Hui Guo

Serrated grain boundaries (GBs) are generally regarded as local geometric features that can improve creep resistance. This study demonstrates that in cast IN 738LC, the formation of serrated GBs and their strengthening effects are governed by multi-scale microstructural heterogeneity. Dendritic elemental segregation after supersolvus solution treatment produces spatially heterogeneous γ′ precipitation, causing coarse primary γ′ precipitates and pronounced GBs serration to develop preferentially in interdendritic regions. The serrated morphology arises from the coupled interaction of anisotropic γ′ growth, preferential solute transport along GBs, and concurrent GB migration locally constrained by precipitate-induced pinning and drag. Slower cooling extends serration over a larger fraction of the GB network, revealing that creep resistance is controlled not only by local serration geometry but also by the population of serrated boundaries. a network-weighted serration factor was proposed to describe the measured creep response. During creep at 760 °C/590 MPa, serrated GBs reduce the minimum creep rate and extend rupture life by redistributing local deformation, suppressing strain localization and dynamic recrystallization, and increasing crack-path tortuosity. These findings establish a direct link between solidification-induced segregation, γ′ precipitation, GB-network morphology, and creep resistance, providing a microstructure-based framework for understanding serrated GBs in cast Ni-based superalloys.

High hardenability is of great importance to ultra-heavy steel plates and can be achieved by tailoring the composition of steel. In this study, the continuous cooling transformation (CCT) curves of two high-strength low-alloy (HSLA) steels (0.16C-0.92Ni steel and 0.12C-1.86Ni steel) were elucidated to reveal the significance of C–Ni collocation on hardenability from the perspective of morphology and crystallography. At a low cooling rate (0.5 °C/s), the 0.12C-1.86Ni steel showed higher microhardness than 0.16C-0.92Ni steel. The microstructure in 0.16C-0.92Ni steel was mainly granular bainite with block-shaped martensite/austenite islands (M/A islands), while that in 0.12C-1.86Ni steel was typically lath bainite with film-shaped M/A islands, denoting that the 0.12C-1.86Ni steel is of higher hardenability. Moreover, the 0.12C-1.86Ni steel exhibited a higher density of block boundaries, especially V1/V2 boundaries. The higher density of block boundaries resulted from the weakened variant selection due to the larger transformation driving force and more self-accommodation of transformation strain induced by the reduced carbon and increased nickel content.

The attainment of both strength and toughness is of vital importance to most structural materials, although unfortunately they are generally mutually exclusive. Here, we report that simultaneous increases in strength and toughness in a high-strength low-alloy (HSLA) steel were achieved by tailoring the heterogeneous microstructure consisting of soft intercritical ferrite and hard martensite via intercritical heat treatment. The heterogeneous microstructure features were studied from the perspective of morphology and crystallography to uncover the effect on mechanical properties. Specifically, the volume fraction of martensite increased with increasing annealing temperature, which resulted in increased back stress and effective stress, and thereby an improved strength-ductility combination. The enrichment of carbon and alloying elements in the martensite was lowered with the increase in annealing temperature. As a result, the hardness difference between the intercritical ferrite and martensite was reduced. In addition, the globular reversed austenite preferentially grew into the adjacent austenite grain that held no Kurdjumov-Sachs (K-S) orientation relationship with it, which effectively refined the coarse prior austenite grains and increased the density of high angle grain boundaries. The synergy of these two factors contributed to the improved low-temperature toughness. This work demonstrates a strategy for designing heterostructured HSLA steels with superior mechanical properties.