Age hardening behaviour of 2014 Al alloy-sic composites: Effect of porosity and strontium addition

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 18, February 2011, pp Age hardening behaviour of 2014 Al alloy-sic composites: Effect of porosity and strontium addition Anshul Badkul, Nidhi Jha, D P Mondal*, S Das & M S Yadav Advanced Materials and Processes Research Institute (CSIR), Bhopal , India Received 2 December 2009; accepted 13 January 2011 The aging characteristics of 2014 aluminum alloy-sic composites with and without strontium addition are studied and compared with that of 2014 Al-cenosphere syntactic foam (Al-Cenosphere composite) and 2014 alloy-sic composite foam in order to examine the effect of strontium addition and porosity on aging behavior. It is observed that primary peak aging time of strontium added 2014 Al-SiC composite is same to that of 2014 Al-SiC composite. But, strontium added 2014 Al- SiC composite shows secondary ageing unlike that of 2014 Al-SiC composite. Again, 2014 Al-SiC composite foam and cenosphere added syntactic foam get peak aged almost at same time, But relatively at slower rate than dense composites. It is observed that the strontium added composite also showed a peak at 10 h for secondary hardening due to delayed precipitation of AlSr 3. Keywords: Age hardening, Microhardness, Porosity, Precipitation, Syntactic foam, Aluminum matrix composites, Aluminum foam Aluminum metal matrix composites (AMMCs) have gained considerable attention because of their improved physical and mechanical properties. AMMCs are used as advanced engineering materials for application in aerospace, defence, automotive and consumer industries 1-5. Ageing treatment has been used as an effective method for obtaining stabilized microstructure and improvement in strength and hardness 6,7. Age hardening efficiency of the composites depends on the shape, size and volume fraction of the reinforcement, distribution of particles and ageing temperature Aluminum composite foam having low density has also gained considerable attention because of the porous nature, and excellent combination of mechanical properties and energy absorption. The mechanical properties of the aluminum foam are influenced by chemical composition and microstructure of matrix with which it is made of, casting process and microstructure including grain size, porosity and density. However, detailed studies on the ageing behavior of Al foam are lacking. Dislocation and excessive vacancies, introduced in the AMMC due to thermal mismatch strain between matrix and reinforcement during quenching, increases the precipitation kinetics vis-a-vis aging *Corresponding author: ( mondaldp@yahoo.com) kinetics Whether, it is also applicable to the Al-foam has not been investigated so far. The rare earth materials like strontium, scandium, etc. lead to grain refinement of aluminum alloy and these could be used for making composites with improved mechanical properties. However, the aging behavior of 2014 Al-SiC composites with strontium addition has also not been explored. In this study, aging behavior of 2014 Al alloy-sic composites with and without strontium addition has been compared. These have been further compared with that of 2014 Al-cenosphere syntactic foam and 2014 Al-SiC composite foam. The ageing behavior of the investigated material was determined by measuring the microhardness as a function of ageing time 18,19. Scanning electron microscope (SEM) was used to examine the microstructure of the material prior to and after ageing. Experimental Procedure Materials The materials examined in this work were 2014 aluminum alloy-sic composite with and without strontium addition and cenosphere reinforced 2014 Al-syntactic foam and 2014 Al-SiC Aluminum composite foam. The composition of 2014 aluminum alloy is shown in Table aluminum alloy-sic

