The control of crystallographic texture in the use of magnesium as a resorbable biomaterial

Magnesium and its alloys are an emerging class of resorbable materials for orthopedic and cardiovascular applications. The typical strategy underlying the development of these materials involves the control of material processing routes and the addition of alloying elements. Crystallographic texture is known to control bulk mechanical as well as surface properties. However, its role in determining the properties of magnesium for implant materials has not been well studied. In this work, an extruded rod of pure magnesium was cut in multiple directions to generate samples with different textures. It was found that texture significantly affected the strength and ductility of magnesium. Corrosion rates in Hank's solution decreased with the increased presence of low energy basal planes at the surface. In vitro cell studies revealed that changes in texture did not induce cytotoxicity. Thus, the control of texture in magnesium based implants could be used to tailor the mechanical properties and the resorption rates without compromising cytocompatibility. This study elucidates the importance of texture in the use of magnesium as a resorbable biomaterial

The importance of crystallographic texture in the use of titanium as an orthopedic biomaterial

Crystallographic texture is perceived to play an important role in controlling material properties. However, the influence of texture in modulating the properties of biomedical materials has not been well investigated. In this work, commercially pure titanium (cp-Ti) was processed through six different routes to generate a variety of textures. The effect of texture on mechanical properties, corrosion behavior, cell proliferation and osteogenesis was characterized for potential use in orthopedic applications. The presence of closely packed, low-energy crystallographic planes at the material surface was influenced by the volume fraction of the components in the overall texture, thereby influencing surface energy and corrosion behavior. Texture modulated osteoblast proliferation through variations in surface water wettability. It also affected mineralization by possibly influencing the coherency between the substrate and calcium phosphate deposits. This study demonstrates that crystallographic texture can be an important tool in improving the properties of biomaterials to achieve the enhanced performance of biomedical implants

Establishing the microstructure-strengthening correlation in severely deformed surface of titanium

Surface nanostructuring of engineering materials can be utilised to enhance materials performance for various applications. The aim of this work was to investigate the evolution of microstructure and its correlation with strengthening mechanisms in nanocrystalline commercially pure titanium (cp-Ti) produced by surface mechanical attrition treatment (SMAT). The individual contributions of dislocation slip and twining as the deformation mechanisms during SMAT have been quantified using X-ray line profile analysis and corroborated with transmission electron microscopy and electron backscattered diffraction techniques. It is found that twining is operative only in the early stages of deformation. The absence of twin-twin intersections suggests that twining is not directly responsible for the initial refinement of grain size. Dislocation slip is the major deformation mode, which leads to the refinement of the microstructure by forming low-angle lamellar boundaries. Continuous dynamic recrystallisation is demonstrated to be the mechanism of nanocrystallisation in cp-Ti using detailed microscopic analysis. In contrast to previous studies, which have neglected the contribution of Taylor strengthening, it is observed that a combination of Hall-Petch and Taylor relationships can explain the strength only if separate set of parameters K (Hall-Petch constant) and (geometrical factor in Taylor relationship) are used for the nanocrystalline surface and severely deformed sub-surface of cp-Ti. Taken together, this work provides new insights into the underlying mechanisms for engineering nanocrystalline materials

Globularization using heat treatment in additively manufactured Ti-6Al-4V for high strength and toughness

A bimodal globularized microstructure in contrast to martensitic laths is known to impart high strength and toughness in Ti-6Al-4V. Heat treatment for the phase transformation of the laths to the globularized microstructure must be preceded by plastic deformation. This work reports an innovative strategy to obtain the bimodal microstructure consisting of globular alpha in additively manufactured Ti-6Al-4V alloy by heat treatment alone. The heat treatment schedule involves repeated thermal cycling close to but below the beta transus temperature to form globular alpha eliminating the need for plastic deformation prior to heat treatment. A new mechanism of globularization other than known in literature is proposed to explain the formation of globular alpha. The inherent dislocation sub-structure of the martensitic laths initiates globularization by thermal grooving and boundary splitting but is unable to completely globularize the microstructure. Mechanisms such as cylinderization and edge spheroidization also do not lead to globularization. The purposefully designed thermal cycling causes oscillations in the volume fractions of alpha and beta phases that in synergism with the slow cooling segments of the cycle globularize the a phase by epitaxial growth. The bimodal microstructure thus produced led to a significant improvement in the ductility by 80% and the toughness by 66%, which are desirable for structural applications. Furthermore, beneficial compressive stresses were generated in the alloy because of cyclic heat treatment. It is envisaged that the exceptional combination of mechanical properties observed here will lead to the fabrication of SLM printed Ti-6Al-4V parts that could leverage the advantages of additive manufacturing with material properties that are comparable to those obtained by conventional fabrication routes. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Elucidating microstructural evolution and strengthening mechanisms in nanocrystalline surface induced by surface mechanical attrition treatment of stainless steel

