The δ-ferrite decomposition has been widely reported in conventional Fe-Cr-Ni austenitic stainless steel during thermal aging. However, a novel phenomenon is found in a Si-modified austenitic stainless steel, that δ-ferrite decomposition is suppressed, replaced by the decomposition of adjacent austenite. Herein, the decomposition behavior of austenite adjacent to δ-ferrite and its effect on impact toughness in a Si-modified austenitic stainless steel during aging at 550 °C up to 3000 h have been investigated. During thermal aging, austenite adjacent to δ-ferrite is decomposed in the following sequence: (1) γ → MC + α transformation takes place. The preferential formation of secondary MC carbides not only rejects Si atoms into the surrounding austenite but also produces a C-depleted zone. The transformation of austenite to ferrite is induced in the Si-riched and C-depleted micro-zone. (2) The growth of secondary MC carbides induces a continuous rejection of Ni and Si atoms into MC/γ and MC/α interfaces, and the strong attractive interaction between Ni and Si provides the chemical driving force for nucleation of MC carbides and G-phase. (3) With prolonged aging time, the lower C, high Ni, and high Si concentrations in the frontier of decomposed austenite will promote the preferential precipitation of MC carbides rather than MC carbides. During impact deformation, microcracks caused by strain incompatibility between secondary MC and α-ferrite, resulting in a slight decrease in impact toughness. As austenite decomposition proceeds, the higher strain incompatibility across MC/α interfaces leads to brittle cleavage fracture, resulting in a significant decrease in impact toughness.

中文翻译：

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Si 改性 Fe-Cr-Ni 奥氏体不锈钢在 550 °C 热老化过程中与 δ-铁素体相邻的奥氏体分解行为

传统 Fe-Cr-Ni 奥氏体不锈钢在热老化过程中发生 δ-铁素体分解已被广泛报道。然而，在硅变质奥氏体不锈钢中发现了一种新现象，即δ-铁素体分解受到抑制，取而代之的是邻近奥氏体的分解。本文研究了硅改性奥氏体不锈钢在 550 °C 时效至 3000 小时期间与 δ-铁素体相邻的奥氏体的分解行为及其对冲击韧性的影响。热时效过程中，与δ铁素体相邻的奥氏体按以下顺序分解：（1）发生γ→MC+α相变。二次MC碳化物的优先形成不仅将Si原子排斥到周围的奥氏体中，而且还产生贫碳区。奥氏体向铁素体的转变是在富硅和贫碳微区中诱导的。 (2) 二次MC碳化物的生长导致Ni和Si原子不断排斥进入MC/γ和MC/α界面，Ni和Si之间强烈的吸引力相互作用为MC碳化物和G-形核提供了化学驱动力阶段。 (3)随着时效时间的延长，分解奥氏体前沿中较低的C、较高的Ni和较高的Si浓度将促进MC碳化物的优先析出，而不是MC碳化物。冲击变形过程中，二次MC与α-铁素体应变不相容而产生微裂纹，导致冲击韧性略有下降。随着奥氏体分解的进行，MC/α界面上较高的应变不相容性导致脆性解理断裂，导致冲击韧性显着下降。