| NiMnIn based alloys have attracted considerable attention due to their multifunctional properties,such as metamagnetic shape memory effect,magnetocaloric effect and magneto-resistance effect.Despite the revelation of many excellent properties,some fundenmental knowledge on these alloys is still missing until now,such as the accurate crystal structure information of modulated martensite,the crystallographic features of microstructure and martensitic transformation.This situation greatly hinders the development of this kind of promising multifunctional alloys.In this dissertation,the crystallographic features,mechanical behaviors and magnetic properties of NiMnln based alloys were studied theoretically and experimentally.First,the crystal structures of NiMnIn alloys were determined by Rietveld method(Chapter 3).Then,the microstructure of martensite(Chapter 4)and the crystallographic features of martensitic transformation(Chapter 5)were systematically studied.Finally,the behaviors and mechanisms of martensite variant rearrangement/selection under two kinds of mechanical loading strategies,i.e.loading at martensite state and loading across the structural transition were explored(Chapter 6).The main results are as follows:Crystal determinations show that the austenite of Ni50Mn50-xInx(0≤x≤25)alloys possesses a highly ordered cubic L21 structure belonging to the space group Fm3m.The non-modulated martensite has a tetragonal L10 structure belonging to the space group P4/mmm.The modulated martensite of Ni50Mn36In14 alloy has an incommensurate 6M modulated structure with the superspace group 12/m{α0γ)00.A three-fold layered superstructure in the three-dimensional space was proposed to approximately describe the incommensurate modulated structure.Microstructure characterizations show that the 6M modulated martensite is in plate shape and self-organized in colonies within which the plates stretch roughly in the same direction.Each colony has four types of orientation variants,which can be classified into three categories,i.e.type-Ⅰ(A:C,B:D),type-Ⅱ(A:B,C:D)and compound twin(A:D,B:C).The twinning shears of type-Ⅰ and type-Ⅱ twins are the same and equal to 0.2392 that is around one order of magnitude higher than that of compound twin(0.0277).Variant interfaces are defined by their corresponding twinning plane Ki in the mesoscopic scale.Type-1 twin is of a coherent interface at atomic scale,whereas the type-Ⅱ and compound twins have "stepped" interfaces.The step heights of the compound twin interface are much larger than those of the type-Ⅱ twin interface.Both the K-S and Pitsch ORs are appropriate to describe the lattice correspondence between austenite and martensite in NiMnIn alloys.However,the strain path related to the Pitsch relation({101}A,<101>A)is evidenced to be the effective for the structural distortion.Four distinct martensite variants within one colony are transformed from the same(or opposite)transformation plane and direction of austenite.Due to cubic symmetry,a maximum 6 distinct martensite colonies and 24 distinct variants within one austenite grain can be obtained.Furthermore,the geometrical compatibility between austenite and martensite of Ni45Co5Mn36.8In]3.2 alloy was explored.The middle eigenvalue of the transformation stretch tensor across the structural transition is close to 1(0.99652)and the habit plane is bordered by single martensite variant with austenite.A stressed transition layer of around 20 nm near the habit plane implies that the criterion of λ2 = 1 is not sufficient to guarantee the geometric compatibility in this alloy.For compressive loading at martensite,variant arrangement is realized by the detwinning process.It is evidenced that a single variant state in some colonies can be obtained when the loading orientation is located in the common positive Schmid factor(SF)zone of the three detwinning systems.For loading across the structural transition,the prestrain is obtained by variant selection in which the number of colonies is significantly reduced and the variant organization within colony is greatly changed.The SF for transformation strain path is introduced to evaluate the possible selection of variants.The present work is expected to provide some fundamental information on crystal structure,microstructure,martensitic transformation crystallography,variant rearrangement/selection behavior of NiMnIn based alloys for the understanding of their multifunctional properties and the further performance optimizations. |
