The ductile-brittle transition for nominally pure polycrystalline nickel

A ductile to brittle transition has been observed for a nominally high purity polycrystalline nickel of ∼200 μm grain size, containing 50 ppm sulphur. The nickel specimens were subjected to heat treatment at 1017 °C for 0.5 h and were then either furnace cooled or water quenched. Small notched bend geometry specimens were tested over the temperature range of −196 °C to 200 °C at a fixed strain rate. Both the work to initiate fracture and the proportion of intergranular fracture were determined. The elemental concentration of sulphur at the grain boundaries has been measured using scanning Auger spectroscopy and STEM-EDX microanalysis. These compositions are compared with predictions from a non-equilibrium segregation model. The segregation of sulphur to the grain boundaries has a strong effect in promoting brittle intergranular fracture and this is attributed to the change in cohesive strength of the grain boundaries as a result of the segregation.