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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

(DSQC) (135) which was

(DSQC) (135) which was applied in the present study for fabrication of the test samples. The differences in strength are a result of the lower cooling rates found in gravity chill cast material. A rate of 1°C/s was measured for DSQC (Figure 4.46), whereas 10 to 1000°C/s was reported for HPDC (143) . The higher cooling rates lead to finer microstructures and higher strengths (143) . The pure alloy IS had a calculated characteristic bending strength of 241 MPa. Due to the limited deflection of the bending test set-up and the relatively high ductility of IS the test bars did not break and therefore the calculated value did not represent σ0 which calculation is based on the stress at fracture. For an alloy similar to IS processed in indirect squeeze casting (ISQC) Kniewallner (51) measured in the tensile tests a mean UTS of 190 MPa (Table 2.1). Kniewallner found inhomogeneities such as oxides and gas inclusions in the fractured surface which led to reductions in strength. No such defects were found in IS in the present work leading to higher fracture strength values. Even though the actual σ0 of IS is higher than the calculated value evaluated with limited deflection, the measured UTS (51) and the specified alloy values (148) indicate that it is legitimate to assume that the real σ0 is higher but close to the value of 241 MPa and it was used for the comparative study. Nevertheless, in future work the bending test set-up will have to be improved in order to allow testing of materials with higher ductility. The limited deflection in the bending test further reduced the scatter in maximum stress values of IS and therefore its Weibull modulus (m) of 88 is an overestimation and not representative. It was reported that m values for the tensile strength of Al alloy castings lie below 40 (6) . For the present comparative study, the maximum value (m = 40) was used as it represents a worst case regarding the comparison with m values of the MMCs, where all test bars fractured prior to the deflection limit being reached. 187

The measured elastic modulus, Edyn , of IS of 69 GPa is marginally lower than recorded in the literature (166) . For metallic materials, E is usually measured in tensile tests. For example Kniewallner (51) found 75±3 GPa for an alloy similar to IS. The difference in modulus may be the result of using the dynamic method which was originally developed for ceramic materials, exhibiting higher moduli and lower damping. Nevertheless, the difference is acceptable and the method was successfully applied to Al-MMCs (175) . The fracture toughness, KIC , of IS could not be measured using the single edge V-notch beam (SEVNB) method (169) , which is a standard method used for ceramics (149) , because the metallic character of the unreinforced alloy prevented the fabrication of an obligatory sharp notch. The compact tension test method (CT) is usually used for metals and precracking is performed using cyclic loading. No KIC values for HPDC Al alloys were found in the literature. Using the CT method, Chan et al. (176) reported values in the range of 12.0 to 17.8 MPa·m 1/2 for gravity cast Al-7%Si-Mg. The lower values were for the as-cast condition and the higher for the T6 condition. The solution heat treatment in the precipitation hardening process of Al-Si-Mg alloys consists of prolonged holding at temperatures about 30-50°C below the solidus. Here, not only solution of Mg takes place but also the morphology of Si is changed from needle-like to a spherical shape. This shape transformation is reported to be used to increase the elongation of casting alloys. As a result of this silicon spheroidization treatment (SST), the elongation of an A356 alloy increased by 70% (177) . The possibility of spheroidization of the IS alloy was investigated and Figure 4.44 shows that the needle-like shape was transformed into a globular one and therefore the KIC of the as-cast alloy may be improved by heat treatment. However, as the target of the present work was a low cost composite, heat treatment was not applied. From Figure 4.81 b) it appears that cracking initiates at the interface of the needle-like Si- precipitates. It can therefore be assumed that the KIC of IS is similar to that reported for as- 188

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