This paper presents a new formula for calculating the hardness of metallic crystals, resulted from the research on the critical grain size with stable dislocations. The formula is 𝐻 = 6 𝑘𝐺/[𝜋(1 – 𝜈)𝑒𝜂], where 𝐻 is the hardness, 𝑘 the coefficient, 𝐺 the shear modulus, 𝜈 the Poisson’s ratio, 𝜂 a function of the radius of an atom (𝑟) and the electron density at the atom interface (𝑛). The formula will not only be used to testify the critical grain size with stable dislocations, but also play an important role in the understanding of mechanical properties of nanocrystalline metals.
