Stress induced martensitic transformation in CuAlZnMn polycrystals investigated by in situ neutron diffraction

Abstract

Abstract. The results of in-situ neutron diffraction investigation of stress induced martensitic transformation in CuAIZnMn shape memory alloy polycrystal are reported. Time-of-flight diffraction spectra were measured during temporary stopovers along two successive tensile pseudoelastic cycles. The integral intensities, positions and widths of multiple individual austenite and martensite ^-reflections from the recorded spectra were evaluated by single peak fits. The evaluated peak profile parameters were used to calculate the lattice strains in the austenite phase and volume fractions of the austenite phase evolving during the pseudoelastic cycles. The obtained /nW-lattice plane responses were related to the elastic anisotropy of the cubic austenite phase and to the transformation anisotropy of the pi->P*i martensitic transformation in CuAIZnMn using a selfconsistent crystallographic model of SMA polycrystals. The mechanism of the load partition among polycrytal grains has been discussed and claimed to be a major obstacle for the stress driven transformability of the polycrystalline Cu-based SMAs It has been found that significant redistribution of stress and large intergranular stresses but no strong transformation induced texture should be expected in Cu based SMA polycrystals transforming pseudoelastically in tension. 1.INTRODUCTIONWhen shape memory alloy /SMA/ polycrystal undergoes stress induced martensitic transformation /SIMT/, the compatibility of strains and continuity of stress across grain boundaries must be fulfilled. Since SMAs are often elastically anisotropic and the SIMT is also a strongly anisotropic process [1,2], it might be in fact very difficult for some SMA polycrystals to sustain larger recoverable strains in polycrystalline state [3], even if their theoretical crystallographic transformation strains are of the order of 10%. We are interested in this paper in the stress redistribution among grains of SMA polycrystal undergoing SIMT in tensile pseudoelastic cycles. Recent developments of in-situ neutron diffraction technique of strain measurement have allowed to study this experimentally as we will demonstrate in this paper.Due to the large penetration depths of thermal neutrons, neutron diffraction is a suitable tool for nondestructive in-situ measurement of elastic strains (and evaluation of stresses) within the bulk of large polycrystal specimens. Diffraction strain measurements rely on the precise measurement of the duaspacing of particularly oriented crystal planes. Knowing the stress free lattice spacing do,hki,, the lattice strain E h u can be calculated according to equation (1).(1)The diffraction evaluated strains are: i) elastic strains only, ii) determined selectively only from those grains of the specimen which are suitably oriented, iii) averaged strain values over those grains, and iv) generally only one component of the elastic strain tensor can be evaluated from a diffraction experiment with a single specimen orientation with respect to the scattering vector. The lattice str...

A study of stress-induced phase transformation and micromechanical behavior of CuZr-based alloy by in-situ neutron diffraction

“…There is an obvious pseudoelastic behavior where the total strain is up to -6.8% beyond the 2% elastic strain limit. This behavior was observed in most of the shape memory alloys [59][60][61][62][63][64]. After unloading in the 2 nd cycle, the residual strain increases from ~-1.4% (in the 1 st cycle) to ~-3.3%.…”

“…The deformation mechanism of the studied alloy was analyzed by the lattice strain (peak positions), full widths at half maximum (FWHM-peak widths) and normalized intensities of the B2 and martensite phases obtained from the measured diffraction patterns [45,53,[59][60][61][62][63]. The lattice strain Ɛ hkl for the different {hkl} lattice planes of the studied phases was calculated by Eq.…”

“…where d hkl is the lattice spacing and d 0,hkl is the lattice spacing under stress-free state [18,[59][60][61][62][63].…”

“…where I hkl and I 0,hkl are the integrated intensities of the different B2 lattice planes under the compressive load and zero load, respectively [59][60][61]. The normalized integrated intensities in the equally oriented hkl austenite or martensite grain families of the austenite or martensite phase scale with the volume fraction variation of the B2 or martensite phase [60].…”

“…In the 1 st cycle, before the appearance of the martensite phase where the stress level is below ~-298 MPa, the B2 phase exhibits an elastic anisotropic behavior where the elastic diffraction constants E hkl are different [45,[61][62]68] [ Fig. 5(a)].…”