Severely plastically deformed Ti‐Ni‐Pd high‐temperature shape memory alloys studied by TEM

Abstract

High‐temperature shape memory alloys (HTSMA) have a broad range of prospective applications [1]. Grain refinement can substantially improve the material strenght and cyclic transformation stability [2]. It is the aim of the present work to study the structural evolution and phase transformations of a Ti 50 Ni 25 Pd 25 HTSMA severely plastically deformed by high pressure torsion (HPT) using transmission electron microscopy (TEM) methods. A Ti 50 Ni 25 Pd 25 alloy prepared by arc melting was homogenized, solution treated, and water quenched. Disc shaped samples taken from the alloy were subjected to HPT (using a pressure of 6 GPa and 40 turns) carried out at room temperature (RT). HPT deformed samples were also subjected to isochronal heating (IH) to a temperature of 460°C followed by cooling to RT. TEM specimens were prepared by ion polishing (Gatan PIPS II) and analyzed in a Philips CM 200 transmission electron microscope. The initial coarse grained TiNiPd HTSMA is fully martensitic since the thermally induced transformation from the cubic B2 austenite to orthorhombic B19 martensite occurs well above RT (the martensite finish temperature is ~ 130°C). Therefore, the HPT is applied to the martensitic state of the initial sample that is showing a self‐accommodated morphology of twinned martensite (cf. Fig. 1). After the HPT, a complex mixture of elongated crystals and amorphous bands has formed that have an average width of 40 and 15 nm, respectively. The thinnest amorphous bands have a width of about 5 nm only. While TEM bright field images hardly facilitate analysis of the deformation microstructure (cf. Fig. 2a), strong differences in scattering of the crystalline and amorphous phase (cf. the selected area diffraction pattern (SADP) of Fig. 2b) can be used to separately image these phases in dark field images: Fig. 2c was taken with strong diffraction spots (cf. C in Fig. 2b) of crystallites closely orientated to a Bragg‐condition. In this case, the dark field intensity strongly depends on the specimen tilt and the position of the objective aperture. Since the lattice reflections form arc shaped segments in the SADP, it is concluded that the most of the crystals have rather similar orientation with respect to each other (the average rotational misorientation is about 20°). Most of the reflections correspond to B19 martensite. Weak reflections of the B2 austenite also arise. Fig. 2d was taken by allowing diffusely diffracted intensity of the amorphous phase to pass the objective aperture that is not superimposed by strong crystalline diffraction spots (cf. A in Fig. 2b; the maximum of the diffusely diffracted intensity was used occurring on a diffraction ring with a radius of about 4.6 nm ‐1 ). As expected, the dark field contrast of the amorphous bands is rather uniform and hardly depends on both the tilt of the specimen and the position of the objective aperture along the amorphous diffraction ring (as long as it is not superimposed by crystalline reflections). The present observations of a lamellar structure of elongated crystallites separated by thin amorphous bands indicates that the amorphization might preferentially occur at martensitic twin boundaries (cf. Fig. 1a) [3,4]. In samples subjected to IH after HPT, an ultrafine grained structure is formed by crystallization of the amorphous phase, as well as by recovery and grain growth that occurs in the austenitic state. During cooling to RT, the forward B2 to B19 transformation occurs in the ultrafine grains that frequently contain a twinned morphology of the martensite (cf. Fig. 3a and b). However, as compared to the case of coarse grains, the forward transformation is hindered (i.e. shifted to lower temperatures and incomplete) yielding retained austenite in some of the grains (cf. Fig. 3c).