ABSTRACT
TiAl6V4 wrought alloy is a standard material used for endoprostheses due to its ideal characteristics in terms of osseointegration. However, the insufficient wear and crevice corrosion resistance of TiAl6V4 are limiting factors that can cause clinical problems. Therefore, the objective of this study was to analyze and identify suitable phases and microstructural states of TiAl6V4 alloy with advantageous implant properties by thermal treatments. By varying the temperature and cooling rate, four heat treatment strategies were derived that produced different microstructural states that differed in morphology, arrangement and proportions of phases present. All TiAl6V4 modifications were characterized regarding their microstructure, mechanical, corrosive and tribological properties, as well as cell adhesion. The acicular, martensitic microstructure achieves a significant hardness increase by up to 63% and exhibits improved corrosion and wear resistance compared to the forged condition. Whereas the modified microstructures showed similar electrochemical properties in polarization tests using different electrolytes (PBS with H2O2 and HCl additives), selective α or ß phase dissolution occurred under severe inflammatory crevice conditions after four weeks of exposure at 37 °C. The microstructurally selective corrosion processes resemble the damage patterns of retrieved Ti-based implants and provide a better understanding of clinically relevant in vivo crevice corrosion mechanisms. Furthermore, a microstructural effect on cell attachment was determined and is correlated to the size of the vanadium-rich ß phase. These key findings highlight the relevance of an adapted processing of TiAl6V4 alloy to increase the longevity of implants.
ABSTRACT
Wear of orthopaedic endoprostheses is associated with adverse local and systemic reactions and can lead to early implant failure. Manufacturing determines the initial subsurface microstructure of an alloy that influences the implant's wear behaviour. Therefore, this study aims at generating enhanced wear resistances by a modification of the surface and subsurface microstructure of a CoCr28Mo6 wrought alloy by applying deep rolling. The state of the art was investigated by means of eleven retrieved CoCr28Mo6 hip implant components from different manufacturers with respect to their subsurface microstructure and micro hardness profiles. CoCr28Mo6 wrought alloy samples (DIN EN ISO 5832-12) were aged at 750 °C for 24 h and/or plastically deformed by deep rolling with varying axial forces (170 N, 230 N and 250 N). The samples were metallographically prepared and investigated using optical and scanning electron microscopy with EDS and EBSD, micro hardness testing, XRD and tribological testing. The retrieved implant components revealed that, independent of the manufacturer, neither the head nor the stem trunnion exhibited a defined subsurface condition. The dominant phase within the implants was face-centered cubic (fcc). Some implants exhibited single hexagonal close-packed (hcp) grains due to a stress-induced phase transformation. The initial CoCr28Mo6 wrought alloy had a fcc crystal structure. After isothermal aging, the matrix entirely transformed to a hcp structure. In the initial fcc-condition, deep rolling generated a plastically deformed surface layer within the first 100 µm and stress-induced phase transformation to hcp was observed. Micro hardness gradients were present in the subsurface of up to 600 µm depth and exhibited a maximum increase of 34% by deep rolling in comparison to the initial fcc-matrix. This trend was confirmed by a correlated increase in residual compressive stresses. In tribological tests under serum lubrication, the modified samples generated lower wear in comparison to the contemporarily used fcc-matrix samples. This study demonstrates that deep rolling is an effective processing to modify the subsurface of a biomedical CoCr28Mo6 wrought alloy in order to increase the wear resistance. The intentional transformation from the fcc to the hcp phase induced by deformation offers great potential for implant application.
