Silicon phases

The amount of silicon that is present in the alloys investigated in this study varies from 0.67 wt% to 12.45 wt%. In eutectic Al-Si casting alloys (which contain approximately 12 wt% Si), two types of silicon may be found: primary silicon and secondary, eutectic silicon (Daykin, 1998; Edwards, 2002; Chen, 2006). Primary silicon is known to have a blocky morphology (Gupta and Ling, 1999) but should not form in hypoeutectic alloys. Eutectic silicon usually has a plate or flake like

morphology but this can be modified to have a much finer structure. Upon etching with a 0.5% HF in water solution, silicon has a grey colour. The size, volume fraction and morphology of the silicon phase are determined by the casting conditions, the chemistry of the alloy (for example the amount of Si and the other alloying additions)

and the heat treatment (Wang et al., 2001b). The size of Si particles can vary from 2

µm (Lee et al., 1995)) to greater than 100 µm (Stolarz et al., 2001).

Increasing the Si content up to the eutectic level (approximately 12 wt %) improves the castability of the alloy by reducing the melting temperature and the temperature range of the ‘mushy’ zone (Polmear, 1993). As a result of a more fluid cast, smaller interdendritic pores form, which can improve the fatigue life of the alloys (Conley et al., 2000). The addition of Si to Al also reduces the coefficient of thermal expansion (Polmear, 1989), this is important for a piston because it limits expansion in the engine and ensures the smooth movement of the piston when the engine warms up from resting temperature to the operating temperature. The more the material expands, the smaller the part must be produced so that it can expand in the cylinder, and then the greater the possibility of piston slap. This has been understood since the early part of the 20th century (Pacz, 1921; Bamberry, 1932).

Silicon is a relatively hard, brittle phase and so improves the wear resistance of the resulting alloy (Harun et al., 1996; Ye, 2003). However, this does make the alloy more difficult to machine and so can increase production costs. Si is also added to improve the strength and elastic modulus of these materials (Elliott, 1983) and these properties are dependent on the morphology and size of the silicon phases (Kim et al., 2002). It has been demonstrated that a finer eutectic silicon structure with a spherical morphology improves both the tensile strength and the percentage elongation of the alloy, whilst coarser particles are more detrimental to these properties (Suarez-Pena and Asensio-Lozano, 2006). Si particle shape and size may be altered through heat treatments such as hot isostatic pressing (HIPping) and solutionizing (Zhang et al., 2002; Lee et al.,2003), which produces a spherodised Si morphology (in a 2-D cross- section), or by the addition of alloying elements (Pacz, 1921).

Sr (Liao et al., 2002) and Na (Verdu, 1996) are typical alloying elements added to Al- Si alloys to alter the morphology of eutectic Si from large plates to finer particles (Conley et al. 2000), which may form part of an interconnected fibrous network of Si particles (Lasagni et al.,2006). Additions of Sr and Na are thought to produce finer Si particles because they lower the eutectic temperature and therefore restrict the growth of Si nuclei (Haque and Maleque, 1998). In this project the alloys have also been

modification mechanism for P is different to that of Na and Sr in that it encourages the growth of Si. The AlP phase acts as a nucleus on which primary ‘blocky’ Si grows. P and Sr or Na can be used together (Polmear, 1989) to give small, blocky Si particles. It should be noted that primary Si is only expected in near-eutectic and hypereutectic alloys and so P will have a limited effect on hypoeutectic alloys. As will be discussed in greater detail in section 2.4, the size, shape and distribution of Si plays an important role in the fatigue life of Al-Si alloys, for example initiation may occur at Si particles and Si particles may also affect crack propagation (Joyce et al., 2002b; Gall et al., 1999).

As well as the large phases mentioned, Si is also known to form as precipitates

(Warmuzek, 2004). Much of the information about Si precipitates is found in work on 2XXX and 6XXX series alloys, which have low Si content (comparable with the lowest Si alloy in this work). The precipitates are reported to form on the {110} and {100} planes (Mondolfo, 1976) (similarly Al2Cu and Mg2Si precipitates also form on

these planes) and can exhibit a size of approximately 1µm when the materials are in the overaged condition (Eskin et al., 1999).

2.2.3

AlCuNi phases

Ni and Cu are both added to Al-Si casting alloys to increase their high temperature strength (Ye, 2003). The mechanism by which they do this is unclear but work by Joyce et al. (2002) indicates that Al-Si alloys containing higher quantities of these elements are more resistant to stress relaxation at 350˚C. This may be attributed to a greater number of large particles, which can both inhibit the movement of dislocations and increase load transfer (Eshelby, 1957), which shields the matrix phase.

Ni and Cu combine with Al to form three different phases (Edwards, 2002); these are the Al3Ni, Al3(NiCu)2 and the Al7Cu4Ni phases. Whilst the name of the Al3Ni phase

suggests that it contains no Cu it actually contains approximately 10 wt% Cu and 29 wt% Ni. The Al7Cu4Ni contains 37 wt% Cu and 18 wt% Ni and so the principal

difference between the alloys is the Ni:Cu ratio. The phases cannot be easily

differentiated by a characteristic morphology. However, with etching in a solution of 0.5% HF in water solution the phases can be separated by colour which is generally related to the Ni:Cu ratio: a dark brown/grey colour indicates a phase with a high

Ni:Cu ratio and a lighter grey colour signifies a low Ni:Cu ratio. These three phases often form adjacent to one another and are usually interconnected (Chen et al., 2006).