Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《材料研究学报》
Abstract： Plate tensile specimens were machined from a Ni- base single crystal (SC) superalloy DD33, holes with various diameters were electrochemically machined (ECM) in the middle of the specimens. The strain fields around the holes during room temperature tensile tests were in-situ observed by an ARAMIS - optical deformation analysis system based on the digital image correlation (DIC) technique and the fracture surface was observed by scanning electron microscope (SEM). It was demonstrated that the yield strength and ultimate tensile strength decreased with the increase of hole diameter from 0.5 mm to 0.9 mm. Strain concentrated in the vicinity of the hole. The maximum strain increased slightly during elastic deformation. However, once plastic deformation began, it increased rapidly above its elastic value. In addition, the maximum strain and strain gradients adjacent to the holes increased with the increase of the hole diameter from 0.5 mm to 0.9 mm. This work is critical for understanding the crack initiation around the cooling holes with different diameters in the SC blade.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Ni-based single crystal (SX) superalloys are widely used for production of blades in gas turbines and aircraft engines for their superior mechanical performance at high temperatures. To obtain high cooling efficiency, most of the SX blades consist of thin wall with cooling holes. However, thermal fatigue cracks are usually observed in blades with this kind of structures. Thus, it must be valuable to investigate the crack initiation and propagation around a hole during thermal fatigue tests in a SX superalloy. In the present work a second generation SX Ni-based superalloy was used. Plate specimens that parallel to directional solidification (DS) direction and along (100) or (110) planes were prepared. A hole with diameter of 0.5 mm was drilled vertical to the surface in the middle of the plate by electro-discharge machining (EDM). Thermal fatigue tests were performed between room temperature and 1100 ℃. Effect of crystal orientation on the crack initiation and propagation was investigated and the reasons were analyzed. It was found that a thin recast layer was produced around holes of EDM drilled. The thickness of the recast layer was 15 mm in the maximum. Crystal orientation has great effect on the crack initiation sites and propagation kinetics. After 80 cyc thermal fatigue tests, in (110) specimens cracks initiated at the edge of the holes that vertical to the DS direction, then grew quickly and propagated along directions about 45° from the DS direction. After 200 cyc tests, cracks developed to more than 2 mm in length. While in (100) specimens no cracks could be observed even after 200 cyc thermal fatigue tests. This difference was mainly due to the combined effects of different thermal stress caused by the anisotropy of single crystals and of the microstructure characteristics.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：选用某第二代镍基单晶高温合金，制备了平行于定向凝固方向且分别沿(100)面和(110)面的2组板式试样,采用电火花方法在试样中心垂直于板面加工孔径为0.5 mm的圆孔。采用室温到1100 ℃的冷热疲劳实验，研究了冷热循环过程中不同晶体学平面的板式试样上圆孔周围裂纹萌生及扩展行为。结果表明，位于不同晶体学平面的板式试样中，电火花方法加工圆孔周围产生了一薄层再铸层，再铸层最厚处约15 um。板式试样所在晶体学平面对圆孔周围裂纹萌生及扩展行为影响显著。冷热循环80 cyc后, (110)面试样中，裂纹在与枝晶生长方向垂直的孔边产生，之后，迅速沿与枝晶生长方向呈45°角扩展。而对于(100)面试样，即使经过200 cyc冷热循环后孔边也未观察到裂纹，造成此差异的本质原因是单晶高温合金晶体结构的各向异性导致的热应力差别与微观组织特征共同作用的结果。
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