Development of a workable radiopaque rare earth doped nitinol

This thesis details the results obtained from an extensive study on the development of a workable radiopaque rare earth doped NiTi alloy through a traditional ingot melting (IM) route and a powder metallurgy (PM) route. The rare earth elements investigated as a potential NiTi ternary alloying element included erbium (Er), lanthanium (La), neodymium (Nd), gadolinium (Gd) and lutetium (Lu). Overall Erbium additions proved to be the most favourable from the ternary alloy workability point of view. The IM route involved the use of three different foundaries to melt the ingots. Two of these foundaries also hot worked the materials. Other hot working results that appear in this thesis were performed in UL on custom built equipment. Overall an interdendritic or intergranular ErNi network was always shown to form around the NiTi dendrites or grains in the IM route. This resulted in an alloy that was extremely difficult, if not impossible, to hot and cold work. Homogenisation heat treatments were discovered that do break up or at least partially break up this network into many individual ErNi spheroids. However, breaking the ErNi network has not, to date, resulted in successful hot rolling of the IM alloys. Extrusion produced far superior results, but no NiTiEr extrudate has been successfully cold drawn to-date. Attempts to add other elements such as chromium (Cr) and palladium (Pd) to improve the network break up through subsequent heat treatments and hot working, worked to a certain degree but ultimately resulted in hardening the alloy and embrittling it. Small additions of boron (B) finally proved successful in improving the workability of the alloy as it is drawn into the ErNi phase and appears to make this phase more ductile. The PM route has produced the most favourable results to-date, where-by 4% recoverable strain has been recorded through tensile testing. This was achieved through the addition of ErNi powder to NiTi powder and the non-reactive spark plasma sintering (SPS) of these powders into an ingot. This was followed by hot rolling the alloy into a strip for tensile testing at a hot rolling temperature up to 880 °C. A second alloy also proved successful, where-by 11% strain to failure was recorded through tensile testing. This alloy was made by spark plasma sintering ErFe gas atomized powder (1.5wt.% Fe) and NiTi powder in a reactive form of sintering. This alloy’s austenite finish temperature was high, so little to no recoverable strain was present. Overall both the IM and PM route have had their successes and failures, but both routes are very much strong runners as potential routes for the production of a workable and cost effective radiopaque NiTi alloy.