Fatigue Crack Growth and Fracture Toughness in Bimodal Al 5083
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Q1.8.1
Fatigue Crack Growth and Fracture Toughness in Bimodal Al 5083 P.S. Pao, H.N. Jones, and C. R. Feng Materials Science and Technology Division, Naval Research Laboratory Washington, DC 20375, U.S.A. ABSTRACT The fatigue crack growth rates and fracture toughness of bulk nanocrystalline Al 5083 having a bimodal grain size distribution were investigated. The nanocrystalline powders were prepared by mechanically ball milling spray atomized Al 5083 powders in liquid nitrogen. This nanocrystalline powder was blended with 50 wt% spray atomized large grained Al 5083 powders. The blended powder was then cold pressed, degassed, and extruded into rods. The bimodal Al 5083 thus produced consists of nanocrystalline grain bands and coarse grain bands. While the yield strength of the bimodal Al 5083 is about 25% lower than that of the all nanocrystalline Al 5083, its tensile ductility is almost 50% greater. In addition, the fracture toughness of the bimodal material is about 85% higher than that of the all nanocrystalline counterpart. Fatigue crack growth rates of bimodal Al 5083 are about 30% lower than those of all nanocrystalline Al 5083. The lower fatigue crack growth rates are accompanied by more tortuous crack paths when the crack propagated through the coarse grain regions in the bimodal Al 5083. INTRODUCTION Thin film nanocrystalline materials are currently under intense development because of their potential in offering unique and better electronic, magnetic, optical, chemical, and/or biological properties. Bulk metallic structural nanocrystalline materials, although they demonstrate good mechanical and wear-resistance properties, have received only limited attention because of the difficulties in scaling up production of nanocrystalline materials. Recently, cryomilling has emerged as a promising technique for producing large quantities of metallic nanocrystalline particulates. Bulk nanocrystalline materials such as Al-7.5Mg and Al 5083, which are produced by cryomilling, exhibit tensile strengths in excess of 700 MPa [1-5]. However, these high tensile strengths are accompanied by low tensile ductility, low fracture toughness, and high fatigue crack growth rates. A recent study has demonstrated that a bimodal microstructure Al-7.5Mg, which consists of a mixture of nanoscale and microscale grains, has significantly higher tensile ductility with only slight reduction in tensile strength when compared to an all-nanocrystalline Al-7.5Mg [6]. Though it hasn’t been established, the bimodal microstructure is also expected to interact and interfere with crack propagation, and thus, may improve the fracture toughness and lower fatigue crack growth rates. In the present investigation, the microstructure, the fracture toughness, and the fatigue crack growth rates of a bimodal microstructure Al 5083, with 50 wt% nanoscale and 50 wt% microscale grains, were studied and were compared to those of allnanocrystalline alloy.
Q1.8.2
EXPERIMENTAL PROCEDURES The bimodal Al 5083 (Al-4.5Mg-0.7Mn-0.3Cr) and nanocrystalline Al 5083 used
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