基于计算机仿真镍基单晶高温合金再结晶过程分析

再结晶是影响高温合金高温蠕变性能的重要因素.以DD6镍基单晶高温合金为研究对象,建立了再结晶过程中形核生长、储能分布、晶粒长大的数学模型,利用计算机模拟技术分析了材料热处理过程中再结晶的整个过程和动力学曲线.该研究为镍基单晶高温合金生产过程中的蠕变性能优化提供了参考.     

Microstructural development in single crystal nickel base superalloy welds

Abstract

The welding behaviour of alloy PWA 1480, a single crystal nickel base super alloy, has been investigated. The ability to successfully weld single crystal nickel base superalloys is desirable for fabrication procedures as well as for potential repair applications in both aircraft and land based turbine systems. The microstructural development in welds of this alloy was characterised and analysed using a geometrical model developed earlier in the study of Fe–Cr–Ni single crystal welds. The microstructural features in the nickel base alloy welds and, in particular, the dendritic growth patterns, were accurately described by this model. However, several potential difficulties with the welding of the nickel base superalloys were identified. First, there is frequent formation of stray crystals which result in the loss of the single crystal nature of the weld. Second, dendritic zones are formed in the weld and these may result in a degradation of the weld properties, even if the single crystal character of the weldment remains intact. In addition, extensive cracking was found in these welds and this subject is dealt with in a companion paper.     

一种镍基单晶高温合金蠕变机制的研究

对一种镍基单晶高温合金在两种蠕变条件下（760℃／780MPa和982℃1248MPa）的变形机制进行分析．结果表明：在中温高应力条件下（760℃／780MPa）,在低应变阶段,位错以堆垛层错的形式切入γ’相;高应变阶段,位错以位错对的方式切入γ’相．在高温低应力条件下（982℃／248MPa）的低应变阶段,母相a／2（110）位错在基体中运动弓出,并绕过γ’相,发生位错反应而形成位错网;高应变阶段与中温高应力条件的切入机制相同．     

Twinning Behaviors During Thermomechanical Fatigue Cycling of a Nickel-Base Single-Crystal TMS-82 Superalloy

This paper provides further insight into the formation of deformation twins at different stages during the whole thermomechanical fatigue cycling in a nickel-base single-crystal TMS-82 superalloy. In general, it is found that twinning behaviors can always be associated with the applied stress orientation. The preferred twinning direction at the primary stage is 〈001〉-compression since the tangled dislocations which appear after the first plastic deformation provide an opportunity for twinning nucleation in compression. At the intermediate stage, the applied stress required for formation of twins in tension is much larger than that in compression; hence, twinning behaviors show distinct tension/compression asymmetry. A thick twin plate and a great many dislocations can be found after fatigue failure, and one can rationalize the reason for this twinning being associated with the TMF procedure. Twins at the tip of the crack in tension occur owing to the existence of compressive strain field.     

Stray grain formation,thermomechanical stress and solidification cracking in single crystal nickel base superalloy welds

Abstract

In the present study, the effects of stray grain formation and thermomechanical stresses on solidification cracking in welds of single crystal Ni-base superalloys have been investigated. Welds were made in an asymmetric crystallographic orientation under three different processing conditions. As welding speed and power increased, stray grain formation became extensive, but only on one side of the weld. Solidification cracking also became more extensive and occurred mostly along the stray grain boundaries. The three welding processes have been simulated using the finite element method (FEM). The calculation results showed that thermomechanical stresses increase with welding speed and power, leading to increased susceptibility to cracking. These results agree well with experimental observations.     

Thermo-mechanical fatigue of single crystal nickel-based superalloy DD8

The thermo-mechanical fatigue (TMF) behavior and the relevant microstructural evolution of the single crystal nickel-based superalloy DD8 were investigated. In-phase TMF lives were much shorter than those of out-of-phase TMF. The mechanism of TMF damage is discussed based on the microstructural evolution during TMF.     

