Functionally graded titanium implants: Characteristic enhancement induced by combined severe plastic deformation.

Shokouh Attarilar,Khaled J Al-Fadhalah,Faramarz Djavanroodi,Masoud Mozafari,Mohamad Taghi Salehi,Antonio Riveiro Rodríguez

doi:10.1371/journal.pone.0221491

Abstract

Commercially pure titanium was processed by equal channel angular pressing (ECAP) and surface mechanical attrition treatment (SMAT) for the purpose of developing functionally graded titanium used for implants and a gradient structure including nanostructured, deformed and undeformed zones were produced on the samples. In particular, it was aimed to design the gradient-structure in the titanium with enhanced properties by applying 4 ECAP passes to form bulk structure of ultrafine-grains and subsequently subjecting SMAT to the surface of ECAPed samples to produce nanostructured surface region. Microstructural examination was made by electron back scatter diffraction (EBSD). Also, microhardness, nanoindentation, topography, roughness and wettability were evaluated. To examine the biological response, human osteosarcoma cells were cultured in contact with the samples in various time periods and morphology change, cell viability and alkaline phosphate activity were conducted also cell morphology was monitored. EBSD showed development of ultrafine-grained structure after 4 passes of ECAP with an average grain size of 500 nm. Applying SMAT resulted in additional refinement in the ECAP samples, particularly in the subsurface regions to a depth of 112 μm. Furthermore, the SMATed samples showed an enhancement in roughness, wettability and hardness magnitudes. Viability enhanced up to 7% in SMATed + ECAPed sample, although the acceptable cell adhesion, improved cell differentiation and mineralization were seen. The combined use of ECAP and SMAT has shown a good potential for optimizing the design of modern functionally graded medical devices and implants.

Highlights

The application of surface mechanical attrition treatment (SMAT) on annealed sample resulted in severe plastic deformation of the subsurface region to a depth of about 100 μm due to residual compressive stresses applied by SMAT, Fig 4(C)
Grain refinement in this region was attained through dislocation cell formation by the gradual alteration of dislocation cell walls to high-angle boundaries (HAGBs) that can be achieved by progressing straining [52]
equal channel angular pressing (ECAP) was performed on CP Ti samples in 450 ̊C up to four passes for attaining favorable mechanical bulk properties, surface mechanical attrition treatment (SMAT) was applied in order to improve surface and mechanical properties, biological experiments were done on different samples

Summary

Introduction

One major disadvantage of pure titanium is the low mechanical properties, as compared to Ti-6Al-4V (Ti64 alloy) and other materials used in biomedical applications. The tensile strength and fatigue endurance limit of pure Ti are relatively low [2]. In this regard, alloying can significantly improve the mechanical properties of Ti, but in the case of Ti64 alloy, there are great concerns about the adverse effects of released aluminum and vanadium ions which lead to cytotoxicity and influence the cellular behavior such as osteoblast metabolism and differentiation [3] and even, DNA and nuclear damage [4]. Alloying is not an ideal procedure for mechanical improvement of biomaterials

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Results

Conclusion