Direct Experimental Determination of Grain Boundary Excess Volume in Metals

Abstract

The grain boundary excess volume, i.e., the grain boundary expansion, e{GB}, was experimentally determined for high-angle grain boundaries in nickel using the direct technique of high-precision difference dilatometry. Values of e{GB}=(0.35±0.04)×10{-10} m and e{GB}=(0.32±0.04)×10{-10} m were obtained by measuring the removal of grain boundary volume upon grain growth for two different types of ultrafine-grained samples. The results are discussed in comparison to values obtained so far from indirect techniques and from computer simulations. It demonstrates the strength of the presented novel, direct approach for grain boundary expansion measurements.

Direct measurement of vacancy relaxation by dilatometry

“…From the length contraction in this stage, values for the excess volume of the grain boundaries can be deduced as reported earlier. 12 Relevant for the present study, however, is the regime (A) where lattice vacancies are mobile and anneal out. In tangential direction a length increase and in axial direction a length decrease occur in accordance with the above presented model of anisotropic annealing of vacancies.…”

“…Samples of HPT-Ni also contain a high abundant concentration of lattice vacancies of several 10 À4 (condition (iii)). 15 Moreover, lattice vacancies in Ni are stable at ambient temperature and become mobile at temperatures around 360 K 16 below the recrystallization stage which starts at about 470 K. 12 Therefore, the length change associated with the annealing out of vacancies is well separated from the length change which occurs due to the removal of grain boundaries upon crystallite growth. This allows a test of the model of anisotropic vacancy annealing.…”

“…11 HPT-Ni is particularly well-suited owing to the following aspects: Besides a macroscopic sample dimensions (condition (i)) and ultrafine crystallite sizes in the regime of 100 to 200 nm (condition (iv)), HPT-processing of Ni yields elongated crystallites with elongation axis lying in tangential direction of the HPT-disk (condition (ii), see Figs. 2 and 1) [12][13][14]. Samples of HPT-Ni also contain a high abundant concentration of lattice vacancies of several 10 À4 (condition (iii)).…”

Thermodynamics of coherent interfaces under mechanical stresses. II. Application to atomistic simulation of grain boundaries

“…The excess defined by Eq. (3) retains the GB excess volume, an important property which was measured both experimentally [9][10][11] and in simulations. [12][13][14] Using this definition of excess quantities, Eq.…”

Interdiffusion in amorphous AlxZr1-x alloys

“…The excess free volume of grain boundaries in polycrystalline metals is 2.4% for Au, 3.8% for Fe, and 12.8% for Al (see compilation of data in Ref. 104) 105 and can reach values of up to 32% in nanocrystalline Ni. 106 The corresponding normalised activation enthalpies for grain-boundary self-diffusion are 8.68, 107 8.46, 108 and 8.21 109 for Au, Fe, and Al, respectively, 110 in which (descending) sequence of normalised grain-boundary diffusion activation enthalpies indeed parallels the (ascending) sequence of excess free volume data given above.…”