Seminar Schedule – Spring 2019
Monday, January 14, 2019
Time: 3:30pm – 5:00pm
Place: TBD
3D-XRD Study of Martensite Band Front in Superelastic NiTi Wire
Petr Sittner, Institute of Physics, Prague, Czech Republic
Macroscopic interfaces between the deformed and undeformed material appearing on the front of deformation bands propagating in solids (Luders bands) always attracted attention of mechanical engineers as well as theoreticians involved in modelling of material deformation. As the strain compatibility has to be maintained at these highly mobile interfaces extending over thousands of polycrystal grains, there have to be sharp gradients of stresses and strains. Experimental analysis is difficult since the macroscopic interface is a 3D object buried in the bulk which disappears on unloading, or at least the stress field around it dramatically changes, as for example in the case of the martensite band front propagating during superelastic tensile deformation of NiTi shape memory alloy wire [1,2]. The stress state in the material around the interface matters – the interface cannot exist without it. Looking from outside by the DIC method [3,4], the martensite band front in stretched NiTi wire appears to be rather broad and perpendicular to the wire axis. Since conventional microscopic methods could not be employed to investigate this buried macroscopic interface, its nature remained relatively unknown for decades.
In this talk, I will present the results of the detailed analysis of grain resolved internal strains, stresses and phase fractions around the martensite band front propagating in 0.1 mm thin NiTi wire we managed to obtain through the analysis of 3D x-ray diffraction /3D-XRD/ experiment at the ID11 beamline [4]. Particularly, we were able to determine full strain and stress tensors in ~15000 polycrystal grains of defined position, size and crystal lattice orientation (mean grain size 5.9 microns) within the martensite band front stabilized by applied tensile stress of ~420 MPa [4]. To derive macroscopic internal stress state in the wire, we performed a scale transitions towards continuum. In this way, we were able to resolve macroscopic internal stress fields surrounding the front with 1 micron spatial resolution and 20MPa stress resolution.
It was found that the martensite band front has a shape of a buried nose cone surrounded by internal stress field. The local stresses in grains ahead of the advancing front redistribute in such a way that the grains located at the interface experience ~200 MPa higher effective stresses compared to the grains located far from the interface. Consequently, those grains transform collectively, while very little is happening elsewhere in the wire. The superelastic deformation of NiTi wire was also simulated by a finite element implemented constitutive SMA model adapted for nonlocal effects [5]. The moving martensite band front was reconstructed [4] confirming its cone shape as well as the sharp internal stress gradient around it. For related videos and x-ray data for downloading see http://ofm.fzu.cz/.
References
- J. A. Shaw, Simulations of localized thermo-mechanical behavior in a NiTi shape memory alloy, Int. J. Plast. 16, 541�562 (2000).
- P. �ittner, Y. Liu, et al., On the origin of L�ders-like deformation of NiTi shape memory alloys, J. Mech. Phys. Solids 53, 1719�1746 (2005)
- L. Dong, R. H. Zhou et al. On interfacial energy of macroscopic domains in polycrystalline NiTi shape memory alloys, Int. J. Solids Struct. 80, 445�455 (2016).
- P. Sedm�k, P. �ittner et al Grain-resolved analysis of localized deformation in nickel-titanium wire under tensile load, Science 353, 559-562 (2016)
- P. Sedl�k, M. Frost, et al. Thermomechanical model for NiTi-based shape memory alloys including R-phase and material anisotropy under multi-axial loadings Int. J. Plast. 39, 132�151 (2012)
For further information please contact Dr. Stelios Kyriakides at skk@austin.utexas.edu or (512) 471-4167.
