Abstract
The role of impurity sulphur in the ductility trough of iron–nickel (Fe–Ni) alloys is investigated using hot tensile tests. A strong detrimental effect of some ppm levels of sulphur is demonstrated. In addition, it is shown that, in the ductility trough, material failure occurs through subcritical grain boundary crack propagation, involving dynamic embrittlement at the crack tip, due to the sulphur. Very high intergranular crack growth rates are observed. This is possible because plastic deformation accelerates the transport of sulphur to the crack tip, by several orders of magnitude, compared to normal bulk diffusion. The ductility is recovered at high strain rates, which correlates with a decrease in the sulphur concentration measured on the fracture surface. It is suggested that the main mechanism of sulphur transport is dragging by moving dislocations.
Highlights
Austenitic materials usually have a good ductility, which makes it possible to cold- or hot-deform them
This phenomenon is referred to as a “ductility trough”, “loss of ductility”, “hot-shortness”, “intermediate temperature embrittlement” or, sometimes, “ductility dip cracking (DDC)”, the latter being more popular in the field of welding
Many studies have shown the prime importance of trace elements, especially impurity sulphur, in the ductility trough of steels [1,2], nickel alloys [3,4,5] and copper alloys
Summary
Austenitic materials usually have a good ductility, which makes it possible to cold- or hot-deform them. Many studies have shown the prime importance of trace elements, especially impurity sulphur, in the ductility trough of steels [1,2], nickel alloys [3,4,5] and copper alloys (see, for example, the review paper by Laporte and Mortensen [6]). Some of those early works on ductility troughs were conducted on model alloys of a very well-controlled purity at the ppm level, so as to identify the effects of individual elements. Some researchers have recently questioned the role of sulphur in the ductility trough—first, because of the absence of a clear correlation between the nominal sulphur content of industrial alloys and the intensity of their ductility troughs [7,8] and, second, because of the extreme difficulty, or even the impossibility, to demonstrate the presence of such impurities segregated at the grain boundaries [9]
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