Abstract
High temperature structural materials must be resistant to cracking and oxidation. However, most oxidation resistant materials are brittle and a significant reduction in their yield stress is required if they are to be resistant to cracking. It is shown, using density functional theory, that if a crystal’s unit cell elastically deforms in an inhomogeneous manner, the yield stress is greatly reduced, consistent with observations in layered compounds, such as Ti3SiC2, Nb2Co7, W2B5, Ta2C and Ta4C3. The mechanism by which elastic inhomogeneity reduces the yield stress is explained and the effect demonstrated in a complex metallic alloy, even though the electronegativity differences within the unit cell are less than in the layered compounds. Substantial changes appear possible, suggesting this is a first step in developing a simple way of controlling plastic flow in non-metallic crystals, enabling materials with a greater oxidation resistance and hence a higher temperature capability to be used.
Highlights
We have shown that the low yield stresses observed in some crystals are associated with electronegativity differences within their unit cells, giving rise to changes of electron density and of local stiffness in different parts of the unit cell
These ideas are consistent with what is observed in other deformed crystals. They enable a very wide range of behaviour to be obtained. This suggests that using controlled electronegativity differences in crystals offers a first step in a general route to being able to greatly increase the toughness of non-metallic materials
A number of layered compounds were chosen so that there was a range of electronegativity differences between the M–X and M–A layers, table S1, so that the effects of electron density being drawn toward the M–X layer could
Summary
Compounds selected for study were chosen from those which have already been synthesised experimentally, according to Fig. 4 of Aryal et al.[5]. Sixth-row elements were avoided due to difficulties associated with producing pseudopotentials containing occupied f states. Families of compounds were selected so that the effect of changing the M atom, changing the A atom or moving from 211 to 312 or 413 structures could be studied. Of particular interest were compounds containing sulphur, which is the sufficiently more electronegative than most A elements that the M–A layer is more electronegative than the M–X laye
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