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 AE 5 MPa ffiffiffiffi m p 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.

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The shape memory properties of Fe-Mn-Si-based alloys containing Nb and C are further improved by pre-rolling of the solutiontreated austenite and the subsequent ageing treatment. For an Fe-28Mn-6Si-5Cr-0.53Nb-0.06C (mass%) alloy, 90% of an initial 4% strain is recovered on heating without any previous training treatment, if the alloy in austenitic state is rolled by 6-14% at 870 K and aged at 1070 K for 10 min to produce NbC precipitates. In the same condition, the alloy develops shape recovery stresses of 255 MPa and 295 MPa for 6% and 14% pre-rolling, respectively, when initially deformed by 4.5%. TEM observations indicate that these improved shape memory characteristics are related to a fine distribution of NbC precipitates in the fcc matrix and their interaction with stacking faults.

We examined the texture evolution in a superelastic Ni 50.7 Ti 49.3 (numbers indicate at.%) alloy under applied uniaxial stress using high-energy synchrotron X-ray diffraction in transmission geometry. Texture information is identified from the intensity variations along Debye-Scherrer rings recorded on area detector diffraction images. The 1 1 0 A austenite plane normals are aligned in the rolling direction and 2 0 0 A is in the transverse direction. Due to the B2-B19 lattice correspondence, the 1 1 0 A peak splits into four martensite peaks 0 2 0 M ,1 1 1 M , 0 0 2 M and 1 1 1 M . The stress-induced martensite is strongly textured from twin variant selection in the stress field with 0 2 0 M aligned in the loading direction while the maxima corresponding to1 1 1 M , 0 0 2 M and 1 1 1 M are at 60 • , 67 • and 75 • from the loading direction. (B19 unit cell setting: a = 2.87Å, b = 4.59Å, c = 4.1Å, ␥ = 96.2 • ). A comparison between the experimental and recalculated distribution densities for the polycrystalline NiTi shows a reasonable agreement. In addition, we compare our experimental results with a micromechanical model which is based on total energy minimization. In this case, we also observe an overall agreement.