Heusler-type phases

The classic Heusler-type alloys are Cu2AlMn and Cu2SnMn (Dwight, 1967 Kouvel, 1967) which will be addressed in Sec. 6. Their unique feature is their ferromagnetism, since the constituent elements are not ferromagnetic. These phases crystallize with the L2j structure which is derived from the B2 structure (see Fig. 1), and which is also adopted by a series of other ternary phases (Dwight, 1967). 

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Fig. 1. Specific yield strength (0.2 % proof stress in compression per unit weight density at 10 4s 1 strain rate in compression) as a function of temperature for the D022 phase Al3Nb [112, 113], the Heusler-type phase Co2TiAl [67], the Laves phasesTiCr15Si05 andTaFcAl [67,114], the two-phase alloy NbNiAl-NiAl with 15 vol.% NiAl in the Laves phase NbNiAl [67,114], and the hexagonal D8g phase Ti5Si3 [100] in comparison to the superalloy MA 6000 (in tension) [115] and the hot-pressed silicon nitride HPSN (upper limit of flexural strength) [116],...

Finally, it is noted that besides the Laves phases the Heusler-type phases NijAlNb and NijAlTa with an L2j structure may... 

A quantum-mechanical interpretation of Miedema s parameters has already been proposed by Chelikowsky and Phillips (1978). Extensions of the model to complex alloy systems have been considered. As an interesting application we may mention the discussion on the stabilities of ternary compounds presented by de Boer et al. (1988). In the case of the Heusler-type alloys XY2Z, for instance, the stability conditions with respect to mechanical mixtures of the same nominal composition (XY2+Z, X+Y2Z, XY+YZ, etc.) have been systematically examined and presented by means of diagrams. The Miedema s parameters, A t>, A ws1/3, moreover, have been used as variables for the construction of structural maps of intermetallic phases (Zunger 1981, Rajasekharan and Girgis 1983). 

The occurrence of a Heusler type ternary compound, ScM2ln, in each of the three Sc-M-In (M = Cu, Ag, Au) ternary systems has been reported by Dwight and Kimball (1987). Their lattice spacings are given in table 17. The alloys were homogenized at 700"C. No phase equilibria in these systems were investigated. 

This Structure type is a superstructure of the Cap2 type. The As atoms occupy the positions of the Ca atoms the Ag and Mg atoms regularly occupy the positions of the F atoms. The structure can also be obtained by removing one of the Cu atoms from the MnCu2Al unit cell and placing the vacancies and the copper atoms in a regular manner. The compounds which crystallize in the structure types MnCu2Al are called Heusler phases. 

Three structure types are found with composition 7533 the PrC02Ga-type, oP8, the YPdjSi-type, oP16, and the MnCujAl (Heusler phase)-type, cF16, discussed with YPdjSn. 

MnCujAl (Heusler phase)-type = CsCl-type derivative... 

The YPdjSn structure with MnCu2Al (Heusler phase)-type is shown in fig. 42. This face-centred structure is a superstructure of the CsCl-type. 

Ish] Ishikawa, K., Kaininna, R., Ohnmna, I., Aoki, K., Ishida, K., Phase StabiUty of the X2AlTi (X Fe, Co, Ni and Cu) Heusler and 52-Type IntermetalUc Compounds , Acta Mater., 50, 2233-2243 (2002) (Calculation, Phase Diagram, Phase Relations, Experimental, Thermo-dyn., 12)... 

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