  The mechanical characteristics of a monolithic (non-porous) crystalline or
amorphous material are described by a well-defined set of quantities. It is
possible to change the mechanical properties by introducing porosity into this
material: as a rule, the strength values decrease with the introduction of
porosity. Thus, porosity can be considered as an additional degree of freedom
that can be used to influence the hardness, strength and plasticity of the
material. In the present work, using porous crystalline NiTi as an example, it
is shown that the mechanical characteristics such as the Young's modulus, the
yield strength, the ultimate tensile strength, etc. demonstrate a pronounced
dependence on the average linear size $\bar{l}$ of the pores. For the first
time, an empirical equation is proposed that correctly reproduces the
dependence of the mechanical characteristics on the porosity $\phi$ and on the
average linear size $\bar{l}$ of the pores in a wide range of sizes: from
nano-sized pores to pores of a few hundred microns in size. This equation
correctly takes into account the limit case corresponding to the monolithic
material. The obtained results can be used directly to solve applied problems
associated with the design of materials with the necessary combination of
physical and mechanical characteristics, in particular, porous metallic
biomaterials.
