KIM Jeong In POSTECH GIFT

Spoluautoři PAK Jun Hak, PARK Kyong-Su, JANG Jae Hoon, SUH Dong-Woo, BHADESHIA H.K.D.H.

The enrichment of the impurities at boundaries reduces their cohesive strength, leading to intergranular fracture under load. Phosphorus is particularly potent in this respect. From an experimental and theoretical point of view, phosphorous segregation has been characterised for static boundaries. One aspect that to our knowledge has never been studied, is what happens to the phosphorus that is at austenite boundaries when allotriomorphic ferrite forms. This phase transformation is reconstructive in nature and hence is not limited by the original austenite boundary, which can be destroyed completely as the ferrite grows into both of the adjacent austenite grains. It is possible that the phase transformation renders the phosphorus benign by leaving it in positions away from boundaries. The issue is now of increasing importance given that there are concerted efforts to exploit the solution strengthening of ferrite using phosphorus as a solute. A binary alloy of iron containing 0.17wt% of phosphorus has been heat treated under a variety of conditions in order to examine segregation behavior of phosphorus to ferrite grain boundaries on cooling from the austenitisation temperature in the present study. The alloy transforms fully into ferrite. It is found that the majority of solute found at the ferrite grain boundaries has its origins in the temperature range where phase transformation occurs, in other words, phosphorus that is accumulated and dragged with the growing ferrite/austenite transformation front. As a consequence, it cannot be suppressed using cooling rates as large as 400 K/s.

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