Crystallographic Study On Microstructure And Martensitic Transformation Of Ni₅₀Mn_(50-x)Sb_x(x=12,13) Alloys

NiMnSb based Heusler type alloys,as a novel multi-functional material,have 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 knowledge on this family of materials,such as the accurate crystal structure of modulated martensite,variant type and orientation relationship(OR)and the underlying mechanism of microstructural evolution during martensitic transformation etc.,is still unknown.This largely restricts the development of NiMnSb based multi-functional alloys.In this thesis,the crystallographic characteristics of NiMnSb modulated martensite,such as crystal structures,microstructural and crystallographic features,martensitic transformation OR and its correlation with variant organization,were systematically characterized by using single martensite sample(Ni50Mn38Sb12).Moreover,the microstructural evolution mechanisms during the martensitic transformation,such as martensitic transformation mechanism,transformation strain characteristics and self-accommodation mechanism,were revealed with two-phase coexisting sample(Ni50Mn37Sb13)at room temperature.X-ray diffraction results revealed that the lattice constants of the Ni50Mn38Sb12 and Ni50Mn37Sb13 martensite are aM=8.5830 A,bM=5.6533 A,cm=4.3501 A,??M=?M=?M=90°and am=8.5788 A,bM=5.6443 A,cM=4.3479 A,?m=?M=?M=90°,respectively.The size of the cell increases with the increase of Sb and decrease of Mn.In the original austenite grain,the 4O NiMnSb modulated martensite presents fine lamellae martensite variants first,then variants into plate,plates into sub-colony and finally sub-colonies into plate colony.Each plate possesses 4 distinct twin related variants A,B,C and D forming type I twins(A and C;B and D),type ? twins(A and B;C and D)and compound twins(A and D;B and C).The complete twinning elements for each twin relation are fully determined.The variant interfaces are defined by the corresponding twinning planes.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.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 plates,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 affects the morphology of the sandwich colonies and results in the high occurrence frequency of the type II twins.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.