Heusler phases

Ternary indides with CsCl superstructures or Heusler phases... 

Dwight and Kimball (1987) prepared the Heusler phases ScAg2Al and ScAu2Al and determined their lattice parameters (see table 15). Data about phase equilibria in these systems are not available in the literature. 

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. 

Bus] reported the Heusler phase MoFc2Al with a lattice parameter a = 591.8 pm in the as-cast alloy, but it was not confirmed in subsequent investigations. Therefore, it is not considered as an equilibrium phase. [1991Bi] observed a grain boundary phase, in rapidly solidified Al-2.2 at.% Mo-(0.1-0.5) at.% Fe alloys that were annealed at 400-450°C, with a bee stmcture and a lattice parameter a = 1272 pm. 

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). 

Keywords ferromagnetism, martensitic transformations, Heusler alloys, phase diagram. 

Heusler alloys have a rich variety of apphcations, owing to some of their unique properties. Some of these phases are half-metallic ferromagnets, exhibiting semiconductor properties for the majority-spin electrons and normal metallic behavior for the minority-spin electrons. Therefore, the conduction electrons are completely polarized. The Ni2MnGa phase is used as a magnetic shape memory alloy and single crystals of Cu2MnAl are used to produce monochromatic beams of polarized neutrons. 

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],...

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