Grain refinement through inoculation treatment is an important process in metal casting, because the refined grains introduce sound castability, enhance chemical and structural uniformity, improve the mechanical properties and increase the formability in the subsequent forming process. A number of effective grain refiners have already been well developed and practically used in light metals/alloys, such as Al and Mg alloys. As one of the most common engineering materials in use, cast Zn possesses many advantages, including energy-saving melting, casting soundness, good corrosion resistance and dimensional tolerance. In addition, the majority of Zn alloy components are wrought products, thus, the formability of the alloy is critical. However, Zn cast products are normally associated with coarse grains, which result in brittleness, low strength and poor formability. Hence, grain refinement of cast Zn alloys has been considered as an effective approach to increase the quality of Zn products at the lowest cost. However, there are few efficient grain refiners available for cast Zn due to the lack of research on grain refinement in this type of alloys.
Based on the currently available grain refinement theories/models that were developed in Al and Mg alloys, four new grain refiners (master alloys), i.e. Zn-10wt.% Ag, Zn-18wt.% Cu, Zn-60wt.% Mg and Zn-6wt.% Al, were originally developed for cast Zn in this PhD project. Foundry test shows that all these four grain refiners can produce significant grain refinement in cast Zn. Furthermore, the grain refining mechanisms were also comprehensively investigated in terms of the three phase diagram related parameters, i.e. growth restriction factor (Q), supercooling parameter (P) and freezing range (∆T), and the crystallography of heterogeneous nucleation. It was found that no single factor can define the grain refining efficiency.
The effect of grain refinement on tensile properties of cast Zn alloys was studied through specially-designed experiments. Two groups of Zn-Mg and Zn-Al alloys with fixed solute contents were prepared using different cooling rates to achieve various grain sizes, which enables to distinguish grain-refinement strengthening from solid-solution strengthening. Then, the Hall-Petch equations were determined in binary Zn-Mg and Zn-Al alloys, respectively. Through controlling the addition levels of Mg/Al in Zn melts, the different grain sizes of another two groups of binary cast Zn alloys (solidified at a fixed cooling rate) were also produced, in which both grain-refinement strengthening and solid-solution strengthening coexist. After extracting the grain-refinement strengthening component, the contribution of solid-solution strengthening to the yield strength was then mathematically quantified. Further, empirical relations, between yield strength, grain size, solute content and intrinsic friction, in different alloys were established. The experimental results indicate that the yield stress of all alloys was increased by > 50 MPa compared with the unrefined cast Zn, while the onset necking only occurred in the cast Zn-Al alloys corresponding to the maximum elongation (~ 4.7%). Fractographic analysis show that the primary brittle mode cannot be changed in all cast dilute Zn alloys, even though the grain sizes were substantially decreased by 90%.
The effect of adding different solutes, including Ag, Cu, Mg and Al, on the grain refining efficiency was investigated. It is found that, in peritectic alloys (Zn-Cu and Zn-Ag), the average grain size of as-cast Zn alloys decreases with increase in the solute content when it is below a critical value. This critical value is 1.7 wt.% Cu in Zn-Cu binary alloy and 3 wt.% Ag in Zn-Ag alloy. Further increase of the solute content led to grain coarsening. Considerable microstructural characterization and crystallographic examination indicated that: although peritectic reaction possibly does not occur due to the relatively fast cooling in practical cast process (no time for the solute to diffuse in solid phase), the pro-peritectic phases, AgZn3 and CuZn4, can act as effective heterogeneous nucleant particles. Thus, the grain refinement resulted from addition of either Ag or Cu is attributed to the cooperative contributions from the solute effect (that restricts the grain growth) and from the heterogeneous nucleation. At high addition levels of solute, formation of coarse pro-peritectic phase particles, which reduces the number of active nucleant particles, is responsible for the grain coarsening of cast Zn alloys.
Then, the crystallographic relations, between AgZn3/CuZn4 and Zn, were investigated. Two new HCP-HCP orientation relationships (ORs) between the pro-peritectic phase (AgZn3/CuZn4) and η-Zn matrix were experimentally determined using electron backscattered diffraction (EBSD) and a Euler-based numerical method. The determined ORs are:
[¯1 2¯1 ¯3]AgZn3 // [¯1 2¯1 0]Zn, (01¯1 1)AgZn3 // (10¯1 1)Zn, (10¯1 0) AgZn3 // (0002)Zn
[¯1 ¯1 20]CuZn4 // [2¯1 ¯1 3]Zn, (1¯1 01)CuZn4 // (¯1 011)Zn, (000¯2)CuZn4 // (01¯1 0)Zn
These two ORs were also theoretically verified based on the edge-to-edge matching (E2EM) model. This further verifies that pro-peritectic phases can be effective nucleants.
Although the most effective grain refiners used in industry are associated with peritectic-based alloy systems, such as Al-Ti, Mg-Zr and Mg-Al2Y. In the present thesis, it was found that the eutectic-forming solute, Mg, can also significantly grain refine the cast Zn. Differential thermal analysis (DTA) of the Zn-Mg alloy, in which efficient grain refinement occurred, evidenced an unexpected peak that appeared before the nucleation of η-Zn grains on the DTA spectrum. Based on extensive examination using X-ray diffraction, high resolution SEM and EDS, it was found that: (a) some faceted Zn-Mg intermetallic particles were reproducibly observed; (b) the particles were located at or near grain centres; (c) the atomic ratio of Mg over Zn of the intermetallic compound was determined to be around 1/2. Using tilting selected area diffraction (SAD) and convergent beam Kikuchi line diffraction pattern (CBKLDP) techniques, these faceted particles were identified as MgZn2 and an HCP-HCP orientation relationship between such grain-centred MgZn2 particles and the η-Zn matrix was determined. The determined HCP-HCP OR (between MgZn2 and η-Zn) is one variant of the ORs reported in the efficient Al2CO-Mg nucleation system. The present determined OR between MgZn2 and η-Zn is actually a different variant of the OR between Al2CO and Mg. Hence, the unexpected peak on the DTA spectrum is believed corresponding to the formation of MgZn2 particles, which act as potent heterogeneous nucleation sites. Together with the solute effect of Mg on restricting grain growth, such heterogeneous nucleation is cooperatively responsible for the grain refinement in Zn-Mg alloys. Moreover, the effect of geometrical features (i.e. size, size distribution and morphology) of the nucleant particles on grain refinement was also analysed using the free growth model.