In-situ XRD Analysis of Nitinol under Tensile Stress

Martin Dudr¹, Jan Drahokoupil^1,2, Luděk Heller³

¹Department of Solid State Engineering, FNSPE, CTU in Prague, Břehová 7, 115 19 Prague

²Department of Advanced Structural Materials, Institute of Physics, CAS, Na Slovance 1999/2, 182 21 Praha 8

³Department of Functional Materials, Institute of Physics, CAS, Na Slovance 1999/2, 182 21 Praha 8

dudrmart@fjfi.cvut.cz

Shape Memory Alloys are intermetallic alloys possessing both the properties of shape memory and superelasticity, which are bound together by a common cause – diffusionless phase transformation called the martensitic transformation. Shape memory effect is to be observed as a recovery of shape after apparent permanent deformation (executed below certain temperature) when heated above certain temperature. Superelasticity (also called pseudoplasticity) allows the material to be apparently elastically deformed in an extent of units of per cent in an environment of sufficiently high temperature.

In the case of nitinol (see below) the martensitic tranformation can be thermally-induced or stress-induced. Parent phase of martensitic transformation (also called austenite) is stable at higher temperatures (lower stresses) and possesses higher symmetry than the daughter phase (so called martensite), which is stable at lower temperatures (higher stresses).

Nitinol is a commercial term for a shape memory alloy based on Ni-Ti in near-equiatomic ratio, commonly comprised also of various other components in minor amounts destined to enhance required thermo-mechanical properties of a given alloy. Its austenitic phase possesses cubic B2 structure (also termed as CsCl structure - space-centered cubic with atoms of one kind in vertices and of the other in the centre) with space group Pm3m. The martensite possesses monoclinic structure B19’ with space group P2₁m.

A series of XRD measurements during different tensile loads on nitinol wire of diameter 0.15 mm were carried out at PANalytical diffractometer at Institute of Physics, CAS equipped with Co anode. A non-standard setting with parabolic mirror in primary beam and X´Celerator detector in diffracted beam was used for reason of intensity.  The wire was in the room temperature in the austenitic phase and was installed in a small tensioning device in the axis of goniometer. The tensioning device enabled to measure current tensile stress and manually set the prolongation. The measurements covered tensile stresses corresponding to austenite, the transformation and martensite, with several measurements being carried out in each phase. However, only the data for austenite were analysed and are reported in this paper (8 measurements between 20 – 586 MPa). An example of measured profile (200 MPa) for austenite is in Figure 1, for martensite (1015 MPa) in Figure 2.

The precise alignment of the sample in diffractometer was complicated by limited possibilities of homemade manipulator of tensioning device.

TOPAS software was used for the data analysis. The Rietveld analysis was supplemented by single line fitting. Rietveld analysis provided: the lattice parameter a₀, volume weighted crystallite size L_VOL, and microstrains e₀.The single line fitting was testing to analyse the crystallographic dependence of studied parameters. The Figure 3 shows the development of lattice parameter obtained by Rietveld refinement of austenitic phase as function of tensile stress.

Figure 1. Measured profile for NiTi austenite under tensile stress 200 MPa	Figure 2. Measured profile for NiTi martensite under tensile stress 1015 MPa

Figure 3. Development of lattice parameter a0 during tensile loading