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Crystallographic study on microstructure and martensitic transformation of NiMnSb meta-magnetic multi-functional alloys

Abstract : NiMnSb based Heusler type alloys, as a novel multi-functional material has attracted considerable attention due to their multiple properties, such as magnetic shape memory effect, magnetocaloric effect, exchange bias effect, magnetoresistance effect. To date, many aspects of the NiMnSb alloys, such as crystal structure, microstructure, magnetic properties and mechanical properties etc., have been widely investigated. However, many fundamental issues of this family of materials have not been fully revealed, which largely restricts the development of this new kind of multi-functional materials. In the present work, a thorough investigation has been conducted on ternary NiMnSb alloys in terms of crystal structures of austenite and martensite; microstructural and crystallographic features of martensite; martensitic transformation orientation relationship (OR) and its correlation with variant organization; transformation deformation characteristics and self-accommodation of transformation strain. The work confirmed that the austenite of NiMnSb alloys possesses a cubic L21 crystal structure belonging to the space group Fm3m (No. 225). The martensite has a four-layered orthorhombic (4O) structure with space group Pmma (No. 051). The lattice constants of the Ni50Mn37Sb13 and Ni50Mn38Sb12 martensite are aM = 8.5830 Å, bM = 5.6533 Å and cM = 4.3501Å, and aM = 8.5788 Å, bM = 5.6443 Å and cM = 4.3479 Å, respectively. The microstructure of the 4O NiMnSb modulated martensite possesses a hierarchical organization feature. Martensite fine lamellae are first organized into broad plates. Each plate possesses 4 distinct twin related variants A, B, C and D forming type I twins (A and C; B and D), type II twins (A and B; C and D) and compound twins (A and D; B and C). The variant interfaces are defined by the corresponding twinning planes. The complete twinning elements for each twin relation are fully determined. The plates are further organized into sub-colonies and sub-colonies into plate colonies. The neighboring plates in one sub-colony and plate colony share one common plate interface orientation. Plate colonies with different oriented plate interfaces finally take the whole original austenite grain. The Pitsch OR, specified as {011}A // {221}M and <011>A // <122>M, is the effective OR between the cubic austenite and the 4O modulated martensite. Under this OR, a maximum of 24 distinct variants can be produced. The 24 variants are organized into 6 distinct variant colonies, 12 distinct sub-colonies and finally 6 distinct plate colonies. The twinning plane of type I twin and the intra-plate plate interfaces all correspond to the same family of {011}A planes of austenite. The formation of martensite variant colonies can be both form intragranular and intergranular during the phase transformation. The sandwich structured variant colony is the basic microstructural unit of the martensite. This structure is composed of twin related variants and possesses the full compatible inner variants interfaces and invariant habit planes. The deformation manner of the twin related variants result in the high occurrence frequency of the type II twins and affects the morphology of the sandwich colonies. The wedge-shaped structure is composed of two compatible sandwiches and conjoined by a midrib plane with a small atomic misfit. All these results indicate that the martensitic transformation is self-accommodated and the microstructure is determined by the self-accommodation of the microstructural constituents. The aim of this work is to provide fundamental crystallographic and microstructural information of NiMnSb alloys for interpreting their magnetic and mechanical characteristics associated with the martensitic transformation and further investigations on property optimization
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Chunyang Zhang. Crystallographic study on microstructure and martensitic transformation of NiMnSb meta-magnetic multi-functional alloys. Materials. Université de Lorraine, 2017. English. ⟨NNT : 2017LORR0030⟩. ⟨tel-01673805⟩

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