Abnormal Grain Growth and Single Crystals in Multicomponent Shape-Memory Alloys

Single crystals of shape-memory alloys (SMAs) usually exhibit prior functional properties, i.e., superelasticity (SE) and shape-memory effect (SME), compared to their polycrystalline counterparts. Thus, it is important and meaningful to produce SMA single crystals with proper techniques that are free from limitation of cost and dimension. Here, a review of recent novel techniques of abnormal grain growth (AGG) through cyclic heat treatment (CHT) and direct annealing (DA) for single-crystal preparation is presented. The driving force of AGG process in CHT is the repeatedly formed subgrain energy, which is of a dynamic nature that combines the effect of internal-stress plasticity and subgrain boundary formation/elimination. Similarly, a driving force by misorientation gradient due to the nanoprecipitates allows AGG occur significantly, by which single crystals can form through DA process. Critical factors such as volume fraction of nanoprecipitate, annealing temperature, and details of functional properties should be also discussed. This review is expected to provide motivation for AGG technology and promote the production and application of SMA single crystals.