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For Vickers hardness, as references, the initial hardness of un-aged Al-Cu was 113 Hv for 5 N load. Similar trend can be shown in aged Al-Cu which was 136 Hv. Both of un-aged and aged-Al-Cu alloys, the value of hardness increased when TiB2 contents increased. The hardness of un-aged Al-Cu-6wt.%TiB2 were recorded 125 with 5 N load, while aged Al-Cu-6wt.%TiB2 were 152 Hv. The addition of TiB2 resulted in an increased of the hardness of Al-Cu alloy. The increase in hardness of the composites can be attributed to the decrease in the grain size to TiB2 particles. It is evident that addition of TiB2 leads to significant increment in the hardening by the precipitates, which can be attributed to additional strengthening due to coherency strains and misfit dislocations formed by the precipitation on TiB2 particles. These results then are illustrated graphically in Table 1 and Figure 1 below.
The properties of a metal are strongly influenced by the grain size and the well-known Hall–Petch equation. It predicts an increase in yield strength with a decrease in grain size (d). This equation has been shown to be applicable to a wide variety of metals.
Where σy, σo, and k is yield strength, friction stress and a constant respectively. In the as-cast condition, the hardening of the Al–Cu–TiB2 composites can be attributed to the fine TiB2 particles and reduction in grain size (due to grain refinement by TiB2). In the case of solutionised composites, the hardness is governed by solid solution strengthening, in addition to Hall-Petch hardening and hardening from TiB2 particles. The increase in the hardness of the solutionised composite with increase in fraction of TiB2 particles clearly gives the contribution of TiB2 particles to the hardness of the composites.
Fig. 1. Vickers hardness (Hv) of Al-Cu with different TiB2 contents with 5N loads
3.2 Effect of ageing
Thus, the presence of higher amount of TiB2 particles enhances the aging kinetics of Al–Cu alloy. The increase in aging kinetics in presence of TiB2 particles could be attributed to the possible nucleation of the precipitates of Al2Cu on the basal planes of the TiB2 particles. In addition, the difference in the coefficient of thermal expansion between Al (23.5 × 10−6 K−1) and TiB2 (7 × 10−6 K−1) causes the development of strain field at the interface between the reinforced particle and the matrix, which leads to an increase in the dislocation density. These dislocations can also act as nucleating sites during precipitation. From the research, tensile strength and Vickers hardness gave highest value compare alloy without aged.
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