Mechanical Properties and Deformation Mechanism of Nitrogen- and Carbon-doped High Entropy Alloys


Student thesis: Doctoral Thesis

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Award date7 Jul 2022


High entropy alloy (HEA) is widely investigated nowadays. Its first research can date back to the 18th century in France by a German metallurgist called Fran Carl Archard. A lot of the current research on HEA focused on the effects of substitutional elements on cantor-alloy-based HEA, namely equiatomic face-centered-cubic (FCC) FeCrCoMnNi allo. A wide variety of HEAs were developed based on this method. Interstitial alloyed HEA was rarely studied in the early stage of HEA research. The first formal research on interstitial alloyed HEA was the investigation on a nitrogen doped AlCrSiTiZr high entropy films by magnetron sputtering in Ar/N2 atmosphere in 2012 by Hsueh et al [1]. In this thesis, a simple process, namely reactive arc-remelting, was used to dope nitrogen into bulk HEA. Using this method, significant amount of nitrogen can be doped homogeneously into HEA in short processing time.

By reactive arc-remelting, 3.51 at% of N was doped into cantor alloy. It was the highest concentration of nitrogen doped into cantor alloy reported so far. Owing to the high cooling rate from water-cooled copper crucible, Cr2N precipitation was suppressed and thus FCC solid solution was found in the nitrogen doped cantor alloy in as-cast state. Taking advantage of the high concentration of nitrogen retained in solid solution state, the yield stress of the N-doped cantor alloy was almost doubled compared with the neat cantor alloy. Planar slip was promoted while activation of twinning was delayed compared to cantor alloy.

Around 0.5at% of nitrogen could be doped into FeCrCoNi, which is another widely studied HEA. This is much less than that can be dissolved into cantor alloy. FCC solid solution was also observed after 1hr of homogenization at 1100℃. The yield stress of FeCrCoNi was increased after nitrogen doping and planar slip was promoted. However, unlike nitrogen doped cantor alloy, nitrogen doped FeCrCoNi showed no sign of twinning. The effects of carbon doping into FeCrCoNi were also investigated. The solid solution strengthening power of carbon in FeCrCoNi was much less than that of nitrogen, and the carbon-containing alloy was prone to carbide precipitation. Planar slip with slight amount of wavy slip were observed at the early deformation stage and twinning was found in the later stage in the carbon-doped FeCrCoNi alloy.