Effect of grain size on yield strength of Ni₃Al and other alloys

This paper analyzes the effect of grain size on yield stress of ordered Ni₃Al and Zr₃Al, and mild steels that show Lüders band propagation after yielding, using the Hall-Petch relation, σ_y = σ₀ + k_yd^−½, and the new relation proposed by Schulson et al., σy = σ₀ + kd ^{−(p − 1)/2} [Schulson et al., Acta Metall. 33, 1587 (1985)]. The major emphasis is placed on the analysis of Ni₃Al data obtained from published and new results, with a careful consideration of the alloy stoichiometry effect. All data, except for binary stoichiometric Ni₃Al prepared by powder extrusion, fit the Hall-Petch relation, whereas the data from boron-doped Ni₃Al and mild steels do not follow the Schulson relation. However, no conclusion can be made simply from the curve fitting using either relation. The results are also discussed in terms of Lüders strain and alloy preparation methods. On the basis of the Hall-Petch analysis, the small slope k_y is obtained only for hypostoichiometric Ni₃Al with boron, which would be related to a stronger segregation of boron in nickel-rich Ni₃Al. In addition, the potency for the solid solution strengthening effect of boron is found to be much higher for stoichiometric Ni₃Al than for hypostoichiometric alloys.