Fracture mechanics and microstructure in NiTi shape memory alloys

• Published in: Acta materialia Vol. 57; no. 4; pp. 1015 - 1025
• Main Authors: Gollerthan, S., Young, M.L., Baruj, A., Frenzel, J., Schmahl, W.W., Eggeler, G.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-02-2009
• Subjects: Fracture mechanics; Intermetallic compounds; Intermetallics; Martensite; Martensitic transformations; Mechanical properties testing; Miniature; Nickel titanides; Scanning electron microscopy; Shape memory alloys; Shape memory alloys (SMA); Stress-induced martensite; Tension test; Stress-induced martensite; Mechanical properties testing; Tension test; Shape memory alloys (SMA)
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2008.10.055

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
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 pseudoelastic 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.