Understanding deformation mechanisms in the Hall-Petch breakdown regime of Ni-W alloys

Abstract

The substantially higher strengths of nanocrystalline metals has been to attributed to the well-known Hall-Petch relationship σ= σ_o+k_HP d^(-1/2) where 𝜎 is the yield stress, 𝜎𝑜 is the lattice friction stress, 𝑑 is the grain size and 𝑘𝐻𝑃 is the Hall-Petch constant. The Hall-Petch scaling breaks down at grain sizes below ~ 10nm. The dominant deformation mechanism in the Hall-Petch breakdown or grain boundary weakening regime are not well-understood, with weakening being attributed variously to diffusion creep, grain rotation, and grain boundary sliding. Typically, specimens with the finest grain sizes are annealed to obtain the coarser grain sizes, and the influence of such annealing on strength is not clear. Several studies have attributed the grain boundary weakening regime to enhanced grain boundary segregation or reduced internal stresses caused by annealing.

This study focuses on characterizing the dominant deformation mechanisms in the Hall-Petch breakdown regime. The Ni-W system was chosen as the model alloy, where W is known to segregate at the grain boundary. The Hall-Petch behaviour was studied using nanoindentation hardness measurements, and the hardness observed in Ni-W alloys (3-15 %W) ranged from 6 to 15 GPa for grain sizes from 29 nm down to 3 nm. Solute segregation was studied extensively to understand the role of solute chemistry at the GB, using Atom Probe Tomography (APT) experiments.

Three alloys with 3, 8 and 15 at% W were studied. Different grain sizes were obtained at a constant W content by annealing at various temperatures. A clear Hall-Petch breakdown was observed in the three compositions. Measurements revealed that solute segregation did not increase upon annealing, so that the GB weakening regime cannot be attributed to variations in segregation.

Annealing introduces multiple microstructural changes simultaneously and it is difficult to separate the contribution from each process. Four alloys with W content 19 ±1 at% and grain sizes ranging from 8 to 3 nm were synthesized by slightly modifying the deposition conditions, without annealing. A clear grain boundary weakening regime was observed, suggesting that the grain boundary weakening regime is not related solely to annealing effects.

The wear behavior was also studied for a 8nm Ni-W alloy deposited on ball-bearing steel used in one of the EOS satellites at ISRO. The wear of ball-bearings limits the life of these satellites. The Ni-W coatings were examined for better wear resistance. The cross-sectional hardness measured for an annealed alloy of 11 nm grain size was  22 GPa. A fivefold increase in the wear resistance was observed compared to current steel, suggesting the viability of such coatings for improving the life of satellites.