Strengthening mechanisms in Ni and Ni-5Fe alloy

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 alpha as a microstructural parameter in the Taylor equation taking into account the dislocation arrangement in the material. alpha 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) similar to 0.15 MPa root m was obtained for both Ni and Ni-5%Fe. Solid solution strengthening obtained from Hall-Petch fitting (sigma(ss )similar to 21.3 MPa) was corroborated with the calculated values from Labusch's model of solid solution strengthening (sigma(ss) similar to 12 MPa for screw and similar to 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.