Understanding the interplay of precipitates and dislocations on the reversible martensitic transformation in cyclically actuated NiTiHf shape memory alloys

非技术性SUMMARYShape记忆合金(SMA)可以像人造肌肉一样，通过一种称为马氏体相变的过程在冷却和加热时收缩和伸展。与肌肉相似，形状记忆合金在一系列操作周期后会出现疲劳。形状记忆合金的疲劳被触发，然后被称为位错的超小晶体缺陷的产生和积累而加剧。最近的工作表明，通过加入纳米颗粒可以显著提高SMA的疲劳寿命。本项目的目的是通过使用先进的原位电子显微镜实时观察纳米颗粒与缺陷的相互作用，为这些纳米颗粒如何影响这些SMA的疲劳提供基本的了解。这些见解将加速抗疲劳和耐用的超弹性SMA的发展，这些SMA可能有益于影响各种关键技术，包括航空航天、能源转换、生物医学、国防和交通运输。该项目还将通过路易斯·斯托克斯少数民族参与联盟计划为代表不足的学生提供多学科的STEM教育和职业发展机会，以及基于这项研究的在线STEM课程材料。技术总结本项目的目标是对热机械诱导的可逆马氏体相变过程中纳米沉淀、位错和马氏体相变之间的相互作用有一个基本的了解。所获得的新知识将阐明它们的相互作用如何控制循环驱动高温形状记忆合金(SMA)的疲劳性能(包括功能和结构)。中心假设是纳米沉淀物将抑制热循环过程中相变诱导的位错增殖、滑移和重排，从而显著改变晶级组织，从而显著改善疲劳性能。将进行的实验任务包括：1)确定析出相特征、相关的弹性应变和局部化学，2)确定析出相对相变诱导位错产生和组织演变的影响，3)评估析出-位错交互作用对奥氏体和马氏体微结构演变和由此产生的功能疲劳的影响，以及4)确定析出-位错交互作用在循环驱动形状记忆合金持久滑移带形成和裂纹萌生和扩展(结构疲劳)中的作用。实验结果将被用来详细描述含沉淀SMAS中的循环可逆马氏体相变和疲劳响应。该奖项反映了NSF的法定使命，并已通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYShape memory alloys (SMAs) can contract and extend, like an artificial muscle, upon cooling and heating through a process called martensitic transformation. Similar to muscles, shape memory alloys experience fatigue after a number of operation cycles. The fatigue of shape memory alloys is triggered and then exacerbated by the creation and accumulation of ultra-small crystalline defects called dislocations. Recent work has shown that the fatigue life of the SMAs can be dramatically improved by incorporating nano-sized particles. The purpose of this project is to provide fundamental understanding of how these nano-particles influence the fatigue of these SMAs by using advanced and in situ electron microscopy to watch the interactions of the nanoparticles with the defects in real time. These insights will accelerate the development of fatigue-resistant and durable super-elastic SMAs that may beneficially impact a wide variety of critical technologies, including aerospace, energy conversion, biomedical, defense, and transportation. This project will also provide multidisciplinary STEM educational and career advancement opportunities for underrepresented students through the Louis Stokes Alliance for Minority Participation program, as well as online STEM course material based on this research.TECHNICAL SUMMARYThe goal of the project is to develop a fundamental understanding of the interplay between nanoprecipitates, dislocations, and martensitic transformation, during thermo-mechanically induced, reversible martensitic transformation. The new knowledge gained will elucidate how their interactions control fatigue performance (both functional and structural) in cyclically actuated high-temperature shape memory alloys (SMAs). The central hypothesis is that the nanoprecipitates will suppress the transformation-induced dislocation multiplication, glide, and rearrangement during thermal cycling, thus dramatically modifying granular-level microstructure, and consequently the fatigue performance. The experimental tasks to be undertaken include: 1) identify precipitate characteristics, associated elastic strain, and local chemistry, 2) determine the effect of precipitates on the transformation-induced dislocation generation and structure evolution, 3) assess the effect of precipitate-dislocation interactions on the austenite and martensite microstructure evolution and the resultant functional fatigue, and 4) identify the role of precipitate-dislocation interaction on persistent slip band formation and crack initiation and growth (structural fatigue) in cyclically actuated SMAs. The experimental results will be used to develop a detailed description of the cyclic reversible martensitic transformation and fatigue responses in precipitate-bearing SMAs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.