Microscale fracture of chromia scales

Abstract

Native protective oxide scales offer resistance against corrosion for high temperature materials, which often work in extreme conditions of varying mechanical and thermal loads. The integrity of such layers is of critical importance, since their damage can lead to significant reduction in material life. Mechanical data such as fracture strain and elastic modulus are required to include oxides in material life estimation models for high temperature materials, but there is lack of such data. Their thickness is in the µm range, which makes mechanical testing for property determination difficult. Here we present a micro-mechanical testing method, based on bending of micro-cantilevers produced by focused ion beam milling, capable of circumventing the limitations of conventional approaches. We apply this method to chromia thermally grown on pure chromium, and measure fracture strains at room and high temperatures (600 °C). The measured fracture strains were found to be higher at room temperature, due to a larger fraction of transgranular fracture. Surprisingly, a large fraction of transgranular fracture was seen even in the presence of stress concentrations at grain boundaries. Removal of the stress concentrations accentuated the propensity for transgranular cracking at room temperature. Realistic values of room temperature elastic modulus were 2 obtained as well. The observed mixed trans-and intergranular cracking points towards the need for dedicated investigations of both oxide grain boundary strength and cleavage resistance of single crystals in order to fully understand the failure mechanisms in thermally grown oxide scales.

Strategies for High-Temperature Corrosion Simulations of Fe-Based Alloys Using the Calphad Approach: Part I

“…the oxide-metal interface, which does not show any indication of grain growth at 600 or 700 C. The growth of this layer will result in parabolic growth kinetics in good agreement with the reported mass gains. [20] However, the work by Iyer et al [70] showed some larger grains at 700 C in the outer part of the scale after longer exposures while the small grains remains intact in the inner part. This is expected to result in a small deviation from parabolic behaviour at higher temperature and longer exposure times, i.e.…”

“…The assumptions explained above for the grain size estimation were based on the results of detailed microstructural investigations of the oxide scale formed on ''pure Cr'' at 600 and 700 C using TEM for exposure time of 24 h [20] and 480 h. [70] In the case of oxide growth on ''pure Cr'' a thin layer containing fine grains can be observed in the bottom of the oxide, [70] i.e. the oxide-metal interface, which does not show any indication of grain growth at 600 or 700 C. The growth of this layer will result in parabolic growth kinetics in good agreement with the reported mass gains.…”