Strengthening mechanisms in Ni and Ni-5Fe alloy

Abstract

Two model systems (pure Ni and Ni-5%Fe) with five different grain sizes varying from submicrons to micrometers were investigated for the evaluative contribution of various strengthening mechanisms operative in both polycrystalline materials at room temperature. The various strengthening mechanisms contributing to the overall yield stress were calculated individually. A good linear fit of the Hall-Petch relationship was attained by (i) subtracting the dislocation strengthening contribution from the experimental yield stress and (ii) considering the dislocation interaction factor α as a microstructural parameter in the Taylor equation taking into account the dislocation arrangement in the material. α decreases with increasing heterogeneity of the dislocation arrangement. Incorporating the above-mentioned assumptions while fitting the Hall-Petch plot within a wide range of grain sizes reduced the ambiguity of obtaining different Hall-Petch constants ($k_{HP}$) for the alloys under investigation. As a consequence, a consistent value of $k_{HP}$ ∼0.15 MPa $\sqrt{m}$ was obtained for both Ni and Ni-5%Fe. Solid solution strengthening obtained from Hall-Petch fitting (${\sigma }_{ss}$ ∼21.3 MPa) was corroborated with the calculated values from Labusch's model of solid solution strengthening (${\sigma }_{ss}$ ∼12 MPa for screw and ∼106 MPa for edge type dislocations, respectively). The good agreement with the modelled lower bound value of ${\sigma }_{ss}$ was attributed to the presence of dislocations with mainly screw character in the experiment.