Making Martensitic Transformation Continuous for Controlled Strain Release

Like dislocation slip and deformation twinning, martensitic transformation (MT) is accompanied by autocatalysis and strain avalanche due to long-range elastic interactions. In this presentation, we will give an overview of alloy design strategies to convert a typical sharp first-order MT in shape memory alloys (SMAs) into a high-order like continuous transformation and, correspondingly, convert strain avalanche into a progressive strain release. We will start with point defect engineering that converts MT into a strain glass transition. We then demonstrate how to engineer nanoscale concentration modulations by spinodal decomposition, film deposition, or dissolution of nanoprecipitates to create a phase stability modulation that offers progressive strain release upon loading or cooling. We finally show how to utilize dislocations, deformation twin boundaries, and nanoprecipitates to regulate MTs for controlled strain release. A precisely controlled strain release will allow one to achieve unique properties including linear superelastic with nearly vanish hysteresis, ultralow modulus, Invar and Elinvar anomalies, and progressive TRIP effect + excellent work-hardenability + prolonged uniform elongation for structural applications. This work is supported by NSF and DOE.