Surface mechanical attrition treatment (SMAT) is a high strain and strain rate severe plastic deformation (SPD) technique for surface nanocrystallization of metals. The aim of this study was to investigate the mechanism of nanocrystallization and strengthening in a medium stacking fault energy 316 L austenitic stainless steel during SMAT. The paramount role of microband and shear band formation in nano crystallization is outlined, as opposed to deformation twinning previously reported in low SFE austenitic stainless steels. Shear bands undergo dynamic recrystallization and recrystallization twinning to produce ultra-fine grains in contrast to twin-twin intersections in low SFE stainless steel. The ultra-fine grains further sub-divide into smaller cells with initially low misorientation. Nanocrystallization occurs when misorientation between these cells increases with further strain. The additivity of strengthening by dislocation density and grain size is studied. Dislocation density was neglected in previous studies while studying strengthening mechanisms in SMAT processed materials. This study illustrates that dislocation density cannot be ignored as the strengthening mechanism in SMAT process. The grain size and dislocation density both significantly contribute to overall strengthening in SMAT processed microstructure. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Engineering the next-generation tin containing beta titanium alloys with high strength and low modulus for orthopedic applications

Metastable beta Ti alloys are the new emerging class of biomaterial for load bearing orthopedic applications. However, these alloys in the single beta phase microstructure have insufficient strength for use in load bearing applications. It is imperative to strengthen these alloys by carefully designed thermo-mechanical processing routes that typically involve aging treatment. In this investigation two newly designed Sn based beta Ti alloys of composition Ti-32Nb-(2, 4) Sn are evaluated. The effects of Sn content on the mechanical properties and biological performance of these alloys processed through designed thermo-mechanical processing route are investigated. The increase in the Sn content led to a reduction in the elastic modulus of the alloy. An increase in the Sn content increased the aspect ratio of the a precipitates, which led to a significant strengthening in the alloy while keeping the elastic modulus low. In addition, the corrosion behavior of the alloy was evaluated in simulated body fluid. The Sn containing beta alloys have an excellent corrosion resistance as desired for an implant material. The corrosion resistance improved with an increase in Sn content. These alloys were also observed to have excellent cytocompatibility as they not only supported the attachment and proliferation of human mesenchymal stem cells but also their osteogenic differentiation in vitro. The combination of high strength, low elastic modulus, superior corrosion resistance and biocompatibility underscores the promise of Sn containing beta Ti alloys for use in orthopedic applications

Surface nanostructuring of titanium imparts multifunctional properties for orthopedic and cardiovascular applications

Commercially pure titanium (cp-Ti) is a metallic biomaterial used in orthopedic and cardiovascular applications. Here, surface nanocrystalline cp-Ti produced by surface mechanical attrition treatment (SMAT) is shown to exhibit multifunctional properties for orthopedic and cardiovascular applications. Nanocrystallization simultaneously enhanced the stem cell response and fatigue resistance in simulated body fluid of cp-Ti collectively required for load bearing orthopedic applications. Stem cell attachment and proliferation was enhanced by 20% and number of cycles to failure increased by 15% after nanocrystallization. Nanocrystalline Ti was also found to be suitable for cardiovascular applications due to its improved hemocompatibility. A 40% reduction in attachment of platelets and their activation was noted on the surface of nanocrystalline Ti. While high surface hardness and compressive residual stress improved the corrosion-fatigue resistance, the biological response of stem cells and platelets was governed by the physico-electro-chemical properties of the surface oxide on cp-Ti. Modulation in properties of the oxide layer altered the protein adsorption, evaluated bymeans of electrochemical impedance spectroscopy and direct protein quantification thereby, augmenting the biological response. Taken together, it is demonstrated that surface nanocrystallization by SMAT is a promising step towards producing high performance Ti implants for orthopedic and cardiovascular applications. (C) 2018 Elsevier Ltd. All rights reserved