Resistance against Creep-Fatigue Realized by Anisotropic and Composite Structure

为了研究提高耐热合金的高温疲劳和蠕变疲劳抗力的方法，对γ相和α 2相的层状组织和柱状（或细长）晶粒等各向异性组织的金属间化合物TiAl和单向凝固Ni基高温合金Mar-M247 LC进行了疲劳和蠕变疲劳试验。以下是TiAl的结果。(1)In在高温疲劳的早期阶段，试样表面的铸造缺陷和晶粒内的层状界面处均萌生微裂纹。(2)As大部分起始于片层界面的微裂纹被阻止，主裂纹由起始于较大铸造缺陷的裂纹形成。（3）当微裂纹扩展速率分散较大时，其畸变速率与宏观扩展试验所得到的断裂力学规律相吻合。 ...更多信息 使用带缺口的样品架。通过单向凝固的Mar-M247 LC的蠕变-疲劳试验，阐明了以下几点：(1)In具有平行于凝固方向的单轴加载的蠕变疲劳（即，O度试样），微裂纹在局部垂直于应力轴方向的波状晶界的小段处萌生。（2）大部分裂纹在裂纹尖端到达垂直段末端时停止。裂纹在晶界外扩展到晶粒内，成为最大的裂纹之一，导致试样最终断裂。（3）还进行了相对于凝固方向改变加载方向（即，使用45度和90度试样）。与加载方向无关，微裂纹总是在垂直段萌生。在相同的蠕变-疲劳条件下，90 °试样的裂纹数最多，垂直段比例最大，其寿命最短。少

In order to examine a method for attaining higher resistance of heat-resisting alloys against fatigue and creep-fatigue at high temperatures, fatigue and creep-fatigue tests were carried out for an intermetallic compound TiAl and a unidirectianally solidified Ni-based superalloy Mar-M247LC.Both alloys are composed of anisotropic structure, such as lamellate structure of gamma and alpha2 phases, and columnar(or elongated)grains. The followings are the results obtainde for TiAl. (1)In the early stage of fatigue at high temperature, microcracks were initiated on the surface of the specimen both at casting defects and at lamellar interfaces in the grain. (2)As most of the microcracks initiated at lamellar interfaces were arrested, the main crack was formed by one of the cracks initiated at relatively large casting defects. (3)While the growth rate of the microcracs was scattered widely, the aberage rate agreed with the fracture mechanics law which was obtained by propagation tests of macro … More rack using notched specimens. It is therefore possible to estimate the fatigue life of TiAl by fracture mechanics.The followings were clarified by the creep-fatigue tests for unidirectionally solidified Mar-M247LC.(1)In creep-fatigue with uniaxial loading parallel to the solidification direction(i.e., O degree-specimen), microcracks were initiated at small segments of wavy grain boundary where the grain boundary was locally perpendicular to the stress axis direction. (2)Most of the cracks were arrested when the crack tips reached the end of the perpendicular segment. The crack, which grew into the grain out of the grain boundary, became one of the largest cracks and brought about the final fracture of the specimen. (3)Creep-fatigue tests were also carried out changing the loading direction against solidification direction(i.e., using 45 degree-and 90 degree-specimens). Independent of the loading direction, microcracks were always initiated at the perpendicular segments. As the proportion of the perpendicular segments becomes the largest, the number of cracks was maximum in 90 degree-specimen, and this specimen showed the minimum life among three specimens under the same creep-fatigue condition. Less