Effects of ternary elements and environment on mechanical properties of TiAl intermetallics

近五年来，两相TiAl合金作为一种新型的轻质高温结构材料，取得了很大的进展和改进。在此基础上，研制出了室温延伸率高达3~4%、强度得到提高的双相工程双相TiAl合金。然而，尽管片状组织具有良好的韧性和高温强度，但它们的延性仍然较差。因此，需要更多的工作来实现片状形式的强度和延性的改进平衡。据报道，铬、钒和锰等合金元素可以提高双相双相合金的室温塑性。近年来，人们发现两相TiAl合金的拉伸塑性和断裂应力对试验环境非常敏感，它们具有较高的…在真空或干燥空气中测试时，比在空气或氢气中测试时更容易产生Er。拉伸伸长率和断裂应力对应变率也很敏感；在空气或氢气中测试时，它们增加，而在真空中测试时，它们随着应变率的增加而减小。这些结果清楚地表明，这类材料容易受到环境脆化的影响。然而，我们最近发现，添加合金可以减少环境对塑性的损失。因此，为了开发力学性能良好的两相TiAl合金，有必要弄清合金元素在两相TiAl合金变形和断裂行为以及环境脆性中的作用，特别是那些片层状的TiAl合金。在本项目中，我们试图阐明铝含量以及过渡元素和非过渡元素的合金化添加对片层组织变形和断裂行为的影响，最后测试环境。在这些结果的基础上，我们试图明确一种特定的合金元素在片层结构的变形和环境脆化行为中的作用。这导致了开发具有足够的强度、塑性和韧性的两相TiAl合金以满足其结构应用的合金化策略的建立，不仅室温塑性和韧性差，而且过高的制造成本一直阻碍着TiAl合金的结构应用。在制造成本方面，可铸造TiAl合金是可取的。然而，在铸态条件下不可避免的片层组织在塑性和韧性方面仍然较差。我们期待该项目的研究结果对改善片层组织的力学性能的合金化策略具有重要意义，从而对可铸造TiAl合金的发展产生相当大的影响。较少

Extensive progress and improvements in two-phase TiAl alloys, which have great potential as a new class of light-weight high-temperature structural materials, have been made in the last about five years. Based on these achievements, several engineering two-phase TiAl alloys in duplex form with room-temperature ductility up to 3-4% and improved strength have been developed. However, the lamellar structures are still poor in ductility, although they are beneficial for toughness and high-temperature strength. More work is thus needed to achieve an improved balance of strength and ductility in the lamellar form. Some alloying elements such as Cr, V and Mn have been reported to increase the room-temperature ductility of the two-phase alloys in duplex form. However, none of them is effective in ductilizing the two-phase alloys in lamellar form.Recently, tensile ductility and therefore fracture stress of two-phase TiAl alloys have been found to be sensitive to test environment ; they are high … More er when tested in vacuum or in dry air than in air or in hydrogen gas. The tensile elongation and fracture stress are also sensitive to strain rate ; they increase when tested in air or in hydrogen gas while they decrease when tested in vacuum with increasing strain rate. These results clearly suggest that this class of materials is susceptible to environmental embrittlement. However, we have recently found that the environmental loss in ductility may be reduced by alloying additions.Thus, it is indispensable to clarify the role of alloying elements in the deformation and fracture behavior and the environmental embrittlement of the two-hhase TiAl alloys, in particular those in lamellar form for the development of two-phase TiAl alloys with wellbalanced mechanical properties. In this project, we attempt to clarify the effects of aluminum content and then alloying additions of transition and non-transition elements and finally test environment on the deformation and fracture behavior of the lamellar structure. On the basis of these results, we try to specify the role of a particular alloying element in the deformation and environmental embrittlement behavior of the lamellar structure. This leads to the establishment of alloying strategy for the development of two-phase TiAl alloys with enough strength, ductility and toughness for their structural applications.Not only poor ductility and toughness at room temperature but also excessive fabrication cost have been hindering the structural application of TiAl alloys. Castable TiAl alloys is desirable in respect of fabrication cost. However, the lamellar structure inevitable under as-cast conditions is still poor in the ductility and toughness. We expect that results of this project will have a great significance in alloying strategy to improve the mechanical properties of the lamellar structure and thereby put a considerable impact on the development of castable TiAl alloys. Less