材料期刊网

以改进的KRC模型决定AG~(γ→α)的方法,计算了Fe-C合金贝氏体相变可能机制:γ→α+γ_1,γ→α+Fe_3C以及γ→α(浓度相同)和α′→α_B~″(贝氏体铁素体碳浓度)+Fe_3C的相变驱动力和长大(形核)驱动力.相变驱动力以γ→α+Fe_3C为最大,γ→α+γ_1次之,γ→α最小.由奥氏体转变成同成分铁素体(γ→α)的长大驱动力远小于γ→α+γ_1的长大驱动力.在贝氏体形成温度范围内,γ→α的驱动力远小于切变机制所需的驱动力.0.1—0.55wt,%C合金在B_s温度时γ→α+γ_1的相变驱动力仅约—45Jmol~(-1).0.8wt %C合金在贝氏体形成上限温度(823K)时γ→α的相变驱动力为137Jmol~(-1),而α→α+Fe_3C的相变驱动力为-527Jmol~(-1);两者相加,即在贝氏体铁素体析出渗碳体的情况下,相变总驱动力也仅有约-390Jmol~(-1).上述结果表明,贝氏体铁素体很难以切变机制形成和长大.

By application of the improved KRC model for determination of △G~(γ→α),the total driving forces for transformation and driving forces for growth(nucle-ation)for the several possible mechanisms of the bainitic transformation in Fe-Calloys,i.e.,γ→α+γ_1,γ→α+Fe_3C and γ→α′(the same composition)and α′→Fe_3C+α_B(the composition of the bainitic ferrite)are calculated.The driving forceof γ→α+Fe_3C is the largest,followed by that of γ→α+γ_1 and that of γ→α′isthe smallest.The driving force for growth of γ→α′is far less than that of γ→α+γ_1.At the temperature range of bainite formation,the driving force for γ→α′ismuch lower than that required for shear mechanism.The driving force for trans-formation of γ→α+γ_1 in 0.1—0.55 wt-%C alloys at B_s temperature is onlyabout -45 J·mol~(-1).At the upper limit temperature of bainite formation,the driv-ing force of γ→α′in 0.8 wt-%C alloy-is about 137 J·mol~(-1),and that of α′→α_B+Fe_3C is—527 J·mol~(-1).If cementite precipitates from the bainitic ferrite,thetotal driving force,i.e.,the sum of the driving forces of the above two processesis only about -390 J·mol~(-1).The above results show that the formation of thebainitie ferrite by shear mechanism is thermodynamically impossible.