The role of intergranular constraint on the stress-induced martensitic transformation in textured polycrystalline NiTi

The in ̄uences of grain boundaries and relative grain misorientations on stress-induced martensitic transformations in NiTi are studied using unique experiments and ®nite element modeling. Tensile and compressive mechanical tests reveal that polycrystalline NiTi with a dominant <111> ®ber texture and single crystal NiTi oriented along the [111] direction exhibit nearly identical stress±strain curves during a stress-induced martensitic transformation. Micro-mechanical ®nite element simulations of ®ber textured polycrystals and single crystals undergoing a multi-variant martensitic transformation con®rm the relative indi€erence of the macroscopic transformation attributes to the presence of grain boundaries. On the microscale, the ®nite element simulations further reveal that the insensitivity of the transformation to intergranular constraint is linked to the local stress disturbance created by transforming grains. The transformation of grains that are favorably oriented with respect to the loading axis creates local stresses that invariably assist the transformation in neighboring grains, e€ectively lowering the in ̄uence of grain misorientations and boundaries on the macroscopic transformation behavior. # 2000 Elsevier Science Ltd. All rights reserved.