Study On Small Fatigue Crack Initiation And Growth Mechanisms Under Microscale Condition

GH4169 nickel-based superalloy is extensively used in aircraft engine turbine disks and blades as structural material. One type of in-service damage to these components is low cycle fatigue cracking in the disc root fixtures. There are some notch-like features in the root fixtures, so lifing of the disc must consider the behaviour of cracks in notched specimens. Although the stages of crack initiation and small crack propagation accounted for more than 70-90% of the total fatigue life, experimental studies on small fatigue crack growth behavior of GH4169 are relatively few in the open literature, not to mention investigations on notched specimens. Hence, the aim of this thesis is to investigate the small fatigue crack initiation and growth mechanisms in the notched specimens of GH4169. The main conclusions have been drawn as following:(1) A complete process from crack initiation and propagation of 304 austenitic stainless steel is recorded by using a new two-part silicon based replica method. When comparing with the traditional acetate tape method, the advantages of the two-part silicon based replica method are accurate, high resolution and has no effect on the fatigue life of the specimen..(2) Fatigue tests of 304 austenitic stainless steel and GH4169 nickel-based superalloy at room temperature are conducted. Results show that the stages of the initiation and slowly growth of small crack take up about 60-80% of the fatigue lifetime. Once the surface small crack length reached the critical crack size of 0.2 mm, it would grow rapidly. When plotting the small crack data as a function of surface crack length, the stress level effect on the small crack growth rate of 304 austenitic stainless steel for the stress range of 330-370 MPa is not distinguishable. Similar results are obtained for the GH4169 nickel-based superalloy with the stress range of 300-340 MPa. When the stress ratio is positive, the small crack growth data will show significant difference only when the difference between high and low stress level is large enough.(3) The effect of grain size on the small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169 is investigated. Results show that fatigue crack initiation mechanisms are dependent on the grain size. Fatigue cracks preferentially initiate at inclusions for the specimens with relatively small grain size, i.e.,9 μm, but they tend to initiate at grain boundaries for the specimens with 25 μm grain size. There is a transition of fatigue crack initiation mechanism between fine grain size and coarse grain size. The small fatigue crack growth rates are almost constant across a wide range of crack lengths. Grain size effect on the small crack growth rates is not distinguishable when plotting the small crack growth rate as a function of surface crack length..(4) Experimental investigations of nickel-based superalloy GH4169 at 650℃ in air show that fatigue cracks tend to initiate at oxidized inclusions. The precipitation of 8 phases may help in the improvement of fatigue resistance and the total fatigue lifetime of GH4169. The crack incubation period takes up more than 90% of the fatigue lifetime. The fatigue crack propagation processes of the direct aging heat treatment specimen and the special heat treatment specimen can be divided into three stages, and the δ phase precipitated along grain boundaries may hinder the oxidation induced embrittlement, resulting in the transition from intergranular fracture of direct aging heat treatment specimen to mixed transgranular-intergranular fracture of special heat treatment specimen. The nominal stress intensity factor range, △K may be an applicable parameter to correlate the small crack and long crack growth rates.