A comparison of the creep properties of an Al-6092 composite and the unreinforced matrix alloy
- PDF / 422,425 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 88 Downloads / 192 Views
I.
INTRODUCTION
ALTHOUGH the experimental data are limited, there appear to be significant similarities between the creep properties of discontinuously reinforced metal matrix composites and the matrix alloys. It has been shown that an Al-6061 alloy reinforced with 30 vol pct SiC particulates and an unreinforced Al-6061 alloy exhibit essentially identical relationships between strain rate and stress when plotted on logarithmic scales, except that the measured creep rates in the composite are approximately two orders of magnitude slower than in the matrix alloy.[1,2] Similarly, a recent investigation of an Al-2124 composite reinforced with 10 vol pct SiC particulates revealed creep properties that were consistent with the unreinforced Al-2124 matrix alloy, except that, as indicated also in an earlier more limited report,[3] the strengthening introduced by the SiC particulates appeared to be lost at strain rates above ;1024 s21.[4] These documented similarities between the creep of unreinforced and reinforced alloys provide strong support for the proposition that the creep of discontinuously reinforced metal matrix composites is controlled by deformation within the matrix.[4] Recently, this concept was extended by noting that the rate-controlling process in the creep of solid solution alloys may be either dislocation climb with a stress exponent of ;5 and an activation energy similar to the value for lattice self-diffusion or viscous glide in which dislocations drag solute atom atmospheres giving a stress exponent of 3 and an activation energy associated with interdiffusion of the solute atoms: following the designations for solid solution alloys,[5] these two processes are termed class M (metal type) and class A (alloy type), respectively. The well-established duality of creep processes in solid solution alloys implies that there may be a similar division into two classes of creep behavior in the metal matrix composites, and the validity of this concept was investigated and confirmed by comparing creep data for two different
Al-based composites.[6] Specifically, it was shown that an Al-7005 alloy reinforced with 20 vol pct Al2O3 particulates exhibited class M behavior with a stress exponent of ;4.4, whereas an Al-6061 alloy also reinforced with 20 vol pct A2O3 particulates exhibited class A behavior with n . 3. Since both composites were fabricated using the same ingot metallurgy procedure, and the composites were identical in terms of type (Al2O3), size (;20 mm), and volume fraction (20 pct) of the reinforcement, these differences in creep behavior were attributed to the presence of ;4.5 wt pct Zn and ;1.0 wt pct Mg as the major alloying elements in the Al-7005 and Al-6061 alloys, respectively. Thus, the two composites exhibited creep behavior which was characteristic of either unreinforced Al-Zn[7,8] or unreinforced AlMg[5,9] solid solution alloys. This article reports the results obtained in a detailed investigation of the creep behavior of an Al-6092 composite and an unreinforced Al-6092 alloy, where both mat
Data Loading...
