Measurements of the Elastic Constants, the Specific Heat and the Entropy of Grain Boundaries by Means of Ultra-Fine Grained Materials

Abstract

U n i v e r s i t d t des Saarlandes, FB 12.1, Bau 2 , 0-6600 Saarbriicken, F.R.G. '~r a u n h o f e r I n s t i t u t fur Zerstbrungsfreie Priifverfahren, Universita't des Saarlandes, Bau 3 7 , 0-6600 Saarbriicken, F . R . G .Abstract. Measurements of the elastic constants,the specific heat and the entropy of grain boundaries have been performed by utilizing ultra-fine grained materials (Pd, 6nm grain size; Mg, 12 nm grain size; CaF2, 7 nm grain size) about 30 to 50 vol% of which consist of grain boundaries. The results obtained suggest that (1) The interatomic potentials in grain boundaries in metals differ from the lattice potentials. Hence, the application of lattice potentials for computer simulation of boundary structures inmetals may be of limited physical relevance.(2) Boundary entropy effects are significant for the interfacial structure above half the absolute melting temperature. (3) Metastable boundary structures with enhanced boundary entropies exist. The entropy enhancement may be several times the entropy of an equilibrated interface in the same material.

Low temperature heat capacity and thermal stability of nanocrystalline nickel

“…Although a variety of physical properties of nanocrystalline materials were extensively studied with an eye to engineering applications, the thermodynamic properties have received less attention. Rupp and Birringer [2] studied the heat capacity of nanocrystalline Cu and Pd, and enhanced heat capacities were observed in the temperature range of 150±300 K. Similar results have been reported by Korn et al [3], Hellstern et al [4], Lu et al [5], Chen et al [6], Wanger [7] and Bai et al [8].…”

Strain rate sensitivity of nanocrystalline Au films at room temperature

“…Therefore, the reduced elastic modulus may be associated with the increased grain boundary volume that is occupied by disordered Au atoms contributing to the increased film compliance [28,29]. This argument, however, does not provide a general mechanism for nanocrystalline materials.…”

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

“…These investigations suggest that the conventional measurement techniques and models used to describe the deformation of bulk materials are not always applicable to thin film systems, While it has long been recognized that a material's plastic properties are controlled by its defects, a material's elastic properties are generally attributed to its bond character [18], However, a few reports have suggested a link between structure and elastic response for some nanostructured matetials. Such elastic properties have been attributed to intercrystalline regions with differing interatomic spacings [5,9,10,19,20], or potentials [20], and porosity [6,21,22,24]. In most cases the link between elastic response and structure was found to be highly dependent upon processing conditions.…”

“…Several models have been developed to explain how defect related stress concentrations cause the elastic properties of nanocrystalline materials to differ from those of similar coarse-grained materials [6,20,21]. These models treat local changes in the interatomic spacing [20] or pore geometry [6,21], caused by stress concentration, as a source of additional compliance.…”

“…These models treat local changes in the interatomic spacing [20] or pore geometry [6,21], caused by stress concentration, as a source of additional compliance. However, the sensitivity of an additional defect compliance to applied stress has not been accounted for in these descriptions.…”