GH4720LI合金熱變形過程動(dòng)態(tài)軟化機(jī)制

GH4720LI合金熱變形過程動(dòng)態(tài)軟化機(jī)制DYNAMIC SOFTENING MECHANISMS OF GH4720LI ALLOY DURING HOT DEFORMATION

以鍛態(tài)GH4720Li鎳基沉淀強(qiáng)化型高溫合金為研究對(duì)象，對(duì)合金進(jìn)行了不同工藝參數(shù)下的熱壓縮實(shí)驗(yàn)。采用OM、SEM和EBSD研究了合金在不同熱變形工藝參數(shù)下的動(dòng)態(tài)軟化機(jī)制，分析了熱壓縮過程中再結(jié)晶晶粒的形成和晶粒內(nèi)亞結(jié)構(gòu)的演變規(guī)律。研究表明，合金在本工作中所有熱變形工藝參數(shù)下均發(fā)生了非連續(xù)動(dòng)態(tài)再結(jié)晶行為。變形組織分析表明，高溫低應(yīng)變速率能夠抑制非連續(xù)動(dòng)態(tài)再結(jié)晶行為的發(fā)生，而提高應(yīng)變速率能促進(jìn)非連續(xù)動(dòng)態(tài)再結(jié)晶行為，且能夠獲得等軸狀尺寸均勻的晶粒組織。未完全溶解細(xì)小γ'強(qiáng)化相的釘扎作用能夠使變形晶粒內(nèi)形成高密度位錯(cuò)亞結(jié)構(gòu)和亞晶界，亞晶界角度通過連續(xù)的吸收位錯(cuò)而不斷地升高，進(jìn)而以“強(qiáng)化相誘發(fā)連續(xù)動(dòng)態(tài)再結(jié)晶”方式形成細(xì)小的再結(jié)晶晶粒組織。不同熱變形工藝下孿晶界的演變規(guī)律分析表明，熱變形溫度與應(yīng)變速率通過影響合金的動(dòng)態(tài)再結(jié)晶行為來改變孿晶界的數(shù)量。

GH4720LI 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 during hot deformation processes of GH4720LI alloy, in this work, the hot deformation behavior of as-cast GH4720LI alloy was studied by isothermal compression tests. OM, SEM and EBSD 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 promoted 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 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.

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