Grain size dependence of fracture toughness and crack-growth resistance of superelastic NiTi

The grain size (GS) dependence of fracture toughness (K-IC) and static crack-growth resistance (K-R) of superelastic NiTi with average GS from 10 to 1500 nm are investigated. The measurements of strain and temperature fields at the crack-tip region are synchronized with the force-displacement curves under mode-I crack opening tests. It is found that with GS reduction down to nanoscale, the K-IC and the size of crack-tip phase transformation zone monotonically decrease and the K-R changes from a rising to a flat R-curve. The roles of intrinsic and extrinsic toughening mechanisms in the GS dependence of the fracture process are discussed. (C) 2015 Elsevier Ltd. All rights reserved.