1. Field of the Invention
The invention is concerned with a shape memory alloy based on nickel and titanium. The invention is further concerned with a method for the production of a memory alloy and its application.
2. Description of the Prior Art
Shape memory alloys based on the intermetallic compound of nickel and titanium and similar related compositions are known in several embodiments. In particular, the martensitic transformation behavior of alloys of stoichiometric or very nearly stoichiometric TiNi composition has been further investigated and described, e.g., R. J. Wasilewski, S. R. Butler, J. E. Hanlon and D. Worden, "Homogeneity Range and the Martensitic Transformation in TiNi", Metallurgical Transactions, 2, 229-239 (Jan. 1971).
It is an established fact that the critical temperature of the martensitic transformation is very strongly dependent on the composition of the material. It is obvious from the TiNi phase diagram that several phases of different physical properties must be reckoned with in close proximity to the 50 atomic percent point, and upon these depends very much whether or not an equilibrium condition is reached. Great difficulties are therefore met in trying to obtain reproducible experimental results. In the region from just under 50 atomic percent up to approximately 52 atomic percent nickel, the martensitic transformation temperature shows a steep drop, and several authors have reported different results corresponding to different experimental conditions (see also U.S. Pat. No. 3,351,463 and C. M. Jackson, H. J. Wagner and R. J. Wasilewski, "NASA-SP-5110", NASA Report 1972).
Production technology has been sought to improve the properties of memory alloys and to produce uniform results in the end product through suitable thermal treatment processes (e.g. U.S. Pat. No. 3,594,239). The service behavior of stoichiometric or near-stoichiometric TiNi alloys depends not only on their compositions but also strongly on their previous metallurgical histories. Heat treatments, deformation cycles, and particularly temperature ranges play a decisive role.
Thus, from the current state of the art, it appears difficult to make memory alloys with material characteristics sufficiently exact and reproducible for industrial application. The strong compositional dependence of the temperature of the martensitic transformation in the immediate vicinity of the intermetallic compound TiNi prevents the economic manufacture of this material as well as its general application in the construction of devices. There is a definite need for a cost-saving manufacturing process and for new alloys with a technically feasible, broadened tolerance range of the composition. It is also desirable to be able to additionally influence the martensitic transformation, while avoiding the sharp dependence on fluctuations of the composition.