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544 H.-S.Chu et al./Scripta Materialia 45 (2001)541-546 5μm 5um (a) (b) Fig.3.The distribution of particles in two composites:(a)10%Al2O;composite,(b)20%Al2O3 composite. 140- 30 ■ 2 (AH)SSOUPIEH 70 6061 alloy -一6061-10vol%Al0 60- 6061-20vol%A10 504 0.1 10 as-quenched Aging time (hour) Fig.4.The aging hardness curves of the 6061 alloy and the composites. aging response of the composites became much smaller than that of the 6061 alloy.A higher Al2O3 particle content resulted in a lower response.Similar observations on degradation of precipitation hardening concerning the Al2O3 reinforced age-hardenable Al composites,have already been presented in the literature [5,8,9].These findings have been shown to relate to the reaction between the precipitation hardening element,Mg, with Al2O3 particles to form the spinel,MgAl,O4.The capacity for precipitation hardening of the composite is correspondingly reduced since the Mg in the matrix is consumed during this reaction.Fig.5 presents the DSC thermograms for 6061 alloy and composites.Two exothermic peaks appear in the 6061 alloy at 245C and 288C The structures corresponding to the exothermic peaks were B"(or GP-ID)and B'phases, respectively [10,11].For the composites,the thermograms were quite smooth with al- most no exothermic peak,implying that the formations of the B"zone and B'phase were depressed.The lack of the B"zone would diminish the hardening capability of the / ! # 6-#H#A7 0 . / / # +# / # +: 0 + F - I!" 0/ 1 :-" HH" 0 1 / ; K3 22< # 6#7 F # / 8 / 1# 9 / 0 1 9 / F 0 / > ;< D # ;< D F : 0 -:: &'& # &( ) *+ ,- ) ,.) /