Strengthening mechanisms (via hardness analysis) in nanocrystalline NiCr with nanoscaled Y2O3 and Al2O3 dispersoids

NiCr-base alloys with nanoscale Y2O3 and Al2O3 dispersoids were fabricated as thin sheets by electron beam physical vapor deposition (EB-PVD). The thermal stability of the nanocrystalline alloys, mapped via microstructural examination, has been correlated to hardness. The alloys showed high hardness values (6 GPa) in the as-deposited state. The two types of dispersoids, Y2O3 and Al2O3, differ vastly in the way in which they contribute to stability, and hence hardness/strength. The fine Y2O3 dispersoids pin the grain boundaries by a Zener type of mechanism and contribute very significantly to strength. In this case, grain size or Hall–Petch strengthening seems to be just twice that of particle strengthening (σHP:σOro = 2:1). Whereas, in the Al2O3-containing alloy, coarsening of the fine Al2O3 particles takes place, leading to the dominant mechanism to be via Hall–Petch strengthening, with particle strengthening playing little or no role (σHP:σOro = 5:1). The important inference that can be drawn out of this study is that, in nanograined materials with nanoscaled dispersoids, there can be nearly equal contributions from both grain size reduction and particle strengthening, whereas in coarser-grained materials, contributions from particle size strengthening are rather minimal, and Hall–Petch strengthening dominates.