Technology of obtaining REM-containing master alloy for silumins modification

. The technology of producing ligatures containing rear earth metals (REM) for modification of silumins has been developed. The optimum content ratio of yttrium, cerium and lanthanum 2: 1: 0.5 (15-20 wt.% Y, 7-10 wt.% Ce, 3.5-5.0 wt.% La) for the synthesis of REM aluminides has been established. The peculiarities of the REM aluminides synthesis is studied using the methods of optical and electron microscopy and X-ray microanalysis, followed by the identification of the structural components of the Al-REM alloy. The microstructure of the master alloy is proved to consists of: REM aluminides (yttrium-containing and cerium-containing), metal base (Al), Al eutectic + REM aluminide and α + Si.


Introduction
There are very few publications on the development of compositions of master alloys from rare-earth metals for the silumins modification [1][2][3][4]. In the majority of works aluminum master alloys containing Ti, C, B, Ca, REM, P, and others are used [5].
The literature review provides no information on the problem of technology development of the Al-Y-Ce-La master alloy synthesis. In this regard the present work gives a detailed identification of the structural components of this master alloy and determines their microand nanohardness values.
Micro X-ray spectral analysis was performed to determine the content of elements on an analytical research complex based on FE-SEM Hitachi Su-70 (Japan) with attachments for energy dispersive (Thermo Scientific Ultra Dry) and wave X-ray microscopic analysis.
The microhardness test was carried out according to the standard procedure on a Vickers instrument of Shimadzu HMV-G brand [20]. Nanohardness was measured on an Integra Prima atomic force microscope [21]. Local analysis was carried out on 3-5 points of sample sections, and the average chemical composition of the structural componentsα-solid solution, eutectic and intermetallic compounds AlxSiyFezREMν -was determined. Table 1 shows the stoichiometry of the synthesized aluminides (yttrium-containing and cerium-containing) in Al-Y-Ce-La master alloy.

Results and discussion
According to the state diagrams, yttrium forms Al3Y aluminide, but cerium and lanthanum -Al11Ce3 and Al11La3 (respectively Al3.66Ce and Al3.66La). The synthesized REM aluminide containing yttrium corresponds to the formula Al3,11REM, but the standard one according to the state diagram -Al3Y. The discrepancy between these values is insignificant (0.11 at.% Al). The resulting cerium-containing REM aluminide has a stoichiometry of Al3,69REM, slightly different from Al3,66REM. The discrepancy in stoichiometry is 0.03 at.% Al. From table 1 it follows that Al, Fe, Si and REM are present in the eutectic in certain amounts, the estimated structure of the eutectic corresponds to: Al+Al3,66РЗМ+Al3Y+Si+FeAl3. The ironsilicon compounds of variable composition AlxSiyFez are also supposed to present.
The results of microhardness measurements showed that the yttrium-containing REM aluminide has a higher microhardness value (on the average 6547 MPa) than the microhardness of cerium-containing REM aluminide (the average 4695 MPa). The microhardness of pure aluminide is 254 MPa, and the microhardness of eutectic (Al + REM aluminide) is 423 MPa. Fig. 1 shows the microstructure and location points of nanohardness measurement of structural components Al-Ce-La-Y of master alloy It was established that nanohardness value of cerium-containing REM aluminide is 8560 MPa. Nanohardness of yttrium-containing REM aluminide is 9627 MPa. Accordingly, the nanohardness -and microhardness of yttrium-containing REM aluminide is significantly higher than that of cerium-containing REM aluminide. The average value of aluminum nanohardness corresponds to 1500 MPa. The average nanohardness value of the eutectic is 4650 MPa. .

Conclusions
Micro X-ray analysis identified the structural components of the synthesized master alloy Al-Y-Ce-La: -metal base consists of pure aluminum and eutectic (Al+AlxREMy); microhardness of pure aluminum is 254 MPa, and nanohardness -1500 MPa; -yttrium-containing REM aluminide has the Al3Y stoichiometry ; microhardness is 6547 MPa, and nanohardness is 9627 MPa; -cerium-containing REM aluminide has the stoichiometry Al11REM = Al3.66REM (Al3.66Ce, Al3.66La); Microhardness -4695 MPa, and nanohardness -8560 MPa. 1) The increase in the addition of master alloy up to 0.5 wt.% contributes to the grinding of structural componentsα-solid solution and eutectic, and to crystallization of highly solid complex-doped aluminide AlxSiyFez with and without REM (in the original alloy). The study was carried out with financial support from the Ministry of Education and Science of the Russian Federation within the framework of the state assignment №11.3014.2017/4.6 "Investigation of the possibilities of obtaining REM -containing ligatures for the modification of metal alloys". The investigations were made using the equipment of the Center for Applied Materials Science of FSBEI HE "TOGU" and with financial support from