Cyclic deformation mechanisms in precipitated NiTi shape memory alloys

• Published in: Acta materialia Vol. 50; no. 18; pp. 4643 - 4657
• Main Authors: Gall, K, Maier, H.J
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 28.10.2002; Elsevier Science
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Dislocations; Exact sciences and technology; Fatigue; Martensite; Martensitic transformations; Materials science; Metals. Metallurgy; Microstructure and cyclic stress–strain response; NiTi shape memory alloys; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Precipitation; Single crystals; Transmission electron microscopy; Microstructure and cyclic stress–strain response; Single crystals; Precipitation; Transmission electron microscopy; Fatigue; NiTi shape memory alloys; Martensite; Dislocations; Nickel base alloys; Intermetallic compounds; Cyclic loads; Martensitic transformations; Experimental study; Stress-strain relations; Transformation temperature; Binary alloys; Titanium alloys; Monocrystals; Shape memory alloy; Cyclic deformation; Microstructure
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/S1359-6454(02)00315-4

Results are presented on the cyclic deformation of single crystal NiTi containing Ti 3Ni 4 precipitates of various sizes. Mechanical cycling experiments reveal that the cyclic degradation resistance of NiTi is strongly dependent on crystallographic orientation. Under compression, orientations approaching the [100] pole of the stereographic triangle possess the highest fatigue resistance. Orientations approaching the [111] pole of the stereographic triangle demonstrate the lowest fatigue resistance. Aging to produce small coherent Ti 3Ni 4 precipitates (10 nm) improves the fatigue resistance of NiTi compared to the other heat treatments (solutionized or overaged) for nearly all orientations. NiTi with 10 nm Ti 3Ni 4 precipitates consistently showed stabilized martensite due to mechanical cycling, and an absence of dislocation activity. Samples with large incoherent Ti 3Ni 4 precipitates (500 nm) consistently showed significant dislocation activity due to mechanical cycling in addition to stabilized martensite colonies. The first cycle stress–strain hysteresis was found to correlate to the fatigue resistance of the material. Samples demonstrating large inherent hysteresis, with different heat treatments and orientations, showed poor fatigue performance. Rational for the observed behaviors is discussed in terms of operant deformation mechanisms and ramifications on modeling the cyclic deformation of NiTi are presented.