INFLUENCE OF TRACE-ELEMENTS ON HOT DUCTILITY OF AN ULTRA-HIGH PURITY INVAR ALLOY

Like steels, austenitic INVAR alloys Fe-Ni 36 % show a large ductility trough between 500 degrees C and 1100 degrees C. To understand hot br ittleness mechanisms and especially trace element effects, synthetic a lloys 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 precipitate s such AlN 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 studie d alloys, and then define the field of temperatures where these mechan isms control ductility.