一种含硼第三代镍基单晶高温合金的初熔及固溶处理研究

采用电子探针测试结合Thermo-Calc软件计算,研究了一种含0.001 wt%硼第三代单晶高温合金枝晶间区域的初熔过程。结果表明,初熔相为M2B,初熔温度为1265 ℃,远低于用差示扫描量热仪测得的共晶熔化温度1352 ℃。采用扫描电镜分析固溶处理过程中合金组织的演化过程,发现共晶组织中马赛克状共晶和扇状共晶率先溶解,粗大γ′最后溶解。初熔区域空冷可发生均质形核,重新凝固后的共晶组织呈圆形,初熔组织中粗大γ′组织较初始凝固的粗大γ′组织更细,而相比残余共晶组织,初熔组织中含有马赛克状共晶和扇状共晶。在通用分步固溶处理制度的基础上,通过低于硼化物初熔的首步处理和适当提高最终固溶温度（ST2）,可以有效避免初熔并完全消除共晶。     

新型抗热腐蚀镍基高温合金K44的高温低周疲劳行为

研究了K44合金900℃低周疲劳性能和断裂行为。研究结果表明,该合金在循环形变过程中,首先表现出起始循环硬化或软化,随后循环稳定及最终失稳断裂三阶段。高应变幅下位错切割γ′相形成层错,降低变形阻力,合金表现出循环软化行为;低应变幅下位错在γ′相前塞积造成位错可动性降低,合金表现出循环硬化行为。疲劳裂纹主要萌生于试样表面或近表面缺陷处,以穿晶方式扩展;合金基体中块状碳化物对裂纹扩展起阻滞作用。     

Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy

The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of ΔK and K_max on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.     

Environmental effects on tensile and low cycle fatigue behavior of single crystal nickel base superalloys

The effect of γ^′ rafting on the tensile and cyclic behavior of the single crystal superalloys PWA1483 and N5 was investigated at room temperature. Both alloys with rafted microstructures exhibited susceptibility to environment i.e. moisture. This environmental effect was manifested by a reduction in ductility and fatigue life accompanied by {1 0 0} cleavage.     

Temperature dependence of deformation mechanism in single crystal Ni-base superalloy

The tensile behavior of a new single crystal Ni-base superalloy was studied at various temperatures.Specimens were strained to fracture in the temperature range from 20 ℃ to 1 000 ℃. σ0.2 is essentially unaffected by temperatures between 20 ℃ and 400 ℃. At higher temperatures it increases until it reaches a maximum at about 800 ℃. Beyond 800 ℃ a sharp decrease of strength is observed. There is a slight fluctuation in ductility between 20 ℃ and 800 ℃. The elongation to fracture increases from 10% to 36% as the temperature increases from 800 ℃ to 1 000 ℃. The deformation is dominated by γ' shearing and the high-density dislocations are observed in matrix channels at low temperatures. The dislocation microstructure is inhomogeneous due to the formation of dislocation concentrations with high-density tangling at intermediate temperatures. The networks deposited at the γ'/γ'interfaces prevent dislocations from entering the γ' precipitates at high temperatures.     

IC10合金热机械疲劳性能与寿命预测

600～1 100 ℃时对IC10合金进行同相位、反相位、135-相位和-135-相位的应变控制热机械疲劳实验.发展了一种三参数幂函数能量方法的寿命预测方法,并用于材料的热机械疲劳寿命预测.对Manson-Coffin方程、拉伸迟滞能模型(Ostergren)、基于微裂纹扩展模型的能量方法和三参数幂函数能量方法热机械疲劳寿命预测能力进行评估,结果表明:材料的热机械疲劳应力-应变滞后回线的形状与温度-机械载荷之间的相位角有关;温度-机械载荷之间的相位角对材料的热机械疲劳性能有一定的影响;三参数幂函数能量方法物理意义明确,形式简单,寿命预测结果分布在2倍的分散带内,可以用来预测IC10合金的热机械疲劳寿命.     

晶体取向对镍基单晶高温合金铸态组织和偏析的影响

用光学显微镜观察了沿 [0 0 1],[0 11]和 [111]方向生长的镍基单晶高温合金的枝晶形态。结果表明 ,以枝晶界面生长的单晶中生长方向完全由择优取向〈0 0 1〉决定 ,[0 0 1]方向中形成平行于轴向的枝晶 ,[0 11]和 [111]方向中由于不同取向的枝晶交错生长 ,形成多种位向的亚晶界。在垂直于枝晶方向的 { 0 0 1}面内测定的一次枝晶间距 ,按 [0 0 1]→ [0 11]→ [111]次序增大。用电子探针测定了不同取向中合金元素的偏析比 ,Al,Co ,W的偏析情况不受晶体取向影响 ,Ti和Cr的偏析比按 [0 0 1]→ [0 11]→ [111]的顺序减小 ,Ta和Mo的偏析程度无确切变化规律。各元素在不同取向中偏析的变化是由元素的扩散能力和枝晶空间构形共同决定的。     

Crack growth behavior of SRR99 single crystal superalloy under thermal fatigue

The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated.Specimens with V-type notch were tested at the peak temperatures of 900,1000,and 1100℃.The crack growth curves as a function of the number of cycles were plotted.With the increase of peak temperature,the crack initiation life was shortened dramatically.Through optical microscopy (OM) and scanning electron microscopy (SEM) observation,it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles.The primary cracks generally propagated along a preferential direction.Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.     

