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Title: Diffraction investigations of high temperature shape memory alloys
Author: Azeem, Mohammed Abdul
ISNI:       0000 0004 2737 1215
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Shape memory alloys are intermetallic materials with a unique ability to revert to a predefined physical shape by virtue of diffusionless transformations. Recent interest by aerospace and automotive industries to exploit the functionalities of these materials in future energy efficient designs has renewed scientific research in this field. However, the current understanding of transformation hysteresis is inhibited by experimental difficulties associated with viewing the transformations and therefore most of our knowledge is confined to symptomatic bulk properties such as those accessible from calorimetry and dilatometry. In the current study, in situ synchrotron X-ray diffraction (SXRD) was used to accurately document the adaptability of unit cells of participating phases during transformation in a series of high temperature shape memory alloys (HTSMAs). Selected alloys based on NiTi, ZrCu and NiMnGa systems were prepared in vacuum arc melter, homogenized and rolled to grain size adequate for SXRD experiments. The resulting diffraction patterns were Rietveld refined and the evolution of unit cell parameters of participating phases were recorded as a function of temperature. It was observed that the lattices of participating phases undergo a significant overall dilation during transformation. The lower symmetry martensite unit cell was observed to undergo unprecedented anisotropic strains, reaching as high as 1.2% in certain alloys. The high temperature higher symmetry austenite was observed to complement the changes in martensite lattice during heating and vice versa was observed during cooling. These changes were mostly observed in final stages of transformation. Surprisingly, a negative coefficient of thermal expansion was observed in the b lattice parameter of the monoclinic martensite in NiTi based alloys. The implications of such strains on the current phenomenological martensitic transformation models that takes into consideration lattice parameters away from the transformation regime for prediction of orientation between participating phases has been discussed.
Supervisor: Dye, David ; Dashwood, Richard Sponsor: Rolls-Royce Group plc ; UK India Education and Research Initiative ; Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral