Effect of La and Ce addition on the microstructural characteristics and tensile properties of 220 and 356.1 alloys

The present work was performed on 220 (2xx) and 356.1 (3xx) alloys to which cerium (Ce) and lanthanum (La) were added in incremental increase of 0.2% with maximum 1% each using high purity rare earth metals (RE-99.9%). The microstructural examination reveals that grain refining caused by the addition of RE may be due to constitutional undercooling (almost 40%), whereas the addition of TiB₂ results in about 90% (heterogeneous nucleation). The addition of RE at any concentration has no eutectic Si modification when the molten metal is properly degassed and filtered before pouring (no RE-based oxides were detected). However, La and Ce have a strong affinity to react with strontium (Sr) leading to the formation of a high density of SrLaCe spherical particles (about 20-50 nm in diameter) with the ratio of La/Ce about 1.5 (SrLa₁₀Ce₆). Both Ce and La react with titanium (Ti) leading to formation of large size gray compacted particles (sludge) instead of the thin white platelets. These particles precipitate a few degrees away from the commencement of solidification, as detected by the analysis of the solidification curves. The results also show that depending on the alloy ductility following heat treatment, the addition of 0.2% of Ce or La would marginally increase the alloy strength. However, any addition beyond that limit will have a detrimental effect on the alloy performance. Cerium and lanthanum are interchangeable in terms of permitting mutual substitution without loss of function or suitability.