The effect of the precipitation of coherent and incoherent precipitates on the ductility and toughness of high-strength
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I.
INTRODUCTION
RESISTANCE to brittle fracture is a major concern in the design of structural materials. From an engineering point of view, the improvement of toughness for structural materials has been attempted by various methods, including the addition of alloying elements, m special heat treatments, t2,3m or thermomeehanical treatments, ~5-9l along with production methods [J~ that reduce impurity elements or nonmetallic inclusions which are harmful to the toughness of materials. From a material science point of view, in order for high-strength materials to obtain reasonable toughness, it is important to understand the effect of precipitate phases on plastic deformation behavior, such as slip character and slip length. For example, high-strength aluminum alloys, of which toughness is a matter of concern, are the 2*** type A1Cu-Mg alloys, 7*** type A1-Zn-Mg-Cu alloys, and A1-Li-type alloys. These are high-strength alloys of the age-hardenable type for aircraft material. High-strength steels, ~2J such as PH15-7Mo, 17-7PH, A-286, and 18Ni maraging steel, are also hardened by the precipitation of intermetallic compounds. These high-strength materials show the lowest toughness and the maximum strength at the peak-aged condition, ~j3,~qlwhere they are hardened by the precipitation of coherent precipitates. The coherent precipitates are easily cut by moving dislocations, and when sheared once, slip deformation is concentrated in a particular slip plane. This leads to inhomogeneous deformation, early crack initiation, I~4-~81and poorer toughness. When, in the overaged condition, the strengthening precipitates are incoherent and can be bypassed by dislocations, homogeneous R. HAMANO, Senior Researcher, is with the National Research Institute for Metals, Tsukuba Laboratories, 1-2-1 Sengen, Tsukuba-shi, Ibaragi-ken 305, Japan. Manuscript received April 29, t991. METALLURGICAL T R A N S A C T I O N S A
deformation and increasing toughness with delayed crack initiation ~181 will result. Accordingly, the high-strength materials are used in the overaged condition with decreased strength. Thus, it is desirable to investigate whether the introduction of incoherent precipitates into the matrix can improve both toughness and strength of the material hardened by the precipitation of coherent precipitates. In order to disperse both incoherent and coherent precipitates into the matrix, materials could be designed to be strengthened by at least two kinds of particles precipitating at different aging temperatures. In the present study, age-hardenable steels derived from NITRALLOY* N t191were used. These steels, containing *NITRALLOY is a trademark of Joseph Ryerson & Son, Inc., Chicago, IL.
nickel, aluminum, chromium, and molybdenum, are strengthened with precipitates of carbide and intermetallic compound. In this article, the effects of the dispersion of coherent and incoherent precipitates on roomtemperature toughness and strength are studied. II.
EXPERIMENTAL PROCEDURES
The alloys used in the study were melted in a vacuum indu
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