Effects of Mg content on aging behavior of
sub-micron Al2O3p/Al-Cu-Mg composites
JIANG Long-tao(姜龙涛), ZHU De-zhi(朱德智), CHEN Guo-qin(陈国钦),
XIU Zi-yang(修子扬), WU Gao-hui(武高辉)
School of Materials Science and Technology, Harbin Institute of Technology, Harbin 150001, China
Received 28 July 2006; accepted 15 September 2006
Abstract: 30%Al2O3p/Al-Cu-2.0Mg composite and Al2O3p/Al-Cu-2.5Mg composite with 0.3 μm-Al2O3 particles were fabricated. Age-hardening behaviors of two composites and the related matrix alloys were studied by means of Brinell-hardness measurement, DSC and TEM. The results show that the hardness of the composite is improved obviously because of the addition of sub-micron Al2O3 particles. But the hardness increment of Al2O3p/Al composite after aging is lower than that of the related matrix alloy. Moreover, the formation of GP region is suppressed by the addition of sub-micron Al2O3 particles, which broadens the exothermic peak of S′ phase. The increment of Mg content has a different influence on accelerating the aging processes of aluminum alloys and the composites, and the hardness also increases.
Key words: aluminum matrix composites; sub-micron Al2O3 particles; aluminum alloys; aging behavior
1 Introduction
Aging behaviors of aluminum matrix composites reinforced by micron grade particles have been well developed for about thirty years[1-3]. Many investigations indicated that the existing of thermal mismatch stresses between the reinforcement and the matrix leads to a lot of dislocations within the composites, and they can provide the nucleation and grown of precipitation phases, which will accelerate the aging precipitation behaviors. Although the properties of composites can be evidently improved with smaller reinforcement particles, the researches on high volume content composites reinforced with sub-micron particles and nano particles have not been taken extensively yet for the limited fabrication methods[4,5]. Ten year ago WU et al[6] designed and developed a 0.15 mm-Al2O3/Al composite, which was characterized by no interface reaction in the composite and no dislocation in the matrix alloy[7-9]. Aging behaviors of sub-micron particles reinforced aluminum matrix composites show obvious difference to that reinforced by micron grade particles [10,11], and the most obvious characteristic is that the aging precipitation phases of matrix become smaller and the amount obviously reduces due to the addition of sub-micron particles. However, the researches on the composites reinforced with sub-micron particles are not taken yet, especially, the effects of alloy components alteration on the aging behaviors of composites have not been reported. In this paper, Al2O3p/Al-Cu-2.0Mg and Al2O3p/Al-Cu-2.5Mg composites were fabricated successfully, and the age-hardening behaviors of the related matrix alloy were studied, so that the experimental data for age-hardening mechanics of the composites reinforced with sub-micron particles can be provided.
2 Experimental
The 30%Al2O3p/Al-Cu-Mg composites with 0.3 μm- Al2O3 particles were fabricated by squeeze casting patent technology[6]. The matrix Al-Cu-Mg alloy was fusion alloying, of which the chemical components are shown in Table 1. The Al2O3p/ Al-Cu-Mg composites and the related matrix alloy aging specimens were solution heat treated at 495 ℃ for 1 h, quenched into water, and then aged in the drying device for 100 h at 130, 160, 190 and 220 ℃, respectively.
Table 1 Chemical compositions of matrix aluminum alloy of composite (mass fraction, %)
The hardness was measured by HBV-30A Brinell & Vickers hardness tester made in Laizhou, Shandong Province. In order to ensure the veracity of experimental data, five different points were measured on each specimen. The DSC experiment was carried on SSC-5200 thermal analyzer, RICOH, and the mass of sample was less than 30 mg. The original temperature of DSC was ambient and terminal temperature was 400 ℃ with a rate of 10 ℃/min. The microstructure was observed using a Philips CM-12 TEM and a JEOL 200CX TEM. Composite aging specimens were prepared using a Gattan-600 ion mill. The matrix alloy specimens were thinned by jet-polishing with a solution of 33%HNO3 and 67%CH3OH at a voltage of 20 V at -20 ℃, and the cooling fluid was alcohol.
