Formation of B19′, B2, and amorphous phases during mechano-synthesis of nanocrystalline NiTi intermetallics

Abstract

Ni-50Ti shape memory alloy was synthesized by mechanical alloying of the elemental powders mixture under an argon gas atmosphere. The structural and microstructural properties of the alloyed powders were evaluated by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Moreover, the Vickers microhardness of the powders was estimated at different milling times. According to the results, by milling progression, the amount of the amorphous phase increased considerably and after sufficient milling time (48 h), the mechanocrystallization of the amorphous phase into the more stable crystalline phases (i.e. B2 and B19′) occurred. It was found that the particles size and microhardness were significantly affected by the formation of the amorphous, B2, and B19′ phases. It was also deduced that, by appropriate heating and cooling cycles, the B2 and thermally-induced B19′ phases can be created. Furthermore, it was inferred that the formation of undesirable intermetallic phases (particularly NiTi 2 ) during the heating cycle was considerably reduced by milling time evolution.

Microstructure Characterization of Nanocrystalline Ni50Ti50 Alloy Prepared Via Mechanical Alloying Method Using the Rietveld Refinement Method Applied to the X-Ray Diffraction

“…The martensitic and austenitic phases were formed at the milling times of 24 h and 72 h due to severe plastic deformation. This is similar to that reported for the Ni-50Ti alloy [17].…”

“…Consequently, the size of particles is appreciably reduced, and their morphology tends to be semi-spherical with a narrow range of a size distribution (Fig. 1, c) [17].…”

“…Among these, the MA method was very effective for the fabrication of Ni 50 Ti 50 nanocrystalline alloys or nanostructured powders. In this technique, high-energy collisions between the milled powder particles, balls, and vial walls generate a large variety of materials such as a nanocrystalline solid solution amorphous phase [16], the NiTi-austenite (B2) and NiTi-martensite (B19) phases [17].…”

Influence of Milling Time on Structural and Microstructural Parameters of Ni50Ti50 Prepared by Mechanical Alloying Using Rietveld Analysis

“…After 48 h, the fraction of the amorphous phase decreases due to its crystallization into more stable crystalline phases (NiTi-austenite and NiTi-martensite), which may be attributed to strain energy and the temperature increase during MA due to severe plastic deformations as reported by Amini et al [19]. However, the amorphous phase is predominant with a relative proportion of about 89.23%.…”

“…Because of the very fine grain size, nanocrystalline materials exhibit diverse and interesting properties, which are different and often considerably improved in comparison with conventional coarse-grained polycrystalline materials [18]. MA of Ni 50 Ti 50 elemental powders mixture can lead to amorphous phase, nanocrystalline solid solution [12], NiTi-austenite (B2), and NiTi-martensite (B19 ) phase [19].…”