Abstract
This paper describes the elastic softening due to the presence of defects in Ni_3Al. We calculated the elastic modulus under isotropic tensile loading by the fluctuation formula. Introducing the defects decreases the elastic constants and it reduces the elastic stability. The origin of the decreasing in the elastic constants is the negative increase of pressure-fluctuation contributions, which is induced by the atomic relaxation around the defects. The elastic softening facilitates the reduction of elastic stability and it gives the small critical volume of the fracture by the isotropic tensile loading.