3 Results and discussion
3.1 Age-hardening rules of Al2O3p/Al-Cu-Mg compo- site and matrix alloy
Figs.1(a) and (b) show the age-hardening curves of 30%Al2O3p/Al-Cu-2.0Mg composite and Al2O3p/Al-Cu- 2.5Mg composite after solution heat treatment at 130, 160, 190 and 220 ℃, respectively.
For Al2O3P/Al-Cu-2.0Mg composite, it can be seen from Fig.1(a) that the aging curves of composite and the related matrix alloy are very similar at 130 ℃. And the shapes of curves are flat and uplifted slightly, but the age-hardening peak has not appeared obviously. When the temperature reaches 160 ℃, the aging curves of composite and the related matrix alloy are similar and the age-hardening peak appears obviously. The age- hardening peak of composite appears at a time of 8.5 h and that of the matrix alloy appears at 10 h. After the peak aging, the hardness of both composites begins to descend slowly. At the state of peak aging, the hardness of matrix alloy is HB103, and the hardness of composite is HB259, which is 150% higher than that of matrix. Moreover, the highest hardness of composite is achieved at the four aging temperatures. As the aging temperature rises continually to 190 ℃, the age-hardening curves of composite and matrix alloy differ a lot with those at lower temperature. It shows intenerating phenomenon after a short time aging. And the hardness of composite decreases rapidly with time elongation, while at a stage, it decreases very slowly. For the matrix alloy, the age-hardening peak keeps ahead, but just a little, with a value of HB118. The highest hardness of matrix alloy is obtained at the four aging temperatures. When the temperature reaches 220 ℃, the alternation rules of age- hardening curves of composite and matrix alloy are similar to those at 190 ℃. The overaging phenomenon of composite appears at the beginning of aging. The matrix alloy becomes soft and the hardness decreases rapidly. While at a certain stage, the hardness decreases slowly and keeps invariable. But the age-hardening peak of matrix alloy keeps ahead obviously, the hardness descends rapidly when the peak aging is past after 3 h. Therefore, the best heat treatment processes of Al2O3p/Al-Cu-2.0Mg composite and the matrix alloy are keeping at 160 ℃ for 8.5 h and keeping at 190 ℃ for 10 h, respectively.
Fig.1 Age-hardening curves of composite and related matrix alloy: (a) Al2O3p/Al-Cu-2.0Mg composite and related matrix alloy; (b) Al2O3p/Al-Cu-2.5Mg composite and related matrix alloy
It can be seen from Fig.1(b) that the alternation rules of age-hardening curves of 30%Al2O3p/Al-Cu- 2.5Mg composite and the related matrix alloy are similar to those of Al2O3p/Al-Cu-2.0Mg composite and the related matrix alloy. But, the change of the alloy component accelerates the aging precipitation course of composite, which causes the aging peak of composite to appear in advance at the same temperature. Therefore, the best heat treatment processes of Al2O3p/Al-Cu- 2.5Mg composite and the matrix alloy are keeping at 160 ℃ for 5 h and keeping at 190 ℃ for 8.5 h, respectively.
The above results indicate that the hardness of composite is one time higher than that of the matrix alloy. The age-hardening effect is not obvious at a low temperature. With temperature increasing, the age- hardening effect improves. While at a higher temperature, the age-hardening ability decreases distinctly. And it is noticed that the addition of particles within the composite can improve the aging behaviors and accelerate the aging dynamics of matrix alloy.
Fig.2 shows the aging-hardening rate curves of two composites and the related matrix alloys under different temperatures. It is found that, as the sub-micron Al2O3 particles are added to the Al-Cu-2.0Mg and Al-Cu-2.5Mg alloy, the hardness of the composite is obvious higher than that of matrix alloy when the aging hardening curves are compared with aging hardening ratio curves of composites and the matrices. At a lower aging temperature, the aging processes of composites and matrix alloys are slow, while at a higher aging temperature, the aging processes of composites and matrix alloys show obviously acceleration.
