Fracture and Toughness of Intermetallics

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FRACTURE AND TOUGHNESS OF INTERMETALLICS HORST VEHOFF MPI fuir Eisenforschung, Max Planck Str. 1, D-4000 Diusseldorf, Germany ABSTRACT The difficulties to obtain valid fracture toughness values in brittle intermetallics are discussed. Different intermetallic alloys with the same specimen size were tested in four point bending to allow the direct comparison of the brittleness of different alloys. The influence of grain size, phase distribution, and temperature on the fracture toughness was measured. The fracture toughness of many intermetallic alloys was found to be extremely rate sensitive. This is discussed in view of dynamic models of the brittle/ductile transition developed recently. NiAl single crystals with < 100 >-specimen axis showed the largest toughness at room temperature and compared to other orientations the highest transition temperature. The apparent activation energy for the brittle/ductile transition depends on orientation. Evidence was given that in multiphase alloys the yielding of a second phase initiates interfacial fracture. For polycrystals it was shown that different processes can cause brittle/ductile transitions. INTRODUCTION New intermetallic materials should combine the advantages in material behavior of ceramics (high strength at high temperature) with a high fracture toughness and sufficient ductility at room temperature. Therefore in many countries the development of new and improved intermetalhc alloys is currently under way as is documented in several conference proceedings on intermetallic alloys [1, 2]. The most challenging problem is still the brittleness at room temperature. The purpose of this overview is to review briefly our current understanding of fracture and problems involved to test small quantities of brittle materials. Different methods for precracking and notching are described together with a description of the method used to obtain fracture toughness values. Fracture toughness data for intermetallic single crystals and for multiphase alloys are presented. The influence of temperature, strain rate and texture on fracture toughness is discussed in view of recently developed models for fracture. TOUGHNESS TESTING IN BRITTLE MATERIALS Intermetallics, depending on crystal structure, microstructure or processing, show fracture toughness values which either rise sharply or smoothly with temperature. For toughness testing in the brittle/ductile transition range, standards known from metals, like precracking in fatigue and a sufficient specimen size according to the ASTM-standard must be used. Many high temperature intermetallics show brittle behavior at lower temperatures with toughness values below 5 M Pa 17t2 In these cases, procedures known from the testing of ceramics must be applied. It is known, that in ceramics the notch radius, r, has only a minor influence on the toughness values, Munz et al [3] have found that KIC is independent of r for radii below 66 pm. For intermetallics corresponding data are not measured till today. But the results presented in Fig. 4 show th

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