This invention relates to a novel micrographic structure for metal alloys which are superplastic in the solid state.
It is known that under certain conditions of temperature and deformation rate some alloys exhibit the phenomenon of superplasticity which is characterized by the ability to obtain a very high degree of evenly distributed deformations, &gt;100% for example under tension, without preferential local deformations, i.e., contraction of area.
This behavior, which can be compared to the viscous deformation of synthetic resin polymers or hot glass, is generally observed under the following conditions:
(a) a deformation temperature higher than 0.5 Tf, Tf being the initial melting temperature of the alloy expressed in degrees Kelvin; and PA1 (b) very low deformation rates relative to the conventional deformation rates.
Consequently, the industrial development of such alloys has been relatively restricted since the rates adopted are very low in comparison with the normal operating rates thereof. This results in extremely long, energy-consuming operations to obtain the desired forms. In fact, for the known superplastic alloys, the rational deformation rates range from 0.1 to 10% min.sup.-1 under optimum conditions.
Significant progress has been made in this field due to the discovery of temporary superplasticity which is induced by extremely fine recrystallation after cold working during the actual hot plastic deformation. This method is the subject of French Pat. No. 2,236,613, filed on July 3, 1973. This method allows a rational deformation rate ranging up to 100% min.sup.-1 to be obtained temporarily. However, the temporary superplastic state has, by nature, a very short life span since the particles formed enlarge very rapidly causing the alloy to lose its capacity for rapid deformation.
This limits the practical applications, therefore, as it has to be carried out under very precise operating conditions and is awkward.