Fig.2 Aging hardening rates of two composites and related matrix alloys under different temperatures: (a) Al2O3p/Al-Cu- 2.0Mg composite and related matrix alloy; (b) Al2O3p/Al-Cu- 2.5Mg composite and related matrix alloy
Compared with the highest hardness of Al2O3p/Al-Cu-2.0Mg composite and Al2O3p/Al-Cu- 2.5Mg composite as well as the related alloy, it is found that with the Mg content increasing, the hardnesses of composite and matrix alloy are both increased. Considering the Cu content unchanged, the increasing of Mg content will cause the amount of S′ phase improved. As the S′ phase is dominant within the alloy, the hardness will increase slightly.
3.2 DSC analysis
The DSC curves on continual heating of 30%Al2O3p/Al-Cu-2.0Mg composite and Al2O3p/Al-Cu- 2.5Mg composite and the related matrix alloys after solution heat treatment are shown in Fig.3. It can be seen from Fig.3(a) that two exothermic peaks exist in the DSC curves of Al-Cu-2.0Mg alloy. It can be determined that the first exothermic peak located at low temperature is generated when the GP region forms, and the other exothermic peak located at high temperature is generated as S′ phase forms. However, only a very flat exothermic peak exists in the composite DSC curve, which forms when the S′ phase precipitates within the composite. And the two exothermic peaks existed in the DSC curve of Al-Cu-2.5Mg alloy are also seen clearly, which are similar to those of Al-Cu-2.0Mg alloy. The exothermic peak temperatures of S′ phase of two composites are put ahead than those of the matrices.
For Al-Cu-Mg ternary alloy, aging precipitation course of supersaturated solid solution is a complex phase transition course and controlled by diffusion. The GPB region is formed by means of vacancy diffusing, and it is affected directly by the density of vacancy. When a lot of smaller particles introduce, many interfaces appear within the composite and dislocations generate at the asymmetry region of particles and matrix. The defaults will dissolve and absorb lots of quenching vacancies, which can make the nucleation and growth up of GPB region difficult. This reflects on the DSC curves that the nucleation exothermic peak is not obvious or delayed to high temperature, or overlapped with the precipitation peak of S′ phase. With the increase of temperature measured, it becomes possible that the indirect diffusion of Cu and Mg atoms within the matrix alloy turns to be direct diffusion. Because of lots of interfaces with larger surface areas and dislocations existed in the composite, the diffusion channels of Cu and Mg atoms increase greatly, and the dislocations can provide vantage point for asymmetry nucleation. The diffusion course of solute atoms is easier and more rapid than that of matrix alloy. This reflects on the DSC curves that the precipitation exothermic peak of S′ phase is put ahead comparatively.
Fig.3 DSC curves of Al2O3p/Al-Cu-Mg composite and matrix alloy with continual heating: (a) Al2O3p/Al-Cu-2.0Mg compo- site and matrix alloy; (b) Al2O3p/Al-Cu-2.5Mg composite and matrix alloy
3.3 TEM observation
The TEM photographs of Al-Cu-2.5Mg alloy, 30%Al2O3p/Al-Cu-2.0Mg composite and Al2O3p/Al-Cu- 2.5Mg composite under the condition of their respective peak aging are shown in Fig.4. Fig.4(a) shows the TEM photograph of Al-Cu-2.5Mg alloy aged at 190 ℃ for 8.5 h. It can be seen that lots of S′ phase particles with lathing shape exist and distribute as oblique crossing. Fig.4(b) shows the TEM photograph of Al2O3p/Al-Cu- 2.0Mg composite aged at 160 ℃ for 8 h. Two different structures exist at the interfaces: one is the smaller precipitation phase nucleated by clinging to interface, and the other is the reactant of gradual nucleation and growth up. A few of precipitation phases with larger size exist in matrix alloy, while more nanometer grade structures magnified like GP region can be seen. Fig.4(c) shows the TEM photograph of Al2O3p/Al-Cu-2.5Mg composite aged at 160 ℃ for 8 h, of which the shape is similar to that of Al2O3p/Al-Cu-2.0Mg composite with peak aging. Few precipitation phases with larger size and many fine GP regions magnified exist, as shown in Fig.4. Moreover, the precipitation phases take on gradual growing trends.
