Effects of processing conditions and test temperature on fatigue crack growth and fracture toughness of Be-Al metal matrix composites

The effects of changes in processing conditions and test temperature on the tension behavior, fatigue crack growth and fracture toughness of a Be-Al metal matrix composite (MMC) containing 70% by volume Be and 30% by volume Al have been determined. Prealloyed Be-Al powders were either hot isostatically pressed (HIP) or CIP/extruded to full density. Testing was conducted in uniaxial tension, three point bend fatigue, and both notched and fatigue precracked toughness, at temperatures ranging from -125 to 225 degreesC. The fatigue crack growth experiments were conducted at R = 0.1, 0.4, and 0.6. Significant effects of processing conditions, test temperature, and R, on the balance of properties were obtained. Tensile and fracture toughness properties of the extruded material were higher than the HIP material. Increasing the test temperature from -125 to 225 degreesC decreased the strength and fracture toughness of both the HIP and extruded materials. Fracture surface observations revealed clear differences between the fracture morphologies in the HIP and extruded materials as well as those at low vs. high temperatures. These differences were used to rationalize the tensile and toughness behavior exhibited. In general, the materials tested exhibited fatigue behavior that is consistent with that of metals, despite the presence of the large volume fraction of Be. a semi-brittle constituent. Values for the Paris law slope and fatigue threshold were in the range of those exhibited by metals. However, changes in test temperature from 225 to 125 degreesC produced significant increases in the Paris law slope for both materials. The results obtained on the Be-Al composite are also compared to that of the constituents (i.e. Be. Al) in order to illustrate some of the benefits obtained by creating the composite structure. (C) 2002 Elsevier Science B.V. All rights reserved.