【作者】 康福伟；

【导师】 沈军； 孙剑飞；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 本文利用喷射成形技术制备了GH742y高温合金沉积锭,并对其进行了热等静压(Hot Isostatic Pressed-HIP)致密化处理(以下简称喷射成形合金)。采用热压缩实验研究了喷射成形合金的变形能力和变形行为,建立了材料本构方程、激活能图及热加工图,确定了其最佳热加工参数范围,并对其进行了锻造加工。利用扫描电镜(SEM)和透射电镜(TEM)研究了喷射成形GH742y合金变形过程中组织演变特点,揭示了其变形特性机制;利用电阻率和正电子湮没技术研究了锻造后的喷射成形GH742y合金时效特性,建立了该状态下合金γ′相长大动力学方程,确定了其合理的热处理工艺;最后测试并分析了热处理后的喷射成形GH742y合金室温和高温力学性能。在研究过程中利用铸造+HIP GH742y合金(以下简称铸造合金)进行了某些对比研究。喷射成形和铸造合金的热变形实验结果表明,在实验条件内,两种状态合金的真应力-真应变曲线变化趋势基本相同,即随着温度降低或应变速率升高,峰值应力增加。在1050℃时,铸造合金应力上升到峰值后突然降低,应变速率越高此现象越明显。喷射成形合金的变形能力远远优于铸造合金的变形能力,在1140℃,1.0~10s-1条件下变形,喷射成形合金最大变形量可达80%,而铸造合金在1140℃,0.01s-1变形条件下的最大变形量仅为35%;实验还发现,喷射成形合金的变形能力随应变速率升高而增加,铸造合金的则与之相反。利用真应力-真应变曲线数据建立了喷射成形GH742y合金的本构方程。结果表明,形式为ln Z = 72+0.018σP的指数函数本构方程不适合描述合金的流变形为,而形式为sinh (0 .0031σp ) = exp(0 .23lnε& +25695T?18.34)的双曲正弦函数本构方程比较合适,经验算该方程与实验结果吻合较好。利用热变形实验数据建立了喷射成形合金变形激活能图和热加工图,该图为制订合理的热加工工艺提供了理论依据。在变形温度1110~1140℃,应变速率1.0~10s-1条件下,激活能图中存在一个小平台区;而此条件也是热加工图中的稳态变形区,且能量耗散率具有较大值,在该区域变形合金具有较好的变形能力,初步确定该变形条件为合金的热加工条件;变形温度1050~1108℃,应变速率0.01~0.1s-1的条件为热加工图中的流变失稳区,合金在此条件下变形试样开裂,应避免在此区域变形。对变形后的喷射成形GH742y合金微观组织观察表明,合金在变形过程中发生的动态再结晶(DRX)组织随变形温度、应变速率的升高以及压下量增加从部分再结晶逐步发展为完全再结晶。高应变速率下的变形温度升高、孪生变形、亚晶内部的位错运动以及空位浓度的增加,都促进再结晶程度的增加,从而喷射成形合金的变形能力随应变速率或变形温度的升高而增强。铸造合金的DRX组织则与之相反,随应变速率升高,DRX程度小且不均匀,合金变形能力变差。喷射成形GH742y合金的峰时效时间(8小时)比铸造合金的锋时效时间(12~16小时)明显缩短,而峰时效硬度值(514.33HV)则高于铸造合金的峰时效硬度值(508.61HV)。计算表明喷射成形合金固溶度(4.668%)比铸造合金固溶度(4.588%)高,而前者空位迁移能(0.165eV)却比后者(1.12eV)低;喷射成形合金的空位浓度、位错密度均比铸造合金的高。这些是喷射成形合金在时效过程中呈现时效加速特征,表现为锋时效时间缩短的主要原因。喷射成形高温合金短期时效过程γ′相长大规律很好地符合LSW熟化理论,即满足r 3∝t的关系。通过计算求得γ′相扩散激活能为133.9kJ/mol,从而得到γ′相长大动力学方程为:从固溶和时效的研究结果确定了两种热处理工艺,HT1:1140℃(6h)+850℃(6h)+空冷(AC),HT2:1140℃(6h)+850℃(6h)+780℃(8h)+AC。喷射成形合金经上述两种热处理后由于一次γ′相、二次和三次γ′相的配合,室温拉伸、高温拉伸及高温持久性能均优于传统铸锻GH742y合金经标准热处理后的性能,其中以HT2处理后的合金性能更好;拉伸断口分析表明断裂机制以韧窝聚集型断裂为主。更多还原

【Abstract】 The billet of GH742y superalloy was prepared by the spray forming processing, and it was consolidated by hot isostatic pressing (HIP) (hereafter referred to spray formed alloy). The workability and deformation characteristics were investigated by means of isothermal hot compression testing. To optimize hot deformation processing variables, the constitutive equations and processing maps of the spray formed alloy were established on the basis of testing data and dynamic materials model. Subsequently, the spray formed billet was hot forged by means of above-mentioned processing parameters. The deformation mechanism and the ageing characteristic of the spray formed and forged alloy were by studied SEM, TEM, electrical resistivity and positron annihilation technology, and ageing kinetics and the optimum heat treatment procedure were determined. The mechanical properties of heat treated alloy were examined at room and high temperatures. By contrast with the spray formed alloy, the HIPed as-cast GH742y (hereafter referred to as-cast alloy) alloy was also investigated.The compression testing results show that the curve shapes of true stress-true strain of two alloys were similar. The peak