Computational and Experimental Design of Novel CoNiGa High Temperature Shape Memory Alloys

技术：形状记忆合金(SMA)是一类重要的智能材料，由于温度、应力或磁场的变化可以触发可逆的马氏体相变，从而产生大量可恢复的形状变化。目前，SMA的实际应用仅限于1000℃以下，即NiTi二元系和Cu2+系的相变温度(TT)极限，这是迄今为止最成功的SMA。近年来，基于CoNiAl/Ga系统的SMA显示出了良好的特性，如通过成分调整实现了较宽的TT控制范围，用于NiTiX高温SMA的Pd、Pt和Au组分比Pd、Pt和Au更便宜，更高的热稳定性，以及对氧/碳的亲和力较小。此外，第三个硬质有序相通常与可变形相处于平衡状态，这为形成纳米复合材料以改善此类合金的循环稳定性、蠕变敏感性和形状记忆(SM)性能提供了可能性。目前工作的最终目标是通过计算/实验相结合的方法来开发CoNiGa高温超导材料。其具体目标是：1)通过计算和实验优化高对称性可转换相的成分，使TTS最大化；2)确定相平衡，以找到找到三个相体系的最佳成分和温度；3)通过预测/实验确定的第二相的析出/溶解动力学，设计热处理制度；4)制备和表征单晶的SM和PE响应，作为第二相的分布和数量以及晶体取向的函数；5)探索通过约束时效选择硬纳米沉淀物变体及其对SM性能的循环稳定性和蠕变的影响；以及6)在韧性第二相的存在下，多晶CoNiGa高温超导合金的成形性得到了改善。这一变革性研究的科学价值是：(A)通过改变目前昂贵、不稳定和脆性的材料，转向具有高热稳定性、低蠕变、老化和循环损伤的敏感性以及良好的延展性以改善交通、航空航天和公用事业应用的多晶的成形性，从而使高温SMA领域发生革命性变化的潜力。(B)通过协同实验计算方法改变新的HSMAs的设计过程；(C)创建具有非常高的温度(500？]7000C)SM和PE性能的全新的形状记忆高温合金家族。非技术性：这些活动的广泛影响体现在以下领域：(A)尽管最近在海外开展了大量关于SMA的工作，但美国仍在活性材料研究方面保持领先地位；(B)开发教学模块，将其纳入本科课程；(C)帮助K-12年级的学生与SMA一起开发科学项目；(D)与新的IGERT和纳米材料证书方案协调，丰富研究生和本科生的研究经验；(E)开发计算材料科学研究生课程；(E)让代表性不足的群体参与；(F)通过专题介绍、出版物和网站向学术界和工业界传播所产生的知识，以及(G)与工业界密切合作。

TECHNICAL: Shape memory alloys (SMAs) are an important class of smart materials that can produce large recoverable shape changes as a result of reversible martensitic phase transformations, which can be triggered by changes in temperature, stress or magnetic field. Currently, practical uses for SMAs are limited to temperatures below 1000 C, i.e. the transformation temperature (TT) limits of NiTi binary and Cu?]based systems, the most successful SMAs to date. Recently, SMAs based on the CoNiAl/Ga systems have shown promising characteristics, such as broad range of TT control through compositional adjustments, cheaper constituents than Pd, Pt, and Au used in NiTiX high temperature SMAs (HTSMAs), higher thermal stability, and less affinity to oxygen/carbon. Moreover, the fact that a third, hard, ordered phase is often at equilibrium with the transformable phases, offers the possibility of forming nanocomposites to improve the cyclic stability, creep susceptibility, and shape memory (SM) properties of such alloys. The ultimate goal of the current work is to develop CoNiGa HTSMAs through a combined computational/experimental approach. The specific objectives are to: 1) maximize TTs through computational/experimental composition optimization of the high symmetry transformable phase; 2) determine the phase equilibria to find optimal compositions and temperatures at which three?]phase systems can be found; 3) design heat treatment schedules through predicted/experimentally determined kinetics of the precipitation/dissolution of secondary phases; 4) fabricate and characterize the SM and PE responses of single crystals as a function of distribution and amount of secondary phases and crystallographic orientation; 5) explore the selection of hard nanoprecipitate variants through constrained aging and their effects on cyclic stability and creep of SM properties; and 6) demonstrate improved formability of polycrystalline CoNiGa HTSMAs in the presence of ductile secondary phases. The scientific merits of this transformative research are: (a) the potential of revolutionizing the field of HTSMAs, by moving away from current expensive, unstable and brittle materials to HTSMAs possessing high thermal stability, low susceptibility to creep, aging and cyclic damage as well as good ductility to improve the formability of polycrystals for transportation, aerospace, and public utility applications.; (b) transformation of the design process for new HTSMAs through the synergistic experimental computational approach; (c) the creation of an entirely new family of SMAs, i.e. shape memory superalloys, with very high temperature (500?]7000C) SM and PE properties. NON-TECHNICAL: The broader impacts of the activities are reflected in the following areas: (a) maintaining US leading role in active materials research despite the vast amount of recent works at overseas on SMAs; (b) development of teaching modules for incorporation into undergraduate courses; (c) helping K-12 students in developing science projects with SMAs; (d) enriched graduate and undergraduate research experiences in coordination with the new IGERT and Nanomaterials certificate programs; (e) development of a graduate course in computational materials science; (e) involvement of underrepresented groups; (f) disseminating the knowledge generated to both academia and industry through the presentations, publications, and a website, and (g) close collaboration with industry.