Predicting the Benefits of Adding Ternary Elements to Al-Sc Alloys
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Predicting the Benefits of Adding Ternary Elements to Al-Sc Alloys Darko Simonovic1,2 and Marcel H. F. Sluiter1,2 1 Materials Science and Engineering, Delft University of Technology, Mekelweg 2, Delft, 2628 CD, Netherlands 2 Netherlands Institute for Metals Research, Mekelweg 2, Delft, 2628 CD, Netherlands
ABSTRACT We seek to explain 1) the effectiveness of substituting Sc with Dy, Er, Y and 2) the loss of properties when Sc is replaced with Yb, Gd and Sm [1]. For a preliminary insight into the stability of structures we utilize the concept of Atomic Environment Type (AET) as pertaining to trialuminides. Electronic density functional total energy calculations at zero temperature are performed to obtain the enthalpy of mixing of quasi-binary Al-Sc-X structures. Estimates of the entropy are used to compute the stability regions of Al3Sc1-αXα L12 phase. We show that Sc is completely miscible with X=Dy, Er, Y and that there is a miscibility gap for X=Yb, Gd and Sm at temperatures near the aging temperature of Al-Sc alloys.
INTRODUCTION Scandium in Al alloys may be used in various ways, e.g. as precipitation-strengthener, as grain refiner, as recrystallization inhibitor and for enhancing superplastic properties. Among alloying elements it gives the highest increase of strength per atomic percent. The extraordinary beneficial effects of Sc are due to it forming a very fine dispersion of coherent Al3Sc precipitates with L12 structure [2,3]. The high price of Sc has limited its practical application. Less expensive third alloying elements that might partially substitute for Sc in Al3Sc precipitates are therefore of considerable interest [1,4]. Ti and especially Zr are the most widely used elements in ternary Al-Sc-X alloys. Zr forms a stable Al3Zr phase with D023 structure, but it can precipitate metastably with L12 structure, e.g. when nucleated and grown on primary L12 Al3Sc precipitates. The small diffusivity of Zr prevents Al3Sc encapsulated in Al3Zr from coarsening. Al3Ti is known to exist in two stable forms: Al3Ti (tI32) stable below 950°C, and Al3Ti (tI8) D022 stable between 950 and 1387°C [5]. Ti also precipitates metastably with the L12 structure in Al-Sc-Ti alloys. Both Ti and Zr can be stabilized into L12 structure with addition of third alloying element [6]. Recently, the substitution with rare earths elements has been claimed to be beneficial [1]. Karnesky et al. showed that substituting 0.2 at.% of Sc with Dy, Er and Y in 0.8 at.% Al-Sc retains hardness loss, whereas for Yb, Gd and Sm hardness is reduced. For Yb the loss of hardness is attributed to shorter aging time (larger diffusion in Al) whereas for Gd and Sm the hardness loss is attributed to reduced solubility in Al3Sc phase (at an aging temperature of 300°C).
It is known that Al- late rare earth element (RE) trialuminides exhibits progressively more cubic character [7]. Er forms a stable L12 structure in Al-Er alloys. Al3Ho and Al3Dy take rhombohedral and hexagonal structures and Al3Y has a hexagonal D019 structure. All three of them
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