Fabrication And Performance Of Gradient Porous NiTi Shape Memory Alloys With Long Bone Structure

The skeleton is an important human tissue, and the quality of people’s life is greatlyaffected by bone defects caused by congenital defect, tumor resection, etc. In clinical work,the long bone repair is a pendent problem due to its structural complexity. In this study, wefocused on the preparation of gradient porous NiTi shape memory alloys with bone structurefor long bone repair, which will expand the application of porous NiTi shape memory alloysin medical field.Firstly, we explored the synthesis technology of porous NiTi shape memory alloys whichimitated the structure of cortical bones and cancellous bones. The effect of sinteringtemperature, sintering time and cold pressing-pressure on the pore morphology and alloyingdegree was studied. We found that the NiTi shape memory alloy with small porosity of13-20%to imitate cortical bones could be sintered with cold pressure of400MPa, sinteringtemperature of1100℃for3h without space holder using vacuum sintering in the tubefurnace. In addition, TiH2was found to be a suitable space holder for the preparation ofporous NiTi shape memory alloys with porosity less than30%, while NH4HCO3was suitablefor making large and connective pores. Adding30-35%NH4HCO3could aid to prepareporous NiTi shape memory alloys with porosity of44-58%which was aimed for bioniccancellous bones.Secondly, we studied the effect of heat treatment parameters and porosity on the phasetransformation and mechanical properties of porous NiTi shape memory alloys. Theestablished relationship between porosity and mechanical properties provides a reference forthe performance evaluation of bionic gradient porous NiTi shape memory alloys. It was foundthat the porous NiTi shape memory alloys exhibited well superelasticity after solid solutionand aging treatment. The elastic modulus and compressive strength decreased with increasingporosity. When the porosity reached49.98%, the compressive strength was192.56MPa,which well met the strength requirement of human bones. As the porosity further increased,the compressive strength could no longer satisfy the strength requirement. The elastic moduli of the porous NiTi shape memory alloys were all below30GPa, in the range of human bonemodulus.Finally, bionic gradient porous NiTi shape memory alloy was successfully preparedthrough a homemade mold for cold pressing and vacuum sintering in a tube furnace. Thealloy with outer layer porosity of13.98%, inner porosity of52.05%, radius and outer wallthickness ratio of5.5and the average porosity of39.36%, exhibited a compressive strength of360.58MPa and elastic modulus of6.69GPa, which could well meet the clinical demand forbionic human bone. Gradient porous NiTi shape memory alloys exhibited a unique crackpropagation model due to the interaction between the inner and outer layers with differentporosities, and their mechanical performances were better than those of uniform porous NiTishape memory alloys with the same porosity.