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Crystallographic study on Ni-Mn-Sn metamagnetic shape memory alloys

Abstract : Being a novel magnetic shape memory material, Ni-Mn-Sn based alloy systems possess multiple physical properties, such as shape memory effect of polycrystalline alloys, giant magnetocaloric effect, large magnetoresistance effect and exchange bias effect. So far, most studies have been focused on the improvement of the multifunctionalities of these alloys, but the fundamental information which is highly associated with these properties is still unclear. Thus, a thorough study on the crystal structures of martensite and austenite, microstructural and crystallographic features of martensitic transformation has been conducted in the present PhD work. The austenite of Ni50Mn37.5Sn12.5 was confirmed to possess a L21 cubic structure (Fm"3" ̅m, No.225). The lattice parameter of austenite in Ni50Mn37.5Sn12.5 is aA=5.9813 Å. The martensite possesses a four-layered orthorhombic (4O) structure (Pmma, No.51). The lattice parameters of martensite in Ni50Mn38Sn12 and Ni50Mn37.5Sn12.5 are a4O = 8.6068 Å; b4O = 5.6226 Å and c4O = 4.3728 Å, and a4O = 8.6063 Å, b4O = 5.6425 Å, and c4O = 4.3672Å, respectively. The 4O Ni-Mn-Sn martensite exhibits a hierarchically twinned microstructure. The martensite is organized into broad plates in the original austenite grain. The plates contain irregularly shaped colonies with two characteristic microstructural patterns: classical lamellar pattern and herring-bone pattern. In each colony, there are four orientation variants (A, B, C and D) and they form three types of twins (Type I, Type II and compound twin). The interfaces between the corresponding variants are in coincidence with their twinning plane K1. The interface planes of the compound twin pairs A-D and B-C can have one or two different orientations, which leads to the two microstructural patterns. The corresponding variants in the neighboring colonies within one broad plate (intra plate colonies) possess close orientations and colony boundary is curved, whereas the inter plate colony boundary is relatively straight. The Pitsch OR, specified as "{1 0 1}" A//"{2 " "2" ̅" " "1" ̅"}" 4O and "<1 0 " "1" ̅">" A//"<" "1" ̅" " "2" ̅" 2>" 4O, was uniquely determined to be an effective OR between the cubic austenite and 4O modulated martensite. Under this OR, 24 variants can be generated within one austenite grain. Such 24 variants are organized into 6 groups and each group corresponds to a martensite colony. The finely twinned martensite structure (sandwich microstructure) is the basic microstructural constitute produced by martensitic transformation. Such a structure ensures an invariant phase interface (habit plane) for the transformation. During the transformation, martensite variants are organized into diamond shaped clusters composed of variant colonies and with wedge shaped structures at the transformation front. Each wedge is composed of two sandwich structures separating by a midrib plane {1 0 1}A. The variant pairs in each wedge should have the same twin type with either Type I or Type II relation to ensure good geometrical compatibilities of the variants at phase interface and at the midrib plane. Within the diamonds, colonies are separated by step-like boundaries with low interfacial energy that evolve into the intra plate colony boundaries and by straight boundaries that become the inter plate colony boundaries. The diamonds elongates along the direction nearly paralleled to the midrib planes of the wedges and plate shape of martensite is finally formed. Such features of the diamond structure in Ni-Mn-Sn alloys are realized by self-accommodation of transformation strains for energy minimization. The present work provides comprehensive microstructural and crystallographic information on martensite and on martensitic transforamtion of Ni-Mn-Sn alloys and it is useful for understanding their multi functionalities associated with martensitic transformation and helpful on property optimization
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Submitted on : Friday, November 9, 2018 - 11:56:13 AM
Last modification on : Sunday, June 26, 2022 - 12:47:06 AM

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Chunqing Lin. Crystallographic study on Ni-Mn-Sn metamagnetic shape memory alloys. Materials. Université de Lorraine; Northeastern University (Shenyang), 2017. English. ⟨NNT : 2017LORR0359⟩. ⟨tel-01917274⟩



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