Estimating the Strength of Single-Ended Dislocation Sources in Micrometer-Sized Single Crystals

摘要

A recent study indicated that the behavior of single-ended dislocation sources contributes to the flow strength of micrometer-scale crystals. In this study 3D discrete dislocation dynamics simulations of micrometer-sized volumes are used to calculate the effects of anisotropy of dislocation line tension (increasing Poissons ratio, u) on the strength of single-ended dislocation sources and, to compare them with the strength of double-ended sources of equal length. This is done by directly modeling their plastic response within a 1 micron cubed FCC Ni single crystal using DDS. In general, doubleended sources are stronger than single-ended sources of an equal length and exhibit no significant effects from truncating the long-range elastic fields at this scale. The doubleended source strength increases with Poisson ratio (u), exhibiting an increase of about 50% at u = 0.38 (value for Ni) as compared to the value at u = 0. Independent of dislocation line direction, for u greater than 0.20, the strengths of single-ended sources depend upon the sense of the stress applied. The value for a, in the expression for strength, t = a(L)mb/L is shown to vary from 0.4 to 0.84 depending upon the character of the dislocation and the direction of operation of the source at n corresponding to that of Ni, 0.38 and a length of 933b. By varying the lengths of the sources from 933b to 233b, it was shown that the scaling of the strength of single-ended and double-ended sources with their length both follow a ln(L/b)/(L/b) dependence. Surface image stresses are shown to have little effect on the critical stress of single-ended sources at a length of approximately 250b or greater. The relationship between these findings and a recent statistical model for the hardening of small volumes is also discussed.