GH4720Li合金热变形过程动态软化机制

Dynamic Softening Mechanisms of GH4720Li Alloy During Hot Deformation

以锻态GH4720Li镍基沉淀强化型高温合金为研究对象,对合金进行了不同工艺参数下的热压缩实验。采用OM、SEM、EBSD和TEM研究了热压缩过程中再结晶晶粒的形成和晶粒内亚结构的演变规律,分析了合金在不同热变形工艺参数下的动态软化机制。研究表明,合金在所有热变形工艺参数下均发生了非连续动态再结晶行为。变形组织分析表明,高温低应变速率能够抑制非连续动态再结晶行为的发生,而提高应变速率能促进非连续动态再结晶行为,且能够获得等轴状尺寸均匀的晶粒组织。未完全溶解细小γ'强化相的钉扎作用能够使变形晶粒内形成高密度位错亚结构和亚晶界,亚晶界角度通过连续的吸收位错而不断地升高,进而以"强化相诱发连续动态再结晶"方式形成细小的再结晶晶粒组织。不同热变形工艺下孪晶界的演变规律分析表明,热变形温度与应变速率通过影响合金的动态再结晶行为来改变孪晶界的数量。

GH4720 Li alloy is a precipitation strengthened Ni-based superalloy and widely applied in high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue, together with corrosion, fracture and microstructural stability for the intended applications. Hot working is an effective way for shaping metals and alloys as well as changing the microstructure and mechanical properties. Lots of typical metallurgical behaviors such as dynamic recovery(DRV), discontinuous dynamic recrystallization(DDRX) and continuous dynamic recrystallization(CDRX) occur, which are related to the hot working parameters, including deformation temperature, strain rate and strain. In order to investigate the effect of deformation parameters on dynamic softening behavior and evolution of twinning for GH4720 Li alloy, the hot deformation behavior of as-forged GH4720 Li alloy was studied by isothermal compression tests. OM,SEM, EBSD and TEM techniques were employed to investigate systematically the dynamic softening mechanisms, formation of DRX grains and evolution of substructure in grains under different deformation parameters. The results showed that DDRX can take place at all studied deformation conditions. The boundary bulging and nucleation of DDRX grains were restrained as a result of decrease of dislocation substructures and subgrain boundaries density consumed by continuous original boundary migration(COBM) in deformed grains at low strain rates and high temperatures, and then the occurrence of DDRX was suppressed. DDRX was promoted as the strain rate was increased and uniform microstructures composed of fine equiaxed grains can be readily obtained as well. The microstructural changes showed that the pinning effect of fine undissolved γ’ precipitates was able to hinder the dislocation movement and promote the formation of high density of dislocation substructures and subgrain boundaries in deformed grains. The increase in sub-boundary misorientation brought about by continuous accumulation of the dislocations was introduced by the deformation, and fine DRX grains formed by particle-induced continuous dynamic recrystallization(PI-CDRX). According to the evolution of twinning under various deformation conditions, the effect of deformation temperature and strain rate on the evolution of twinning was characterized by the occurrence of DRX behavior.