Influence of grain morphology on strengthening of aluminium and magnesium alloys

Abstract

The Hall-Petch equation is commonly employed to establish a connection between the yield strength/hardness of metallic materials and the grain diameter (d). However, for alloys having grains with dendritic morphology, a more accurate representation of the Hall-Petch equation involves substituting d with the secondary dendritic arm spacing (SDAS), denoted as λ2. This thesis underscores the significance of considering a perimeter-associated parameter, rather than relying solely on d or λ2, to account for strength contribution from morphological changes in various alloys. To express the complexity of dendritic / grain morphology in 2 dimension, a perimeter associated parameter (p') has been proposed. And, a comprehensive generic Hall-Petch relation expressed as 𝜎=𝜎o+𝑘′′√𝑝′, where p' signifies the area per unit perimeter which can be accurately measured using digital imaging tools and 𝑘′′ is the slope representing Hall-Petch constant, is being proposed. To substantiate the validity of this proposed relation, controlled solidification experiments were conducted on Al-Si and Al-Ce binary alloys, encompassing a range of cooling rates from 0.000167 °C/s to 0.34 °C/s. The degree of dendritic structure in primary α-Al was deliberately manipulated through solute content, such as the addition of Si or Ce. Notably, the relationship between λ2 and the corresponding p' is observed to be non-linear, both experimentally and theoretically. This underscores the critical importance of incorporating the perimeter-associated parameter p' as a fundamental strength metric in the Hall-Petch relation. The resulting changes in hardness, in conjunction with evolving grain morphologies, were meticulously analysed. The experimental data from a spectrum of observed morphologies (facetted, rosette, and dendritic) fit more effectively with our proposed generic Hall-Petch relationship compared to conventional representations. To assess the applicability of the new Hall-Petch relationship across different alloy systems, magnesium (Mg) alloys were also investigated. Wedge mould castings of commercially available AZ91D alloy and Mg-5 and 9 wt.% binary alloys were examined. The diverse cooling rates induced by the wedge mould, along with varying solute content, led to a range of morphologies in the magnesium grains. Tensile tests were conducted on both as-cast and solution-treated AZ91D alloy to examine the relationship between d, λ2, p', and yield strength (YS). The proposed generic Hall-Petch relationship yields better coefficient of regression when p' is considered instead of d or λ2. This underscores the significance of incorporating perimeter-associated parameters to accurately quantify the variation in the morphology of grain structures across different alloys. During the examination of dendritic structure morphology in Al-Ce binary alloys, high thermal stability of Al11Ce3 was recognised. Spheroidization experiments were conducted to evaluate the coarsening resistance of the Al11Ce3 phase in Al-5Ce alloy, with comparisons made to Al-5Si alloy through circularity measurements. The circularity of Al11Ce3 changed from 0.64 to 0.66, while the circularity of eutectic Si changed from 0.55 to 0.8 during isothermal annealing at 400°C. To exploit the superior thermal stability of Al-Ce alloys, new alloys incorporating various compositions of Sc, Zr, Si and Mg for their mechanical properties at elevated temperatures. Specimens were fabricated using gravity and high-pressure die casting techniques. The addition of 0.2 wt.% Sc and 0.2 wt.% Zr resulted in the formation of nanoscale Al3Sc,Zr precipitates with an average particle size of approximately 3 nm at the peak aged condition of 300°C. Moreover, the addition of 0.4 wt.% Sc and 0.2 wt.% Zr refined the grain structure, leading to the formation of grains with an average size of approximately 20 μm. Specimens cast through HPDC were tested at room and elevated temperatures ranging from 150 to 300°C. The yield strength (YS) of Al-10Ce-0.2Sc-0.2Sc was measured to be 115 MPa with a retention factor of 0.9 at 300°C.