Crystal-to-amorphous transformation of NiTi induced by cold rolling
- PDF / 866,312 Bytes
- 5 Pages / 593.28 x 841.68 pts Page_size
- 22 Downloads / 227 Views
M. Nastasi Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 9 March 1990; accepted 23 March 1990)
A NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 1013 to 1014/cm2, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.
I. INTRODUCTION
Amorphous materials have been traditionally synthesized by condensing a random structure from a liquid or vapor phase. In the last decade, various solidstate techniques have been developed to produce an amorphous phase from a crystalline phase. A basic premise of the solid-state amorphization is to increase the free energy of the crystalline phase to a state higher than that of the amorphous phase. To prevent the nonequilibrium high-energy crystalline phase from transforming to an equilibrium stable state, amorphization is always carried out at low temperatures where atomic mobility is sufficiently low to prevent diffusion that leads to the equilibrium state. Under this kinetic restriction, the thermodynamic requirement for amorphization can be fulfilled in many intermetallic, ceramic, and semiconductor systems by means of highenergy particle irradiation, hydrogenation, ion-beam mixing, ion implantation, mechanical mixing, and annealing of diffusion couples. For each amorphizing technique, extensive studies have been carried out to understand the thermodynamic driving force and the underlying mechanism.1
observed before amorphization was identical to that of Kr-irradiated Zr3Al, indicating that a critical dilation may induce the elastic instability independent of the method. For B implantation into Nb,5 the mean square displacement of atoms from their lattice site, derived from the measured x-ray diffuse intensity, increases to a large maximum value before amorphization takes place. In this instance, the mean square displacement averaged along the principal crystallographic axes is equivalent to the lattice dilation. It has also been reported that NiTi6 and GaSb7 can be amorphized by applying a shear stress under high pressure. These results can be understood in terms of elastic instability caused by the external shear stress instead of the internal stress induced by irradiation or hydrogenation. Consistent with experimental observation, molecular dynamics simulation of amorphization by interstitial insertion8 and chemical disordering9 shows a strong correlation between amorphization and an increase in the mean square displacement and dilation, respectively. These results all imply
Data Loading...
