Solid-state phase transformations (SSPTs) being the most widely versatile routes to tailor microstructures are less utilized in the design of nanostructured (NS) alloys, resulting in a lack of understanding of the significance of SSPTs in tuning mechanical properties. Here, we combine nanostructuring with reverse austenite transformation to make a phase-transforming NS Fe alloy consisting of ultrafine ferrite and austenite grains. By comparison with the coarse-grained (CG) counterpart, the NS sample exhibits high strengths (i.e., the yield and the ultimate tensile strengths are 816 and 1170 MPa, respectively), good ductility with a uniform elongation of 53 %, and superior strain hardening capacity, under multiple strengthening mechanisms and the activation of transformation induced plasticity. Regarding the mechanism in forming the NS sample, grain refinement is revealed to markedly affect the austenite formation kinetics by generating sluggish growth behavior accompanied by sufficient solute partitioning. Then, we clarify the difference in mechanical responses of the phase-transforming NS and CG samples using a physically based microstructures-properties model. Further based on the thermo-kinetic theory of generalized stability, we show that the simultaneous increase of strength and ductility in the NS sample relative to the CG sample is originated from the deformation physics with higher driving force and higher generalized stability. From the thermo-kinetic perspective, our work demonstrates that the SSPTs hold substantial promise in optimizing the microstructures and the mechanical properties of NS Fe alloys and are expected to find wide application in the development of other NS materials.

中文翻译：

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相变纳米结构铁合金变形机制的热动力学见解


固态相变 (SSPT) 是最广泛通用的定制微观结构的途径，但在纳米结构 (NS) 合金的设计中较少使用，导致人们对 SSPT 在调整机械性能方面的重要性缺乏了解。在这里，我们将纳米结构与逆奥氏体相变相结合，制造出由超细铁素体和奥氏体晶粒组成的相变 NS Fe 合金。与粗晶 (CG) 样品相比，NS 样品表现出高强度（即屈服强度和极限拉伸强度分别为 816 和 1170 MPa）、良好的延展性（均匀伸长率为 53%）和优异的应变在多种强化机制和相变诱导塑性激活下的硬化能力。关于形成 NS 样品的机制，晶粒细化被揭示通过产生缓慢的生长行为以及足够的溶质分配来显着影响奥氏体形成动力学。然后，我们使用基于物理的微观结构-性能模型阐明了相变 NS 和 CG 样品机械响应的差异。进一步基于广义稳定性的热动力学理论，我们表明NS样品相对于CG样品的强度和延展性同时增加源于具有更高驱动力和更高广义稳定性的变形物理。从热动力学角度来看，我们的工作表明，SSPT 在优化 NS Fe 合金的微观结构和机械性能方面具有巨大的前景，并有望在其他 NS 材料的开发中得到广泛应用。