In order to study the mechanical properties in the field of high temperatures (850, 870 and 890°C) and explain the reasons of properties instability, the L96 alloy specimens, cast by semi-continuous method, are subjected to tests at maximum tension rate of 3 mm/s. Two fracture types take place, namely, very deformed 'cup-shaped' failure and 'point' failure, that is characteristic for superplasticity effect. The nonlinear methods of fracture mechanics are used in analysis of the mechanical properties. It is found that the castings of alloy have the mechanical properties enabling a plastic working.