stresses of two alloys decreased with the increase of temperature or the decrease of strain rate. It is found that the spray formed alloy exhibits an excellent hot-workability compared to the as-cast counterpart. The spray formed specimens didn’t crack at the engineering strain up to 80%, but all as-cast specimens cracked at engineering strain beyond 35%. Also, the result shows that the higher the strain rate used, the better is the deformability of the spray formed alloy. However, the as-cast alloy is reverse.The hyperbolic sine function of sinh(0 .0031σp)instead of exponent equation of ln Z = 72+0.018σP, fitted for the spray formed alloy.The activation energy map and processing map of the spray formed alloy were established according to the constitutive equations. A small platform region exhibits at 1110~1140℃,1.0~10s-1 in the activation energy map. However, flow stability occurs at 1110~1140℃, 1.0~10s-1 in processing map, with to efficiency power dissipation being max. in this region. The flow stability region corresponds to the small platform region, and the spray formed alloy possesses better workability in the region. Flow instability occurs at 1050~1080℃, 0.01~0.1s-1 in processing map, the deformed specimens cracked in this region.The microstructure observation indicates that the dynamic recrystallization (DRX) of the spray formed alloy occurs in deformation, which related with temperature, strain rate and strain. With increasing of strain rate, testing temperature and strain, the extent of DRX is increased, which causes development of deforming workability with increasing of strain rate. Nevertheless the DRX extent of the as-cast alloy decreased with the increase of strain rate, resulting in the worse workability.In contrast to as-cast alloy, the peak aging time of the spray formed alloy was significantly shortened, and the peak ageing hardness of the former was lower than that of the latter. The difference of two alloys in ageing characteritics was attributed to high solid solubility, high vacancy concentration, low vacancy migration energy and high dislocation density for the spray formed alloy.For the spray formed alloy, it is found that theγ′size growth fits with LSW ripening theory, viz r 3∝trelationship. The calculation shows that, theγ′phase growing activation energy is 133.9kJ/mol, thereby theγ′phase growth kinetics equation as follows:Two heat treatment procedures are determined according to solution and ageing treatment tests. In the first treatment, the alloys were solutionized at 1140℃for 6h and air cooled, and aged at 850℃for 6h and then air cooled. In the second treatment, aging at 780℃for 8h and air cooling was added on the basis of the first treatment. The mechanical properties of the heat treated the spray formed alloy were studied. The results show that its properties are better than those of the as-cast, which is attributed to cooperation among the primary, secondary and tertiaryγ′phases. The fracture surfaces are characterized by dimple aggregation.更多还原