Fracture Mechanics and Microstructure in NiTi Shape Memory Alloys

S. GOLLERTHAN; M. L. YOUNG; A. BARUJ; J. FRENZEL; W. W. SCHMAHL; G. EGGELER

ACTA MATERIALIA

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2009 vol. 57 p. 1015 - 1025

1359-6454

Crack extension under static loading in pseudoplastic and pseudoelastic binary NiTi shape memory alloy (SMA) compact tension (CT) specimens was examined. Two material compositions of 50.3 at.% Ni (martensitic/pseudoplastic) and 50.7 at.% Ni (austenitic/pseudoelastic) were investigated. The SMAs were characterized using differential scanning calorimetry to identify the phase transformation temperatures and tensile testing to characterize the stress–strain behavior. A miniature CT specimen was developed, which yields reliable critical fracture mechanics parameters. At 295 K, cracks propagate at similar stress intensities of 30 +- 5 MPa SQR(m) into martensite and pseudoelastic austenite. Integrating the miniature CT specimen into a small test device which can be fitted into a scanning electron microscope shows that this is due to cracks propagating into regions of detwinned martensite in both materials. Investigating a pseudo-elastic miniature CT specimen in a synchrotron beam proves that martensite forms in front of the crack in the center of the CT specimen, i.e. under plane strain conditions.