The Influence of Microstructure on Brittle Fracture Toughness
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The Influence of Microstructure on Brittle Fracture Toughness
G.T. HAHN
This lecture presents viewpoints of the role of microstructure in brittle fracture that have emerged in the past two decades. The fracture mechanics concept of crack arrest is inserted into the Griffith theory of microcracks to describe the resistance of microstructural boundaries. The implications of crack blunting are related to the essential role of carbide particles and other brittle phases in the steel. Dislocation pile-ups and the ferrite grain size are accorded a diminished role. The brittle fracture stress is related to the dimension of the largest "eligible" parent particle and the resistance experienced when the advancing microcrack crosses the boundary. "Eligibility" is connected with the probabilities of nucleating a crack and finding a boundary with minimal resistance. The boundaries of carbide particles, carbide films at the grain boundaries, brittle inclusion particles, ferrite grains, pearlite colonies, and bainite packets are possible barriers. The use of the growing data base of Griffith energy values--or equivalently, the local arrest toughness value--to identify controlling microstructural features is demonstrated. The lecture touches on the relation between the transition temperature and the brittle fracture stress and draws attention to the latter's dependence on the size of the stressed volume. Finally, the role of the stressed volume in analyses of the brittle fracture K~c-value and a statistical treatment that clarifies its origin are discussed. These analyses indicate that a small number of the largest particles may have disproportionate influence on K~c, and that the microstructural features that effect "eligibility" may have a modest effect on Klc.
The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. G. T. HAHN, a native of Vienna, Austria, came to this country at the age of 8. He received his primary and secondary schooling in Manhattan and at the Brooklyn Technical High School. He studied mechanical engineering at New York University (B.M.E., 1952) and metallurgy at Columbia University (M.S., 1955) and the Massachussets Institute of Technology (Sc.D., 1959). In 1951 and 1952 he worked for short periods of time for the NYU Research Division and the Westinghouse Research Laboratories. From
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1953 to 1955 he served in the Titanium Section of the Air Force Materials Laboratory. In 1960, after graduate studies and a 1-year post doctorate at M.I.T., he assumed a research position at Battelle's Columbus Laboratories where he headed the Metal Science Group from 1966 to 1979. This period also included brief assignments with Battelle's Geneva Research Center (1963) and the Argentine Atomi
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