Grain boundary decohesion by nanoclustering Ni and Cr separately in CrMnFeCoNi high-entropy alloys

The loss of ductility with temperature has been widely observed in tensile tests of single-phase face-centered cubic structured high-entropy alloys (HEAs). However, the fundamental mechanism for such a ductility loss remains unknown. Here, we show that ductility loss in the CrMnFeCoNi HEA upon deformation at intermediate temperatures is correlated with cracking at grain boundaries (GBs). Nanoclustering Cr, Ni, and Mn separately at GBs, as detected by atom probe tomography, reduces GB cohesion and promotes crack initiation along GBs. We further demonstrated a GB segregation engineering strategy to avoid ductility loss by shifting the fast segregation of principal elements from GBs into preexisting Cr-rich secondary phases. We believe that GB decohesion by nanoclustering multiprincipal elements is a common phenomenon in HEAs. This study not only provides insights into understanding ductility loss but also offers a strategy for tailoring ductility-temperature relations in HEAs.

Ductile-brittle transition temperature shift controlled by grain boundary decohesion and thermally activated energy in Ni-Cr steels

Temper embrittlement induced by segregation of metalloid solutes to grain boundary (GB) was evaluated by a shift of the ductile-brittle transition temperature (DBTT). DBTT was found to be linearly correlated with the amount of metalloid on the GB (Xgb) for both dynamic and static displacement rates (dδ/dt) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δep) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope (α) of DBTT vs. Xgb was found to be linearly dependent on Δep regardless of the segregated solutes. In high hardness steels, the slope is independent of dδ/dt, while in medium hardness steels the α is dependent on dδ/dt. An Arrhenius plot of dδ/dt vs. the reciprocal DBTT was used to drive the thermal activation energy (Eact), which represents a barrier to plasticity. It was found that Eact correlates to a reduction in the GB fracture surface energy. The Eact depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.

Tensile Deformation Behavior of Medium Manganese Steels with High Carbon Concentrations and Austenitic Microstructures

: Tensile properties of the Fe–1.44C–8Mn–1.9Al (mass %) steel with a fully austenitic microstructure, obtained by rapid quenching from the austenite range, was determined at room temperature. Tensile tests were performed using specimens prepared in two different routes involving an exchanged sequence of machining and heat treatment as the last processing steps prior to tensile tests. In spite of the occurrence of deformation twinning, total tensile elongations in both processing routes remained below 18%. Abrupt stress drops during tensile tests suggested the occurrence of discontinuities in tensile specimens. Detailed examination of the fracture surface indicated a mixed intergranular–transgranular fracture mode. Furthermore, a high density of surface cracks was observed near the outer surface in the gauge section of fractured tensile specimens. The origin of surface cracks could not be identified. The coincidence of surface cracks with the grain boundaries, especially those nearly perpendicular to the tensile direction, is thought to be responsible for the accelerated grain boundary decohesion and the premature fracture of tensile specimens.

Microstructure and Mechanical Properties of Heat-treated T92 Martensitic Heat Resistant Steel

T92 samples were solutionized at 1,050 °C, 1,100 °C and 1,150 °C for 20 min and then tempered at 730 °C, 745 °C and 760 °C for 60 min. Optical microscopy studies were carried out to understand the microstructural evolution due to heat treatment. These heat-treated samples comprised of lath martensite microstructure in all the cases. Prior austenite grain size of the heat-treated samples increased with solutionizing temperature. Tensile properties were evaluated using micro-tensile samples. Hardness values of the heat-treated samples were estimated using Vickers hardness tester. Interestingly, for all the given tempering condition, the hardness values showed an increasing trend with solutionizing temperature while their tensile strength values tend to decrease. Fractograph analysis depicted that increasing the solutionizing temperature led to grain boundary decohesion.