Viscoelastic Deformation and Fracture of Porous Nickel Titanium after Tension and Cyclic Bending
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VISCOELASTIC DEFORMATION AND FRACTURE OF POROUS NICKEL TITANIUM AFTER TENSION AND CYCLIC BENDING E. S. Marchenko, Yu. F. Yasenchuk, G. A. Baigonakova, S. V. Gyunter, and A. A. Shishelova
UDC 538.971
The paper deals with the deformation curves and fatigue tests of the porous nickel–titanium (NiTi) alloy produced with self-propagating high-temperature synthesis. Deformation curves obtained for tension and three-point bending of porous NiTi plates demonstrate their viscoelastic deformation due to the austenite-tomartensite phase transformation. Scanning electron microscopy observations of the fractured surface show the regions of quasi-brittle fracture of martensite and ductile fracture of austenite. Brittle fracture is observed for nonmetallic inclusions and the porous structure coating of the intermetallic compound. Fatigue tests show that the brittle phases and inclusions have no critical negative effect on the strain and fatigue properties of the NiTi alloy. It is determined that 70% of the porous alloy specimens withstand 106 cyclic bending owing to the reversible austenite-martensite phase transformation in the parent phase which is one the components of the multi-phase porous alloy. Keywords: TiNi alloy, self-propagating high-temperature synthesis, structure, surface, viscoelastic deformation, fatigue strength, wire, fracture pattern.
INTRODUCTION The porous nickel–titanium (NiTi) alloy produced with self-propagating high-temperature synthesis (SHS), is the promising material for the bone defect replacement owing to its viscoelasticity coherent with biological tissues subjected to cyclic loading [1, 2]. This is possible due to the reversible, diffusion-free austenite-martensite phase transformation (A→M→A), which prevents the defect accumulation of the internal structure and allows the TiNi alloy to withstand a 4–8% recoverable strain without fracture [3]. For clinical applications of the SHS-produced TiNi alloy, it is advisable to explore its deformation behavior and resilience under static and dynamic loads. The deformation curve of the load is the most important and indispensable priority in studying the strength properties of all materials. Many ductile or elastic alloys exhibit, to some extent, a viscoelastic behavior and a viscous flow region of about 1–5% strain on the deformation curve [4–6]. In this region, a primary irreversible defect accumulation occurs in the internal structure, resulting in further elastic deformation of the hardened alloy. The self-propagating high-temperature synthesis is one of powder metallurgy techniques that produces TiNi alloys with a protective coating and 50–80% porosity allowing them to be well integrated into biological tissues. The low thickness and high density of this coating, its nanocrystalline intermetallic carbides and nitrides, and good substrate diffusion provide the appropriate corrosion fatigue [7]. This coating is however, rather brittle, which is well observed during an impact fracture of the porous alloy.
National Research Tomsk State University, Tomsk
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