Extruded Mg based hybrid composite alloys studied by longitudinal impression creep
📝 Abstract
The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been examined in the longitudinal direction, i.e., the plane containing random fibre orientation was parallel to the loading direction, in the temperature range of 175-300 C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. At 175 C, normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the stresses employed. At 240 C, normal creep behaviour is observed up to 80 MPa and reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and again decreasing, is observed above that stress. At 300 C, reverse creep behaviour is observed at all the stresses employed. This pattern remains the same for all the composites. The reverse creep behaviour is found to be associated with the fibre breakage. The stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the stress exponent varies from 3.9 to 7.0, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Qc was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres at all the temperatures and stress levels investigated.
💡 Analysis
The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been examined in the longitudinal direction, i.e., the plane containing random fibre orientation was parallel to the loading direction, in the temperature range of 175-300 C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. At 175 C, normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the stresses employed. At 240 C, normal creep behaviour is observed up to 80 MPa and reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and again decreasing, is observed above that stress. At 300 C, reverse creep behaviour is observed at all the stresses employed. This pattern remains the same for all the composites. The reverse creep behaviour is found to be associated with the fibre breakage. The stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the stress exponent varies from 3.9 to 7.0, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Qc was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres at all the temperatures and stress levels investigated.
📄 Content
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Extruded Mg based hybrid composite alloys studied by longitudinal impression creep A.K. Mondal, S. Kumar Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India
The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been examined in the longitudinal direction, i.e., the plane containing random fibre orientation was parallel to the loading direction, in the temperature range of 175– 300 C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. At 175 C, normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the stresses employed. At 240 C, normal creep behaviour is observed up to 80 MPa and reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and again decreasing, is observed above that stress. At 300 C, reverse creep behaviour is observed at all the stresses employed. This pattern remains the same for all the composites. The reverse creep behaviour is found to be associated with the fibre breakage. The stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the stress exponent varies from 3.9 to 7.0, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Qc was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres at all the temperatures and stress levels investigated.
- Introduction
In recent times, there has been a great upsurge in using magnesium for structural applications, the demand mainly coming from automotive industry owing to environmental concerns, increasing safety and comfort levels, a significant improvement in the corrosion resistance of high purity magnesium alloys, rising fuel prices and lowering of prices of primary magnesium metal. For powertrain components, several creep-resistant magnesium alloys have been developed [1]. However, for temperatures above 200 C, metal matrix composites (MMCs) need to be developed. Particulate reinforced Mg-MMCs might actually deteriorate the creep properties [2,3]. Therefore, short fibre reinforced MMCs have to be developed for these applications. However, they are expensive and have anisotropic properties. Therefore, hybrid composites (HC) would be the best choice. The partial replacement of expensive short fibres by cheap particulates reduces the cost as well as anisotropy. The addition of particulates also helps in keeping the fibres apart, which improves mechanical properties [4,5]. Friend et al. observed that the partial replacement of Saffil short fibres by SiC particulates in 7039 Al alloy-based hybrid MMCs did not affect the strength and elastic moduli but improved fracture toughness [6]. Park observed an improvement in wear resistance of 6061 Al alloy-based hybrid MMC reinforced with Al2O3 short fibres and SiC whiskers than that in MMCs reinforced either with Al2O3 short fibres or SiC whiskers alone [7]. However, there have been only a few studies on Mg alloy-based hybrid MMCs [8–16].
Creep studies are very important for MMCs being developed for engine components, as they are subjected to high temperature in these applications. These properties of MMCs are currently determined in conventional tensile and compressive creep tests [15–18]. There has been only one study on the creep behaviour of Al alloy-based MMCs using indentation creep [19]. However, to the best of our knowledge, there has been no report on the creep behaviour of MMCs using impression creep, though it has been identified as a versatile method for determining creep properties of materials long ago [20,21]. In the present study, creep behaviour of a creep-resistant AE42 alloy reinforced with 20 vol.% Saffil short fibres as well as various volume fractions of Saffil short fibres and SiC particulates has been studied in the longitudinal direction, i.e., the plane containing random fibre orientation was parallel to the loading direction, using impression creep technique.
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Fig. 1. XRD patterns obtained from all the composites. 2. Experimental procedure
The magnesium alloy AE42 (Mg–4.0 wt.% Al–2.0 wt.% Rare Earth (RE)–0.2 wt.% Mn) based composites are used in the present investigation. The alloy is reinforced with Saffil short fibres having 3–8 lm diameter and 200 lm length and SiC particulates having a maximum diameter of 40 lm, in various combinations. The various combinations are 20 vol.% Saffil short fibres, 15 vol.% Saffil short fibres and 5 vol.% SiC particulates, 10 vol.% Saffil short fibres and 10 vol.% SiC particulates, and 10 vol.% S
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