Beneficial effects of Sc/Zr addition on hypereutectic Al–Ce alloys: Modification of primary phases and precipitation hardening
Introduction
In recent years, considerable attention has been given to Al–Ce alloys, in which Al11Ce3 phases formed during the solidification possess excellent properties, such as the negligible solubility of cerium (Ce) in aluminium (Al), low coefficient of thermal expansion, and excellent casting ability [[1], [2], [3], [4], [5], [6], [7]]. Due to the special properties of Al11Ce3 phases, Al–Ce alloys are adaptable to heat resistant aluminium alloys and appropriate for the aerospace and automotive industries [[8], [9], [10]]. Massive primary intermetallics are not conducive to load transfer, thus causing stress concentrations and microcracks [11,12]. Some strategies have been proposed to realize the modification and refinement to optimize the morphology of Al11Ce3 phases, such as laser additive manufacturing [13,14], permanent magnet stirring [15], hot extrusion [16], severe plastic deformation [17,18] and cold rolling and annealing [19].
Although changing the process conditions will improve the morphology of Al11Ce3 phases, this improvement in the morphology is difficult to achieve and increases cost unnecessarily. Generally, adding trace elements is an economical and efficient way to refine the primary phases in hypereutectic systems. Among microalloying elements, rare earth scandium (Sc) is the best element in Al alloys. Sc can significantly refine the grain size, inhibit the recrystallization, increase the strength of alloys, improve the weldability and reduce the hot tearing [[20], [21], [22], [23], [24]]. In recent years, the modification of Sc on secondary phases in cast Al alloys has also attracted extensive attention. Wu et al. [25] found that Sc segregation and composition supercooling in front of Mg2Si phases led to the combination of grain refinement of primary α-Al phases and modification of Mg2Si phases. Zhang et al. [26] pointed out that Sc atoms were concentrated in interdendritic regions in Al-7 wt % Si alloy, which resulted in a decrease in the size of eutectic Si phases. Zirconium (Zr) is generally added together with Sc in Al alloys. The Zr atom replaces the Sc atom (up to 35 at %) to form Al3(Sc, Zr) phases, reducing the lattice constant of Al3Sc. In addition, the aggregation tendency of Al3(Sc, Zr) phases at elevated temperatures is much smaller than the aggregation tendency of the Al3Sc phases [[27], [28], [29]]. Jiang et al. [30] found that the addition of Sc and Zr resulted in the refinement of θ′ precipitates by acting as preferential nucleation sites and segregation at the coherent interfaces, thus preventing the coarsening of θ′ precipitates in the Al–Cu alloy. In addition, the segregation of the major alloying elements to Al3(Sc,Zr) precipitates in the Al–Zn–Mg–Cu-Sc-Zr alloy was observed [31]. The above studies deepen our understanding of the role of Al3(Sc,Zr) phases in Al alloys.
In this paper, Sc/Zr-modified hypereutectic Al-15 wt % Ce cast alloys were investigated through their microstructure, tensile properties, fracture behaviour and thermal stability. This work has two main research objectives. The first is focused on understanding the effect of Sc/Zr addition on the microstructure and mechanical properties during solidification, which has not been systematically reported before. On this basis, the study analyses mainly the modification of Sc/Zr addition on Ce-rich phases. The second aim is to investigate the thermal stability of the alloy by the Al3(Sc, Zr) precipitation hardening effect, which has not been explored in hypereutectic Al–Ce alloys. This study is an initial feasibility study and provides great potential to extend the applications of aluminium alloys, especially in high-temperature fields.
Section snippets
Alloy preparation
According to the Al-Sc-Zr phase diagram, the maximum solubility of Zr atoms in Al3Sc phases is 50 at %. If the Sc/Zr atomic ratio exceeds 4:1, the L12 structure can be maintained [28]. Various Sc/Zr additions are designed (Table 1), and the Sc/Zr atomic ratios are 4.25, 4.57, and 4.36. Al-15 wt % Ce (-Sc-Zr) alloys were prepared from pure Al ingot (99.99 wt %) and master alloys of Al-20 wt % Ce, Al-2 wt % Sc and Al-10 wt % Zr. The alloys were melted by graphite crucibles in an electrical
Macrostructure and microstructure
The cross-sectional macrostructures of the 0Sc-0Zr, 0.13Sc-0.06Zr, 0.23Sc-0.10Zr, and 0.49Sc-0.21Zr alloys are illustrated in Fig. 1. The macrostructure of the four alloys is a completely equiaxial grain structure without a columnar grain structure. Since all alloys are hypereutectic alloys, the grains are actually a mixture of primary structures and several eutectic colonies. Compared with the 0Sc-0Zr alloy, no significant difference in macrostructure is observed in the 0.13Sc-0.06Zr alloy.
Modification effect on primary Al11Ce3 phases
In hypereutectic Al–Ce alloys, bulky primary Al11Ce3 phases with a “Chinese script” morphology cause stress concentrations [[8], [9], [10], [11]]. As shown in Fig. 2, primary Al11Ce3 phases with an average length of 79 μm are sharp at the edges and corners. In addition, primary Al11Ce3 phases form in the early solidification process, which results in the obstacle to the flow of liquid metal in the feeding channel and the formation of casting defects such as cracks and looseness. According to
Conclusions
The effects of Sc/Zr addition on the microstructure, mechanical properties and thermal stability in the Al-15 wt % Ce alloy were investigated. The main conclusions are as follows.
- (1)
The Sc/Zr addition can refine the primary Al11Ce3 phases of the Al-15 wt % Ce alloy. Sc/Zr atoms can be adsorbed on the surfaces of Al11Ce3 phases and prevent their growth via adsorption effect in the 0.10Sc-0.06Zr alloy. Compared with the 0Sc-0Zr alloy, the average size length of the primary Al11Ce3 phases in the
CRediT authorship contribution statement
Jieyun Ye: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft. Kun Dai: Investigation, Formal analysis, Data curation. Zhigang Wang: Writing – review & editing. Jiqiang Chen: Validation, Formal analysis, Investigation. Minqiang Gao: Conceptualization, Methodology, Writing – review & editing. Renguo Guan: Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key Research and Development Program of China [grant No. 2018YFB2001800] and Science and Technology Research Project of Jiangxi Provincial Education Department [grant No. GJJ200847].
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