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耐热的相关金属学问题 材料结构在高温环境中的演化及其 对力学性能的影响 北京科技大学 杨善武 内 容 范性变形的一般机制与强化措施 改善材料高温力学性能的主要措施 高温力学行为及其对应的结构演化 范性变形的一般机制与强化措施 范性变形的微观机制 金属与合金的宏观范性变形在微观上对应 位错的滑移(包括螺型位错在高温的交叉 滑移)、攀移(发生于高温)、晶体的孪 生(主要发生于低温)和多晶体沿晶界的 滑动(发生于高温),其中以位错滑移为 最常见的方式。 刃型位错滑移 孪生 刃型位错攀移 “趁热打铁”的道理 升高温度导致钢的变形抗力(位错运动阻 力)减弱 温度超过一定值后,钢基体由体心立方结 构转变为面心立方结构,更易于变形 升温导致的软化效应(回复、再结晶、析 出相粗化、晶粒长大等)可以抵消变形导 致的硬化(加工硬化) T 金属单晶体的切变强度随温度的变化趋势 原因:热激活可以帮助位错克服短程阻力, 但对长程阻力无作用。 硬度 时间 回复过程中硬度随时间的变化 再结晶体积分数 时间 再结晶体积分数随时间的变化 范性变形的一般机制与强化措施 强化金属的主要措施 以阻碍位错运动为基本出发点,强化措施 主要包括:位错强化、细化晶粒的强化、 固溶强化(高温适用) 、析出强化(高温 适用) 和相变强化(为前几类强化方式的 综合)。 改善材料高温力学性能的主要措施 强化晶界 降低界面能 增加位错运动的晶格阻力(P-N力) 在材料内部引入稳定的应力场 度 强 晶界与晶内强度随温度的变化 晶界工程-降低界面能的有效途径 利用面心立方结构的金属易于发生退火孪 生这一现象,通过反复进行的变形和退火 处理,可以在材料中得到大量=3, 9, 27的 重位点阵晶界,从而降低界面能,提高晶 界的热稳 定性。 Grain boundary character distribution of the as-received (AR) and GBE-treated samples as a function of initial percentage reduction in thickness (5%, 9%, and 13%). The symbols in open, diagonal cross, cross, and single horizontal bar denote the number of thermomechanical cycles from 1 to 4 applied to the samples. Fraction of low-R CSLBs of the as- received and GBE treated samples, GBE1 and GBE2. L. Tan et al. Effect of thermomechanical processing on grain boundary character distribution of a Ni -based superalloy. Journal of Nuclear Materials 371 (2007) 171175 Deviation angle from exact R3 of the as- received and GBE-treated samples, GBE1 and GBE2. Thermal stability of the grain boundary character distribution of GBE-treated samples, GBE1 and GBE2, after exposure at 850 for 672 h. 位错运动的晶格阻力(P-N力) 其中为位错半宽度 由上式可以看出,位错运动的晶格阻力与温度无关,在 高温仍能得到维持。 高温力学行为及其对应的结构演化 在材料内部引入应力场的途径 固溶强化 主要是利用低固溶度元素的过饱和固溶 析出强化 主要依赖于弥散、细小的共格或半共格析 出相产生的共格应力场 ( d ) 不同类型的相界面 (a)完全共格;(b)部分共格;(c)切变共格;(d)非共格 An illustration for the concept to lengthen thermal fatigue lives. N. Fujita et al. Changes of microstructures and high temperature properties during high temperature service of Niobium added ferritic stainless steels. Materials Science and Engineering A351 (2003) 272/281 Effects of Nb and Mo on the 0.2% proof strength at 900 for 13Cr steels. Relationship between addition contents and amounts in solid solution of Nb and Mo for 13Cr steels. Relationship between 0.2% proof strength at 900 and Nb addition contents of 13Cr steels as annealed and aged at 900 for 500 h. Relationship between Nb amounts in solid solution and Nb addition contents of 13Cr steels as annealed and aged at 900 for 500h. Transmission electron micrographs and their analysis results by electron diffraction of 0.01C/13Cr/0.5 Nb steels as annealed (a) and aged at 900 for 500 h (b). Corresponding EDS results of (a) and (b) Transmission electron micrographs and their analysis results by electron diffraction and EDS of 0.01C/14Cr/0.1Ti /0.3Nb/0.5Mo steels as annealed (a) and aged at 900 for 500 h (b). Changes of Nb and Mo amounts in solid solution with aging time at 900 for 0.5Nb and 0.3Nb/0.1Ti/0.5Mo steels. Changes of 0.2% proof strength at 900 with aging time at 900 for conventional 19Cr/0.4Nb and 14Cr/0.3Nb/0.1Ti/0.5 Mo steels. Comparison of thermal fatigue lives between conventional 19Cr/0.4Nb and 14Cr/0.3Nb/0.1Ti/0.5Mo steels. The thermal fatigue tests were performed with sheet specimens with 2 mm thick which were heated and cooled repeatedly between 200 and 900 in a constraint ratio of 100% of thermal expansion. Change in yield strength at 700 with aging time. G.M. Sim et al. Effect of Nb precipitate coarsening on the high temperature strength in Nb containing ferritic stainless steels. Materials Science and Engineering A 396 (2005) 159 165 Change of Nb amount in precipitates measured by ICP with aging time. Relationship between high temperature strength and Nb content in solid solution in Nb containing steels. Average diameter of precipitates with aging time in Nb containing steels. (a) NbC particle after aging for 60,000 min in 0.01C0.38Nb steel, (b) HR-TEM micrograph showing interface between NbC and the Fe matrix, (c) SAD pattern of interface (arrows indicate overlapped spot) and (d) schematic illustration of SAD pattern. (a) Fe2Nb particle after aging for 60,000 min in 0.01C0.38Nb steel, (b) HR-TEM micrograph showing interface between Fe2Nb and the Fe matrix, (c) SAD pattern of interface (arrows indicate overlapped spot) and (d) schematic illustration of SAD pattern. 小结 对于铁素体不锈钢而言,Nb的固溶强化是 提高其高温强度的有效机制。但是,Nb的 析出与析出相的粗化会导致钢的高温强度 迅速下降。而适当降低Nb含量并复合添加 Mo、Ti有利于得到持久稳定的强化。析出 类型及析出基体界面的共格程度对析出 相的长大、粗化速度有重要影响。 (a) (b) 含Nb钢在850变形25%后弛豫不同时间的样品在650(a) 和700(b)重加热过程中的硬度变化 真应变0.4弛豫不同时间的样品在650重加热过程中的硬度变化 b 0.1 m e 0.5 m a a) a) 0.5 m c 0.5 m d 0.1 m f 0.1 m 不同弛豫时间的样 品重加热等温前的 透射电镜形 (a),(b) 弛豫0s; (c),(d) 弛豫60s; (e),(f) 弛豫1000s (a)(b) 0.1 m (c) 弛豫不同时间的样品650重加热等温0.5h后的位错组态 (a)弛豫0s;(b) 弛豫60s;(c) 弛豫1000s 0.2 m 0.3m 变形奥氏体中的位错网络在贝氏体相变后的形态 (b) 0.1 m (a) 0.1 m 弛豫60s的样品650重加热等温
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