Lecture 9
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describe the basic chemical and mechanical properties of NiTi
• 50% Ni, 50% Ti (atomic %) • High T resistance, high corrosion resistance, tough, ductile • shape memory (pseudoplastic) • superelastic (pseudoelastic)
identify the intermetallic NiTi phase in the Ni-Ti phase diagram
towards the middle, has an elongated triangle shape (edge of triangle pointing to the right)
describe the 2 unique properties of nitinol in a stress - strain - T diagram
1) pseudoelastic : at constant T ; first elastic A, then at cst stress A to detwinned M. Reverse : at lower cst stress, M to A and then down to original position -> no residual deformation. 2) pseudoplastic : at lot T we have twinned M. We shape it by applying a load -> detwinning (stress plateau). Then we heat up and detwinned M goes to A. B cooling down, shape doesn’t change but we go to twinned M.
specify applications that use eihter one or the other effect
shape memory : - clip to fix cranial prosthesis - hooks for anchorage of prosthesis pseudoelastic : - wire for orthodontics (braces) - nerve probe - fix heart defect
give information about shape setting
heat up too 450° then quench (drop into cold water) -> this fixes the crystal defects. The metal will always go back to this shape (except if we heat up to 450 again, or do many heat treatments)
explain how energy is stored in a pseudoelastic material
energy is stored during the deformation induced phase transformation (internal stresses). austenite -> detwinned martensite (bent) -> austenite
describe the T-dependance of the crystallographic phases
martensite (monoclinic) : at low T austenite (cubic) : at high T NO diffusion ! Just lattice symmetry that changes
what are the two ways for the phase transformation from austenite to martensite to happen ?
stress - strain applied T decreased
explain the physico-chemical method DSC
there is a blank (reference) and our sample. We perform a heating / cooling cycle and measure the heat flow between the two.
interpret a DSC diagram of NiTi
heating : M to A, with an endotherm peak (towards the bottom). Cooling : A to M, with an exotherm peak
explain the term “nickel cliff”
Martensite start temperatures depend strongly on the Ni % after ~49.8% -> very steep curve : M_s decreases fast when %Ni increases a little bit
explain pseudoplasticity and -elasticity by means of crystallographic phase transformations
1) pseudoplasticity : A -> cool -> twinned M -> deform -> detwinned M -> heat -> A (original shape) 2) superelasticity : A -> load -> detwinned M -> unload -> A
influence of T on the stress plateau for superelasticity ?
Higher T = higher stress plateau
differentiate elastic and pseudoelastic deformation in an atomic model
Hooke : load -> stretch the bonds, they act like a spring and can go back (except if plastic) Pseudoelastic : load -> phase transformation (bonds are not stretched) -> shows no fatigue
represent T-dependant phases, elasticity, pseudoplasticity and -elasticity by means of a statistical-mechanical model
Looks like inverted batman cap : left and right are detwinned M, middle is A. Energy VS shear angle. T (horizontal lines) : High = A, medium = coexistence, low = M pseudoplasticity : load -> detwinning to preferred M variant -> heating to A -> cooling to all M variants (equal poportions) elastic : small loads = reversible deformations without changing phase pseudoelastic : - high T : stress induced M -> reversible when unloading (x axis is diagonal) - low T : not reversible, just preferred M variant
ho to derive the corresponding stress-strain diagrams from the statistical-mechanical model
free energy F = U - TS stress = dF / de (strain)
elucidate the stress-strain digram of NiTi at various temperatures, how does Young’s modulus change with T ?
(TRY TO DRAW THEM) at T lower than A, or higher than A Modulus increases when T increases (counter intuitive)
picture reasons for the excellent biocompatibility of NiTi
outer titanium oxide layer (prevent Ni from leaking out) it is stable long term (corrosion resistance)