纳米Fe Ni,其α和γ相区均较大块的扩大 ,并呈现奥氏体的稳定化 ,但逆相变近似大块合金 ,文章对此作了解释。测得一定晶粒大小及一定成分的Fe Ni冷却时马氏体相变的临界温度 ,显示纳米材料中会发生马氏体相变。Al Cu薄膜中 ,Cu在晶界偏聚量大 ,在饱和浓度处形成非共格Al2 Cu ,并无过渡相 ,且脱溶温度 (固溶线 )比大块低85K。定性地讨论了晶界包括晶界偏聚和晶界扩散对无扩散型相变和扩散型相变的作用。提出一个纳米金属相变的热力学模型 ,和纳米材料马氏体形核能垒的计算 ,用以描述纳米晶晶粒大小对马氏体相变的影响。

In nano sized crystals,there often appeared the different crystal structure from that in bulk materials,such as fcc structure in Co and Fe at room temperature.In nanocrystalline Fe Ni,there exhibit extended phase areas of α and γ,the stabilization of austenite,and the similar reverse transformation to that in bulk alloy.Explanations to these phenomena were given in this article.Critical temperatures of martensite transformation during cooling were detected in Fe Ni with certain grain size and composition,which revealed that martensitic transformation might occur in nanocrystalline material.In Al Cu film,a large amount of copper segregated at grain boundaries and the incoherent Al 2Cu particles formed at the sites with saturated concentration of segregation.No intermediate phase was found before the precipitation of Al 2Cu in Al Cu film and the solid solubility temperature(solvus) is 85K lower than that in bulk alloy.The functions of grain boundary,together with the grain boundary segregation and diffusion,in the diffusionless and diffusional transformations were qualitatively discussed.A thermodynamic model for the phase transformation in nanocrystalline metal and the calculation of the nucleation barrier for martensite formation in nanocrystalline material were suggested to describe the size effect on phase transformations.Perspective in further studies was presented.