Cryogenic nanoindentation size effect in [0 0 1]-oriented face-centered cubic and body-centered cubic single crystals

Abstract

Cryogenic nanoindentation experiments performed on [0 0 1]-oriented single crystalline Nb, W, Al, and Au in an in situ nanomechanical instrument with customized cryogenic testing capability revealed temperature dependence on nanoindentation size effect. The Nix-Gao model, commonly used to capture indentation size effect at room temperature, does not take into account thermal effects and hence is not able to explain these experimental results where both hardness at infinite indentation depth and characteristic material length scale were found to be strong functions of temperature. Physical attributes are critically examined in the framework of intrinsic lattice resistance and dislocation cross-slip probability.

Hardness-Depth Relationship with Temperature Effect for Single Crystals—A Theoretical Analysis

“…In Figure 3, we present the H − h relationships obtained from the calibrated theoretical model and experimental data of single crystal Cu [24], Al [16], CaF 2 [25] and W [16]. As one can see the results match reasonably well for the four materials, and an obvious indentation size effect is informed at different temperatures.…”

“…In this section, four sets of experimental data for single crystal Cu [24], Al [16], CaF 2 [25] and W [16] are applied to verify the rationality and accuracy of the developed model. Before the calibration of model parameters, it should be noted that H f for Cu and Al is so small that is generally not considered [26,27].…”

“…Therefore,h * (T) tends to decrease at high temperatures that results in the weakened indentation size effect at elevated temperatures. Furthermore, increasing temperature not only leads to the decrease of H 0 (T) and m(T), but also weakens the lattice friction, thus, it becomes rational to experimentally observe that H(T) decreases with the increase of T for most crystalline materials [16,24,25].…”

“…However, when compared with the test performed at room temperature, both the increasing rate of materials hardness and ultimate bulk hardness are noticed to get weakened at elevated temperatures [9,15]. For instance, Lee et al [16] investigated the dependence of indentation size effect on T for [0 0 1] -oriented single crystalline Nb, W, Al and Au, and demonstrated that for all of them both the hardness at infinite indentation depth and intrinsical materials length scale are strong functions of T. Similar experimental phenomena [17] have also been observed in the indentation test of polycrystalline Co, Ni and Pt that the indentation size effect becomes moderated when T increases from room temperature to T m /3.…”

“…(Color online) Comparison of the H 1/m n − 1/h relationships at various temperatures between theoretical results (lines) and experimental data (dots) for (a) Cu[24], (b) Al[16], (c) CaF 2[25] and (d) W[16].…”