Creep behavior of an AZ91 magnesium alloy reinforced with alumina fibers
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I. INTRODUCTION
SUBSTANTIAL progress has been made recently in interpreting the creep behavior of metal matrix composites (MMCs) at elevated temperatures. The important results from these investigations may be summarized briefly as follows. (1) When the steady-state or minimum creep rate, «˙ , is
plotted logarithmically against the applied stress, s, a marked curvature is generally observed when these data extend over more than 5 orders of magnitude of strain rate.[1,2] This curvature leads to exceptionally high values for the apparent stress exponent, na , and the apparent activation energy for creep, Qa , at low strain rates. (2) The occurrence of curvature in the logarithmic plots of «˙ vs s suggests that creep occurs under the action of an effective stress given by s 2 s0, where s0 is a threshold stress.[1–5] When a threshold stress is incorporated into the analysis, the values for the true stress exponent, n, and the true activation energy for creep, Q, are often consistent with those anticipated for creep in the matrix materials.[2,4] (3) Many MMCs are fabricated using powder metallurgy (PM) procedures, and in these materials, the threshold stresses may arise from attractive interactions between mobile dislocations and a dispersion of fine incoherent oxide particles introduced during the atomization process.[1] This is consistent with the observation that similar threshold stresses are also recorded in the unreinforced matrix alloys when they are fabricated using identical PM procedures.[6,7] (4) Similar curvatures in the logarithmic plots of «˙ vs s are obtained in MMCs fabricated using ingot metallurgy (IM) procedures where no oxide particles are present. In these materials, the threshold stresses may arise from
YONG LI, Research Assistant Professor, and TERENCE G. LANGDON, Professor, are with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted November 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
an attractive interaction between mobile dislocations and precipitates present in the lattices of the matrix alloys.[8,9] (5) The creep behavior of MMCs is controlled by deformation in the matrix materials.[8–11] Thus, MMCs fabricated with Al-6061 and Al-7005 as the matrix alloys, having magnesium and zinc as the major alloying elements, exhibit creep properties consistent with unreinforced AlMg and Al-Zn alloys, respectively.[8,9] As in solid solution alloys,[12,13] the creep properties of MMCs divide into two distinct types: class M (metal type) with n . 5 and class A (alloy type) with n . 3.[9] (6) In some MMCs, the creep resistance is higher, and the activation energy for creep appears to be larger than in the unreinforced matrix alloy even after the threshold stresses are incorporated into the analysis.[7,14] This difference arises when a significant load transfer partitions the external load between the matrix and the reinforcement. In the presence of load transfer, the creep data may be successfully r
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