Young, M. L.; Wagner, M. F. -X.; Frenzel, J.; Schmahl, Wolfgang W.; Eggeler, Gunther
Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading.
In: Acta Materialia, Vol. 58, Nr. 7: S. 2344-2354
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An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with50.9 at.\% Ni was examined using synchrotron X-ray diffraction during insitu uniaxial tensile loading (up to 1 GPa) and unloading. Bothmacroscopic stress strain measurements and volume-averaged latticestrains are reported and discussed. The loading behavior is described interms of elasto-plastic deformation of austenite, emergence of R phase,stress-induced martensitic transformation, and elasto-plasticdeformation, grain reorientation and detwinning of martensite. Theunloading behavior is described in terms of stress relaxation andreverse plasticity of martensite, reverse transformation of martensiteto austenite due to stress relaxation, and stress relaxation ofaustenite. Microscopically, lattice strains in various crystallographicdirections in the austenitic B2, martensitic R, and martensitic B19’phases are examined during loading and unloading. It is shown that thephase transformation occurs in a localized manner along the gage lengthat the plateau stress. Phase volume fractions and lattice strains invarious crystallographic reflections in the austenite and martensitephases are examined over two transition regions between austenite andmartensite, which have a width on the order of the wire diameter.Anisotropic effects observed in various crystallographic reflections ofthe austenitic phase are also discussed. The results contribute to abetter understanding of the tensile loading behavior, bothmacroscopically and microscopically, of NiTi shape-memory alloys.