2 80 INDIAN J. ENG. MATER. SCI., FEBRUARY 2011 Table 1 Chemical composition of 2014 Al alloy (in wt %) Si Fe Cu Mn Mg Cr Zn Ti Al composites with and without strontium addition are prepared by stir-casting technique. In this technique, aluminum alloy ingot pieces were heated to its molten state. After that, SiC particles were dispersed in the melt along with strontium particles (1 wt %). A vortex was created using mechanical stirrer. Stirring is continued for about 10 min after addition of SiC and Sr particles for uniform distribution in the melt. Castings were prepared by pouring the melt in preheated permanent mould of 20 mm diameter and 100 mm length Al-cenosphere syntactic foam was also prepared using the same methodology. During preparation of 2014 Al-SiC composite foam, foaming agent of CaH 2 (1 wt %) was added into the composite melt through mechanical stirring. After completion of the mixing, melt is held at 670 C for 30 s for foaming and finally the foam is rapidly cooled with force air. Age hardening Ageing of the samples involved solutionizing at 495 C for 8 h and then cooled in water followed by artificial ageing at 185 C for varying length of time (2-10 h). The microstructural changes after age hardening are studied using scanning electron microscope (SEM). For SEM examination, all the samples are mechanically polished using standard metallographic technique and then etched with Keller s reagent. Prior to SEM examinations, samples were sputtered with gold. Age hardening behavior was studied through micro-hardness measurement on the samples as a function of aging time. Micro-hardness testing was done at a constant load of 10 gf to have smaller indentation as compared to the cell wall thickness and also to get microhardness near to the matrix-reinforcement interface. The surface being tested is polished using standard metallographic technique and etched with Keller s reagent. The micro-hardness is taken on the matrix region of the composites or foam. In case of composites and foams microhardness were taken in the matrix region at random locations with respect to the interface so that some of the indentation come near to the interface. At least 25 readings of micro-hardness were taken on each sample and their average was considered for the analysis. Results and Discussion Material and microstructures The microstructures of 2014 Al-SiC composite with and without strontium addition are shown in (Figs 1a and 1b) respectively. It is noted that in both the cases, SiC particles are fairly and uniformly distributed. But, in case of strontium added 2014 Al-SiC composite, the grains are relatively finer than that of 2014 Al-SiC composite without strontium addition. Furthermore, relatively greater degree of agglomeration of SiC particle is noted in case of strontium added composite. The interface bonding between the matrix and SiC particles are quite sharp indicating reasonably good bonding (Fig. 1c) in both the cases. The microstructure of 2014 Al-cenosphere syntactic foam also exhibits uniform distribution of cenosphere in the matrix (Fig. 1d). The microstructure of 2014-SiC composite foam is shown in (Fig. 1e). It exhibits cells (marked C ), cell wall (W) and clear plateau region (P) and SiC particle within the cell wall and plateau region. In the foam, the interface between SiC particles and matrix is associated with porosity (gaps) indicating weak bonding. It is further noted from Fig. 1e that the cell wall thickness of Al-SiC composite foam is ~100 µm. The density, average cell size, SiC percentage, cenosphere shell thickness, porosity of the investigated materials is shown in Table 2. It is noted that the porosity in 2014 Al-cenosphere are much finer than that in 2014 Al- SiC composite foam, but, the former one has relatively less porosity than the later one. Age hardening behavior of the investigated materials was studied by examining the variation of average microhardness of the matrix as a function of aging time as shown in (Fig. 2). It could be noted from this figure that 2014 Al-SiC composite in dense condition aged much faster than those of 2014 Al-cenosphere syntactic foam or 2014 Al-SiC composite foam. The primary aging kinetics (when CuAl and CuAl 2 precipitates formed) of the dense 2014 Al-SiC composite is same and invariant with strontium addition Al-SiC composites with and without strontium addition peak aged at same time. But some differences are observed in the aging behaviour of these materials. Strontium added 2014 Al-SiC composites exhibits less hardness under primary peak aging condition as compared to that observed in composite without strontium. Additionally, the strontium added composite shows the tendency of

3 BADKUL et al.:age HARDENING BEHAVIOUR OF 2014 AL ALLOY-SIC COMPOSITES 81 Fig. 1 Microstructure of (a) 2104 Al-SiC composite (b) 2014 Al-SiC composite with Sr addition (c) Interface between 2014 Al matrix, SiC particle (d) 2014 Al-Cenosphere Syntactic foam and (e) 2014 Al SiC composite foam Table 2 Density, cell size, SiC percentage, cenosphere shell thickness and porosity of investigated material Materials Density g/cm 3 Cell size (mm) SiC percentage (%) Cenosphere shell thickness (µm) Porosity (%) 2014 Al-SiC Al-SiC-Sr Syntactic Foam ± ± SiC Composite Foam ± secondary hardening at aging of 10 h. After peak aging, both the composites show almost same rate of decrease in micro-hardness signifying that the growth rate of precipitate in over aged regime (after primary aging) is invariant to strontium addition. Lower micro-hardness values in the primary aging in strontium added 2014 Al-SiC composite may be due to greater degree of clustering (Fig. 1b) and some amount of copper in the matrix did not get precipitated due to the presence of strontium. Thus,