Fig.4 TEM photographs of Al-Cu-2.5Mg alloy, 30%Al2O3p/ Al-Cu-2.0Mg and 30%Al2O3p/Al-Cu-2.5Mg after respective peak aging: (a) Al-Cu-2.5Mg alloy; (b) Al2O3p/Al-Cu-2.0Mg; (c) Al2O3p/Al-Cu-2.5Mg
By comprehensive analysis, the aging rules of composite are obviously different with those of matrix alloy. The addition of lots of particles changes the alloy microstructures intensively, which shows the exiguity of the dislocations and the introduction of a large amount of interfaces as well as the reactants generated in the interfaces during the course of casting. At the stage of the peak aging, the S′ phase of matrix alloy precipitates much more and the size is larger. But the composite at peak aging exhibits a different characteristic that the sizes of precipitation phases are small and lots of fine structures of GP regions magnified are observed. The TEM photographs at peak aging of both composites have the same characteristic. It is considered that the peak aging is formed due to the highest hardness caused by a great deal of GP regions existed. Because of its complete coherence with the parent phases and a high density, a lot of elastic strain fields exist in the matrix of the composite, which baffles the movement of the dislocations intensively and improves the hardness of the composite. With the aging time prolonging, the structure of GP region grows up and the precipitation of S′ phase is accelerated largely. With the growth of the precipitation, the degree of coherence decreases, the density is lower and the strengthening effects wear off. So the hardness decreases gradually as shown in the hardness curve. Moreover, the aging processes are shortened largely with increasing the aging temperature. It is shown that the transition phase is changed shortly at high aging temperature and the equilibrium phase forms, which is incoherent with matrix, and the hardness of composite decreases. It can be seen that the primary precipitation structures are fine at peak aging and keep coherence with matrix as well as serious crystal lattice aberrance. Otherwise, the solution strengthening effect of matrix is also better and the integration action makes the hardness of macro properties the highest. Moreover, the reason why the hardness increment of matrix and composite induced by the addition of Mg element is larger has been discussed above. The Al-Cu-Mg alloy is characterized by the strength and hardness increasing with the Mg content, and the dominant strengthening effect of S′ phase enhances, which is approved by contrasting the amount and size of precipitation phases of Al2O3p/Al-Cu-2.0Mg composite with those of Al2O3p/Al-Cu-2.5Mg composite. It can be seen that the hardness of alloy and composite after heat treatment (aging) can be improved by increasing Mg content of matrix alloy.
4 Conclusions
1) With the increasing Mg content of matrix alloy, the aging processes of sub-micron composites and the related matrix alloys are accelerated on different degrees, and the aging peak is put ahead greatly. The highest hardness of composites and matrix alloys are increased with Mg content.
2) The GP region and exothermic peak formed by S′ phase exist obviously within the matrix alloy by DSC analysis. The GP region of composite is suppressed, while the exothermic peak of S′ phase is put ahead and broadened.
3) TEM observations indicate that the hardening effect of alloy is dominant by the precipitation of transition phase, and the hardness improvement of composite is attributed to the aging strengthening and solution strengthening.
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(Edited by HE Xue-feng)
Foundation item: Project(50071019) supported by the National Natural Science Foundation of China; Project(HIT2002.34) supported by the Science Research Foundation of Harbin Institute of Technology, China
Corresponding author: JIANG Long-tao; Tel: +86-451-86402372-5055; Fax: +86-451-86412164; E-mail: jlongtao99@163.com