Bonding phenomena of transient liquid phase bonded joints of a Ni base single crystal superalloy

The bonding phenomena of Ni base single crystal superalloy CMSX-2 during transient liquid phase (TLP) bonding have been investigated using MBF-80 and F-24 insert metals. TLP bonding of the superalloy was carried out at 1373-1548K for 0-19.6ks in vacuum and the (100) plane of each test specimen was always aligned perpendicular to the joint interface. The dissolution width increased when the bonding temperature and holding time increased. The eutectic width decreased linearly with the square root of holding time during isothermal solidification. After homogenization treatment, the microstructure, distributions of hardness and alloying elements in the bonded interlayer become similar to those of the base metal.     

第二代单晶高温合金DD6的高周疲劳行为

研究Hf含量为0.34%和0.10%Hf的第二代单晶高温合金DD6在700℃的高周疲劳行为:采用扫描电镜对疲劳裂纹的萌生与失效机制进行分析,采用透射电镜研究变形后的显微组织.结果表明:含0.34%Hf的DD6合金比含0.10%Hf的合金的疲劳寿命稍有降低;疲劳裂纹萌生于试样表面、亚表面,沿{111}面扩展,而并不沿垂直应力轴方向扩展;断口上有典型的疲劳条带;Hf含量为0.34%和0.10%的DD6合金的高周疲劳断裂机制为类解理断裂.透射电镜分析表明,合金在循环变形中形成了不同形式的位错.     

High-temperature oxidation of a single crystal Ni-base superalloy

The isothermal oxidation behaviour of a single crystal Ni-base superalloy (CMSX2) between 900 and 1300°C in air at atmospheric pressure or in pure static oxygen at 0.2 atm. pressure has been studied for test durations between 8 to 1000 hours. The extremely low oxidation rate did not permit accurate measurements of oxidation kinetics. However, oxide scale examinations by means of SEM, EDS, TEM, X-ray diffraction, RHEED, SIMS and metallography allowed to discuss the oxidation mechanisms of the studied alloy. At temperature higher than 1100°C, the very good oxidation resistance is due to the growth of a thin Al₂O₃ scale while at lower temperature it may be attributed to the growth of a subscale of a complex oxide (Cr, Ti, Al) (Ti, Ta)O₄ possessing a rutile structure. The effect of chemical homogeneity was studied in comparing the behaviour of as-cast and homogenized specimens.     

第二代单晶高温合金的低周疲劳行为

研究了含0.34%Hf DD6单晶高温合金在760℃下的低周疲劳行为,对其疲劳裂纹的萌生与扩展进行了分析。结果表明,含0.34%Hf DD6合金比含0.10%Hf的疲劳寿命稍有降低;其损伤以弹性损伤为主。疲劳裂纹萌生于试样表面、亚表面或试样内部后,先沿垂直应力轴方向扩展,后沿{111}面扩展。其断裂机制为类解理断裂。断口上有典型的疲劳条带。断裂组织分析表明,位错主要在基体通道中扩展,变形后期切过γ'相,形成滑移带。     

热障涂层对单晶高温合金多轴热机疲劳性能的影响

采用电子束物理气相沉积工艺在CMSX-4单晶合金空心试管表面制备了Y₂O₃(7%~8%,质量分数)-ZrO₂热障涂层,然后在500~1000 ℃范围内对带/不带涂层试样进行了机械应变控制的热机械疲劳(TMF)和热梯度机械疲劳试验(TGMF)。结果表明,热障涂层可在一定程度上提高单晶合金试棒的疲劳寿命,同时,在相同的应变幅值下,同相位带热障涂层试样的热梯度机械疲劳寿命略高于反相位带涂层试样。试样的断口分析结果表明,同相位热机疲劳和反相位热机疲劳断口呈现显著差异,反相位疲劳试样断口具有明显的疲劳辉纹,材料蠕变和氧化对断口影响较小;同相位试样断口则明显与材料的蠕变和氧化相关。