Grain size effects in nanocrystalline materials

Abstract

Nanocrystalline materials have a grain size of only a few nanometers and are expected to possess very high hardness and strength values. Even though the hardness/strength is expected to increase with a decrease in grain size, recent observations have indicated that the hardness increases in some cases and decreases in other cases. A careful analysis of the available results on the basis of existing models suggests that there is a critical grain size below which the triple junction volume fraction increases considerably over the grain boundary volume fraction and this is suggested to be responsible for the observed softening at small grain sizes.

Defect-dependent Elasticity: Nanoindentation as a Probe of Stress State

“…The response of the film when compressive (tensile) stress is applied is consistently above (below) the averaged unstressed response as shown by data point 3 (6). This measurement sequence (points [1][2][3][4][5][6][7][8] illustrates that the change in measured elastic response with applied stress is reversible. The response of the film before applying stress (1) is the same as after applying stress (5).…”

“…In particular the correlation between mechanical response, stress state and deposition conditions have been documented for several materials but are not well understood [1,2]. Deformation properties unique to the nanoscale have been attributed to a material's high defect densities and to changes in the mechanisms accommodating deformation [2][3][4][5][6][7][8][9][10]. Such mechanical behavior has been observed in bulk samples with grain sizes below 10-20 nm for which a significant volume fraction of atoms reside in the intercrystalline grain boundary regions [3,4,[7][8][9][10].…”

“…Deformation properties unique to the nanoscale have been attributed to a material's high defect densities and to changes in the mechanisms accommodating deformation [2][3][4][5][6][7][8][9][10]. Such mechanical behavior has been observed in bulk samples with grain sizes below 10-20 nm for which a significant volume fraction of atoms reside in the intercrystalline grain boundary regions [3,4,[7][8][9][10]. For thin films the constraints of film thickness, adhesion to the substrate and non-random texture extend the range of grain sizes over which such 'nanocrystalline' mechanical properties are observed [1,2,5,8].…”

Microhardness of binary near-equiatomic Ti-Ni alloys after severe cold rolling and post-deformation annealing

“…Recently, it was found that thermomechanical processing consisting of severe plastic deformation (SPD) and post-deformation annealing (PDA) allows creation of a nanocrystalline structure in Ti-Ni SMA, which results in a significant improvement of their functional properties [1][2][3]. In a series of works, the stability of the amorphous/nanocrystalline structure formed by severe cold rolling (CR) was studied during room-temperature aging [4], and a peculiar dome-shape that nanocrystalline metallic alloys do not follow the classical Hall-Petch behaviour and that below a certain grain size, softening instead of hardening is observed [5,6]. In this work, in the efforts to compare these phenomena, the microhardness of Ti-50.26%Ni samples subjected to severe CR and annealing to form nanocrystalline, submicrocrystalline and recrystallized austenite structures was measured and analysis of the experimental data was performed using two generic models to describe the abnormal Hall-Petch behaviour of the nanocrystalline materials: model of the intercrystalline regions and model of the melting temperature grain-size dependence [7][8][9].…”

“…The normal H-P relationship is generally explained on the basis of a dislocation pile-up at grain boundaries. However, below a critical grain size (around 10 nm for metals), no dislocation activity is observed and plastic deformation occurs mostly by grain boundary sliding [5,6,8,9,12] with increasing influence of: (a) interface regions mobility [8,9,13,14] and (b) melting temperature decrease [15][16][17][18], both resulting in material softening if grains become sufficiently small.…”