Structure and thermomechanics of selective laser melted nickel-titanium

Abstract

NiTi has been established for applications in load-bearing implants due to its mechanical properties, which mimic the characteristics of bone better than any other known biocompatible metal or metallic alloy. Further, NiTi is well known for pseudoelasticity and pseudoplasticity, i.e. the possibility for shape recovery after deformation. Both macroscopic effects are based on a thermoelastic martensitic phase transformation, i. e. rearrangement of atoms on the sub-nanometer scale. For load-bearing implants, the appropriate mechanical stimulation of bony tissue enhances osseointegration. NiTi scaffolds exhibiting pseudoelasticity allow the cyclic mechanical stimulation of tissue in its proximity, as an induced deformation is recovered if the stress is removed. This is hypothesized to lead to improved bone ingrowth, better bonding between implant and surrounding tissue and ultimately to an enhanced implant performance. As the additive manufacturing technique of selective laser melting (SLM) allows the straightforward fabrication of dense as well as porous NiTi constructs, this work deals with SLM-processing of the NiTi alloy regarding scaffolds as medical implants. The first part of the thesis is concerned with the impact of processing parameters onto the resulting material properties, because selective laser melting is known to alter material characteristics in an anisotropic manner. In dense parts, variation of the processing parameters shifted the phase transformation temperatures of up to 50 K. This shift resulted from preferential nickel evaporation and allowed the fabrication of parts with pseudoelastic and with pseudoplastic properties at body temperature from the same lot of powder. While the scanning speed determined the amount of lost Ni, the laser power applied was crucial for the resulting microstructure. The grain size increased about a factor of 3 and the grain width increased about a factor of 10 with raised applied laser power. Also the crystallographic texture, i.e. a preferred crystal orientation in the building direction, increased. The grain size distribution changed thereby from unimodal to bimodal. The enlargement of grains > 40 µm and the bimodal grain size ditribution indicateded secondary grain growth, i.e. Ostwald-ripening, during SLM fabrication. In case of the unimodally distributed grain sizes, the microstructure was in accordance to the ASTM standard F2063-05 regarding medical applications of NiTi alloys. The second part of the thesis deals with the characterization of SLM-built porous NiTi scaffolds. The scaffolds morphology showed deviations from the intended design, as excess material was accumulated particularly underneath the struts. This led to increased material volume and decreased porosity within the scaffold. The actual porosity of the investigated specimen corresponded to about 76 %, while an open pore volume of about 84 % was aspired. As the scaffolds are intended to mechanically stimulate surrounding tissue by mechanical micro-motions, the local deformations upon uniaxial scaffold compression were analyzed by synchrotron radiation based micro computed tomography in combination with three-dimensional non-rigid registration. Displacements and strains within the scaffold were identified on the micrometer scale and visualized. Compressive and tensile strains occurred simultaneously during scaffold deformation. Uniaxial compression of 6 % led to local compressive and tensile strains of up to 15 %. In addition, an in-situ SRµCT setup was applied to study the shape recovery process of the pseudoplastic scaffold during heating. The inhomogeneous shape recovery process starting on the scaffolds' bottom, proceeding up towards the top and terminating at the periphery of the scaffold was demonstrated.