4 82 INDIAN J. ENG. MATER. SCI., FEBRUARY 2011 dislocation density is expected to be reduced significantly as compared to that in dense one. This, along with relatively weak bonding between SiC/mullite and matrix causes slower rate of ageing. The under ageing and over ageing characteristics of all these foams are noted to be almost same. The residual stress (σ r ) generated due to thermal mismatch of the reinforcement and the matrix is expressed as 20,21 : σ r = αgb (1) Fig. 2 Variation of microhardness as a function of ageing time the microstructure shows relatively less amount of precipitate in strontium added composite as compared to that in the other one. The precipitation of AlSr 3 and/or CuAlSr intermetallics starts at later stage as the diffusivity of strontium in aluminum is much less as compared to that of copper in aluminum. This is because of the fact that strontium is considerably different from aluminum in terms of valency, crystal structure, atomic diameter etc. which controls the diffusivity. Additionally, finer grain size results in greater extent of grain boundary area. Similarly, relatively weak interface bonding (especially in the clustered regions) causes wider interfacial area and more porosity. All these factors act as sites for dislocation sinks and there is a possibility of less dislocation strengthening in the matrix of strontium added composite. This also causes delay in secondary ageing in strontium added composite and lower micro-hardness in primary peak ageing. The porous structure, on the other hand, aged at slower rate as compared to the dense composites. Both foams, i.e., 2014 Al-SiC composite foam and 2014 Al-cenosphere syntactic foam, exhibit almost similar nature of ageing behavior, where in the matrix of 2014 Al-SiC composite foam exhibits higher micro-hardness as compared to the matrix of 2014 Al-cenosphere syntactic foam. This may be attributed to relatively greater extent of difference in thermal expansion coefficient between 2014 Al alloy and SiC as compared to that between 2014 Al alloy and mullite, and SiC particles are harder and stiffer than mullite. This leads to higher degree of residual stress in 2014 Al-SiC composite foam as compared to that in 2014 Al-cenosphere syntactic foam. Slower rate of ageing in these materials as compared to that in dense material is attributed to the presence of huge porosity which acts as dislocation sink and thus, the Where α is the difference in thermal expansion coefficient of the matrix and reinforcements, G is the bulk modulus, b is the Burger s vector of the matrix and T is the temperature difference due to cooling. The thermal expansion coefficient SiC and mullite are /K and /K respectively. As a result, SiC reinforced composite will provide ~20% more residual stress in the matrix as compared to mullite reinforced composites. Out of these, some amount stress of gets released in case of foams. As a result, overall improvement in dislocation density within the matrix is expected to be very marginal and thus considerable differences in ageing kinetics of these two kinds of foams are noticed. The microstructure of 2014 Al-cenosphere syntactic foam after ageing for two hours is shown in (Fig. 3a). It indicates less extent of precipitation and the extent of precipitation increases significantly when the material is aged for 8 h as shown in (Fig. 3b). The precipitates get coarsen when the syntactic foam aged for 10 h (Fig. 3c). Similar types of observation are noted in case of 2014 Al-SiC foam. This demonstrates that both these materials over aged at 10 h of ageing and peak aged at 8 h of ageing time. The microstructure of 2014 Al-SiC composite after aging for 2 h exhibits huge amount of very fine (nano size) precipitates throughout the matrix (Fig. 4a). Some of these precipitates grow and more new precipitates are formed when the material is aged for 6 h as shown in (Fig. 4b). Further ageing for 8 h results in significant coarsening of the precipitates, which formed earlier as shown in (Fig. 4c). In case of strontium added 2014 Al-SiC composite similar trend is observed. Interestingly, when this material is aged for 10 h more precipitate of AlSr 3 are formed in the matrix as shown in (Fig. 4d). The microstructure of strontium added 2014 Al-SiC composite after ageing for 8 h is shown in (Fig. 4e), which exhibits relatively less amount of precipitates as compared to that in

5 BADKUL et al.:age HARDENING BEHAVIOUR OF 2014 AL ALLOY-SIC COMPOSITES 83 Fig. 3 Microstructure of aged 2014 Al - cenosphere syntactic foam after ageing time of (a) 2 h, (b) 8 h and (c) 10 h Fig. 4 Microstructure of 2014 Al SiC composite at different ageing time (a) 2 h, (b) 6 h, (c) 8 h (d) and of 2014 Al SiC composite with strontium addition at 10 h and (e) 2014 Al SiC composite with strontium addition at 8 h

6 84 INDIAN J. ENG. MATER. SCI., FEBRUARY 2011 Fig. 5 EDX of 2014 Al-SiC precipitates for (a) 6 h and (b) 10 h (Fig. 4d). These could be assessed from EDX analysis of the precipitates. The precipitate after ageing for 6 and 8 h do not show the presence of strontium (Fig. 5a), where as EDX of precipitate after ageing for 10 h depicts the presence of strontium. This demonstrates that AlSr 3 precipitates formed later after ageing to 10 h which leads to secondary hardening. Delayed precipitation of AlSr 3 may be attributed to significantly higher atomic radius of strontium, which reduces its diffusivity within aluminum matrix. Conclusions Following conclusions can be drawn from this study: (i) Addition of Sr may change the microstructure of the 2014 Al-SiC composite but the primary aging characteristics remains almost unchanged. But, the addition of Sr causes secondary hardening due to formation of AlSr 3 Precipitates at later stage. (ii) 2014 Al-SiC composite foam and 2014 Al-cenosphere syntactic foam shows almost similar kind of aging behavior and the aging kinetics of these materials are almost same. But the aging kinetics of these foam materials is slower than the dense materials. This may be attributed to the annihilation of a fraction of dislocation due to presence of higher porosity. (iii) Porous materials aged slower than the dense materials. References 1 Rajput V, Mondal D P, Das S, Ramakrishnan N & Jha A K, J Mater Sci, 42 (2007) Gui M & Kang S B, Mater Lett, 46 (2000) Kim B G, Dong S L & Park S D, Mater Chem Phys, 72 (2001) 42.

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