What is The ability of an alloy to withstand mechanical stresses without permanent  deformation is reﬂected by its: and explain in detail?
Elastic strain—Amount of deformation that is recovered instantaneously when an externally applied force or pressure is reduced or eliminated.
Strain may be either elastic, plastic, elastic and plastic, or viscoelastic. 
Elastic strain is reversible. The object fully recovers its original shape when the force is removed. Plastic strain represents a permanent deformation of the material; it does not decrease when the force is removed. When a prosthetic component such as a clasp arm on a partial denture is deformed past the elastic limit into the plastic deformation region, elastic plus plastic deformation has occurred, but only the elastic strain is recovered when the force is released. Thus, when an adjustment is made by bending an orthodontic wire, a margin of a metal crown, or a denture clasp, the plastic strain is permanent but the wire, margin, or clasp springs back a certain amount as elastic strain recovery occurs.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 48,51