Plastic Flow and Fracture Processes in Powder Metallurgical Nickel-Base Superalloys

摘要

The advent of powder metallurgical methods for nickel-base superalloys has made it possible to use high-strength alloys for large components such as turbine disks. Although much of the manufacturing and process technology for such products is reasonably well understood, many of the features of the structure-property relationships in these alloys remain unclear. This document describes the results of a three-year effort undertaken to provide insight into the factors that control specific properties in advanced powder metallurgical nickel-base superalloys. The first-year effort examined the stress-rupture characteristics of smooth sections of a number of alloys including AF115 and MERL 76. It was concluded that the ability of a material to store dislocations controls the creep rate but that the fracture processes were dictated by grain boundary structures. Models that satisfactorily accounted for the behavior pattern were developed. The second-year effort extended the study to notch-rupture properties which were found to depend upon a larger number of variables. An interesting parallel between short notch lives and the fracture times of smooth sections tested under stress relaxation was found. Based on this observation, certain mechanical cirtieria for poor notch characteristics could be identified.