MICROSTRUCTURE AND FUNCTIONAL PROPERTY CHANGES IN THIN Ni-Ti WIRES HEAT TREATED BY ELECTRIC CURRENT — HIGH ENERGY X-RAY AND TEM INVESTIGATIONS

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni–Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni–Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni–Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni–Ti wire are comparable to those observed in straight annealed wires.

[1]  Olivier Mathon,et al.  Invited article: the fast readout low noise camera as a versatile x-ray detector for time resolved dispersive extended x-ray absorption fine structure and diffraction studies of dynamic problems in materials science, chemistry, and catalysis. , 2007, The Review of scientific instruments.

[2]  J. Perkins,et al.  Shape Memory Effects in Alloys , 1975 .

[3]  Jason C. Cole,et al.  DASH: a program for crystal structure determination from powder diffraction data , 2006 .

[4]  S. Miyazaki,et al.  CRYSTAL STRUCTURE OF THE MARTENSITE IN Ti-49.2 at.%Ni ALLOY ANALYZED BY THE SINGLE CRYSTAL X-RAY DIFFRACTION METHOD , 1985 .

[5]  Ruslan Z. Valiev,et al.  Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing , 2003 .

[6]  D. Schryvers,et al.  R-Phase Structure Refinement Using Electron Diffraction Data , 2002 .

[7]  C. Ro,et al.  X-ray spectrometry. , 2008, Analytical chemistry.

[8]  Z. A. Munir,et al.  Electric current enhanced defect mobility in Ni3Ti intermetallics , 2004 .

[9]  J. Humbeeck,et al.  Grain growth and precipitation in an annealed cold-rolled Ni50.2Ti49.8 alloy , 2007 .

[10]  L. Schetky.  Shape-memory alloys , 1979 .

[11]  W. J. Buehler,et al.  Crystal Structure and a Unique ``Martensitic'' Transition of TiNi , 1965 .

[12]  Mark R. Daymond,et al.  In situ neutron diffraction studies of martensitic transformations in NiTi polycrystals under tension and compression stress , 2004 .

[13]  X. Ren,et al.  Physical metallurgy of Ti–Ni-based shape memory alloys , 2005 .

[14]  D. Parkin,et al.  Crystal-to-amorphous transformation of NiTi induced by cold rolling , 1990 .

[15]  V. Brailovski,et al.  Characterization of amorphous and nanocrystalline Ti–Ni-based shape memory alloys , 2009 .

[16]  M. Gao,et al.  Effect of electric current pulse on tensile strength and elongation of casting ZA27 alloy , 2002 .

[17]  I. Bataronov,et al.  Physical basis of the electroplastic deformation of metals , 1996 .