PSI - Issue 33

V. Di Cocco et al. / Procedia Structural Integrity 33 (2021) 1035–1041 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Natali proposed a new model to determine the phase transition in a NiTi alloy, then it was verified through experimental tests. In the present work an integrated model which is able to predict the microstructure evolution and the mechanical properties of a shape memory alloy characterized by a pseudoelastic behaviour and able to predict the effect of hysteresis and the effect of cycling both in terms of microstructure evolution and mechanical behaviour in reversible range of stresses is proposed. 2. Materials and methods An equiatomic pseudo-elastic NiTi alloy which exibit a pseudo elastic behaviour was made in the laboratory by using a vacum fournace. The fournace is able to eleminate the oxigen by using an initial fully nitrogen atmosphere, prior to generate a the vacuum conditions. The casting is generated by a centrifugal force wich pushes the alloy from crucible (made on Yttria) to a casting mold made on graphite. Specimens, characterized by a uniaxial flat dog bone shape, have been machined by using an electroerosion technique. The dimensions of specimens and the tools used to cyciling tests allows to undergo the specimens in the X-Ray chamber of diffractogram machine in load conditions, in order to evaluate the X-Ray spectra. XRD tests were carried out by using a Philips X-PERT diffractometer equipped with a vertical Bragg–Brentano powder goniometer. A step–scan mode was used in the 2θ range from 41° to 45° with a step width of 0.02° and a counting time of 2 s per step. The employed radiation was monochromated CuKα (40 kV – 40 mA). The calculation of theoretical diffractograms and the generation of structure models were performed using the PowderCell software. The loop experiment is carried out controlling the displacement of the machine croos-head, form the initial positions (zero point) up to an epsilon gross εg=10% corrisponding to 1.6mm of cross-head displacement and inverting the cross head displacement to undeformed condition. Firstly a initial diffraction has carried out on the unloaded material in order to evaluate the correct initial austenitic microstructure, checking the presence of austenite.Then, the loop is performed step by step, holding in the stepping deformation in order to evaluate the difraction spectra. In Table 1 the investigated steps are shown.

Table 1. Investigated steps (ε ENG in percent)

Engineering deformation in unloading condition ε ENG [%]

Engineering deformation in loding condition ε ENG [%]

0.0

--

2.50 3.33 4.16 5.00 5.83 6.66 7.49 8.32 9.15

9.15 8.32 7.49 6.66 5.83 5.00 416 3.33 2.50 0.00

10.00

For each loading step, the loading frame containing the specimen was removed from the testing machine, under fixed deformation, and both XRD measurements.