Advanced TEM Investigations on Ni-Ti Shape Memory Material: Strain and Concentration Gradients Surrounding Ni 4 Ti 3 Pre
- PDF / 2,934,839 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 127 Views
S3.6.1
Advanced TEM Investigations on Ni-Ti Shape Memory Material: Strain and Concentration Gradients Surrounding Ni4Ti3 Precipitates
Dominique Schryvers, Wim Tirry and Zhiqing Yang Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium ABSTRACT Lattice deformations and concentration gradients surrounding Ni4Ti3 precipitates grown by appropriate annealing in a Ni51Ti49 B2 austenite matrix are determined by a combination of TEM techniques. Quantitative Fourier analysis of HRTEM images reveals a deformed nanoscale region with lattice deformations up to 2% while EELS and EDX indicate a Ni depleted zone up to 150 nm away from the matrix-precipitate interface. INTRODUCTION NiTi alloys with near-equiatomic composition can exhibit shape memory and superelastic properties resulting from a temperature or stress induced austenite-martensite phase transformation. The behaviour and characteristics of this transformation are strongly influenced by the presence of Ni4Ti3 precipitates in the B2 austenite matrix and which can be obtained by appropriate annealing procedures. The atomic structure and morphology of these precipitates have been investigated before [1,2,3]. Due to the anisotropic change of the unit cell dimensions and lattice parameters the precipitates form with a lens shape inside the cubic matrix. Their influence on the transformation temperatures and the occurrence of multiple step transformations was mainly investigated by differential scanning calorimetry (DSC) measurements and conventional transmission electron microscopy (TEM) [4-7]. Small precipitates with a diameter of the central disc up to 300 nm remain coherent or semi-coherent and can act as nucleation centers for the formation of the so-called R-phase [4,6], a rhombohedral distortion preceding the martensitic transformation. Larger precipitates lose their coherency with the matrix and the stress field is partially relaxed by the introduction of interface dislocations [9,10], though they can still act as nucleation centers [4]. This behaviour is explained by the fact that the lattice mismatch between precipitate and matrix induces a stress field in the surrounding matrix favouring particular variants of the product phases. Also the change of Ni concentration in the matrix, due to the higher Ni content in the precipitates, can be expected to have an influence on the local transformation temperatures as is the case for concentration changes at the bulk level [4,8]. However, up till now no quantitative experimental measurements of the strain or concentration gradients exists. In the present work high resolution transmission electron microscopy (HRTEM) is used to measure the actual lattice deformations in the matrix around the Ni4Ti3 precipitates. Relative differences in interplanar spacings are determined by Fast Fourier techniques applied to the HRTEM images. To determine the presence of a possible variation in Ni concentration in close proximity of a precipitate nanoprobe electron energy l
Data Loading...
