Important Factors Affecting the Stability of Substructure in Heat-resistant Materials

在项目期间，我们证实了在耐热钢、铝铜合金和工业镁合金中观察到的蠕变过程中亚晶的长大(即亚结构的退化)和Ostwald熟化都是应变相关的，尽管到目前为止还没有注意到应变对组织退化的影响。此外，还研究了亚晶界逃逸析出相钉扎的临界应力。结果表明，亚晶尺寸在低于临界应力的应力范围内保持不变，在高于临界应力的应力范围内开始增大。在理论基础上，临界应力应与试验温度无关，但实际测得的临界应力与温度有关。为了弄清这种温度依赖性的原因，测量了析出物的平均尺寸和颗粒间距作为测试温度的函数。实验性的…更多的结果表明，由于在施加应力前加热到测试温度时发生了颗粒粗化，颗粒间距随试验温度的升高而增大。分析表明，临界应力只有明显的温度依赖性。由于T91钢的组织退化与亚晶界的迁移有关，因此可以认为T91钢的组织退化是由位错运动(塑性应变)促进的。实际上，这种结构退化的起源似乎与应变增强的Ostwald熟化不同，但我们可以认为这两种现象的起源是相同的，因为阻碍次边界迁移的析出物的生长也可以借助于次边界的运动。通常，许多耐热材料含有相对较大尺寸失配的溶质原子。这是因为这种类型的溶质能有效地强化其基体，但这些溶质容易引入位错运动辅助的溶质迁移，最终导致析出相的应变促进生长。到目前为止，较大尺寸失配的溶质最好被添加到耐热材料中以增强其基质，然而，这些元素容易产生应变增强的析出物粗化。到目前为止，在合金设计中还没有考虑应变增强的组织退化，但重要的是重新考虑合金元素，考虑导致析出物应变增强粗化的清除作用。较少

In the project term, we confirmed that the growth of subgrain during creep (i.e., degradation of substructure) and the Ostwald ripening observed in a heat resistant steel, Al-Cu alloys and a commercial magnesium alloy were all strain dependent notwithstanding that attention was not paid to the effect of strain on the structural degradation so far. Furthermore, the critical stress necessary for subgrain boundary to escape from pinning of precipitates was examined. The results showed that the subrain size remained unchanged in a stress range below the critical stress while it begins to increase in a stress range above the critical stress. On the theoretical basis, the critical stress is expected to be independent of testing temperature, however, the measured critical stress was temperature-dependent. In order to clear the reason for this temperature dependence, the average size and inter-particle spacing of precipitates were measured as a function of testing temperature. The experimental … More results showed that the inter-particle spacing increased with increase in testing temperature because particle coarsening occurred during heating to testing temperature before stress application. The analysis showed that the temperature dependence of critical stress was only apparent. The magnitude of critical stress was almost equal to the theoretical one calculated from the model proposed in Mater.Trans.44(2003),239-246.Since the structural degradation of T91 steel is associated with migration of subboundary, we can view that the structural degradation was promoted by dislocation motion (plastic strain). Actually, the origin of structural degradation of this type is seemingly different from the strain enhanced Ostwald ripening, however, we can view that the origin of both phenomena is the same because the growth of precipitates which hinder the sub-boundary migration can also be assisted by the motion of subboundary.Generally, many heat-resistant materials contain solute atoms with a relatively large size misfit. This is because solutes of this type are effective to strengthen their matrix, however, these solutes are liable to introduce the solute transport assisted by dislocation motion, and eventually lead to strain-enhanced growth of precipitates. So far, solutes with a relatively large size misfit were preferably added to heat-resistant materials to strengthen their matrix, however, these elements are susceptible to strain enhanced coarsening of precipitates. Strain enhanced structural degradation was not taken into account so far in alloy designing, however, it is important to re-consider the alloying elements taking into account the scavenging effect which causes the strain enhanced coarsening of precipitates. Less