: Like steels, austenitic INVAR alloys Fe-Ni 36 % show a large ductility trough between 500°C and llOO°C. To understand hot brittleness mechanisms and especially trace element effects, synthetic alloys were prepared using ultra-high purity iron and nickel doped with selected amounts of carbon, sulphur, boron, aluminium, and nitrogen. Four kinds of synthetic alloys were studied to establish the intrinsic influence of sulphur, the combined effects of sulphur and precipitates such AiN or BN, and the effect of boron in the presence of sulphur, on ductility and fracture mechanisms. We propose several mechanisms to explain the loss and the recovery of ductility for each of the studied alloys, and then define the field of temperatures where these mechanisms control ductility.

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