Abstract
Low cycle fatigue (LCF) and fatigue crack propagation (FCP) tests under creep conditions were carried out on HK40, HP, and their modified alloys containing a small amount of Zr, Nb, and/or Ti. The failure mechanism was established based on the microscopic observations by the scanning and transmission electron microscopies. Also, the mechanical factors affecting the LCF life were discussed, considering the crack propagation law. The main results obtained are as follows;(1) The addition of Nb, Ti and/or Zr improves the LCF life and the FCP resistance under creep conditions. In particular, the effect of Zr is marked.(2) Cracks propagate along the intensive sub-boundaries, on which coarse M23C6 carbides precipitate during the LCF test.(3) Zirconium, Nb and Ti form fine and dispersed secondary carbides (MC) in the matrix of material. These fine carbides lead to a uniform dislocation structure without intensive sub-boundaries and then result in the dispersed precipitation of coarse M23C6 carbides. Accordingly, the addition of these elements improves LCF and FCP properties.(4) From a mechanical viewpoint, the LCF life can be determined by the fatigue and creep crack propagation laws and the strain energy densities which are obtainable based on J-integral analysis on a cracked body.
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