Wear behavior of Ti6Al4V biomedical alloys processed by selective laser melting, hot pressing and conventional casting

The aim of this work was to study the influence of the processing route on the microstructural constituents,hardness andtribological(wear and friction)behavior of Ti6Al4V biomedical alloy.In this sense,three different processing routes were studied:conventional casting,hot pressing and selective laser melting.A comprehensive metallurgical,mechanical and tribologicalcharacterization was performed by X-ray diffraction analysis,Vickers hardness tests and reciprocating ball-on-plate wear tests ofTi6Al4V/Al2O3sliding pairs.The results showed a great influence of the processing route on the microstructural constituents andconsequent differences on hardness and wear performance.The highest hardness and wear resistance were obtained for Ti6Al4Valloy produced by selective laser melting,due to a markedly different cooling rate that leads to significantly different microstructurewhen compared to hot pressing and casting.This study assesses and confirms that selective laser melting is potential to producecustomized Ti6Al4V implants with improved wear performance.

Microstructure and properties of Ti64.51Fe26.40Zr5.86Sn2.93Y0.30 biomedical alloy fabricated by laser additive manufacturing

From the perspective of biomechanics and forming technology,Ti−Fe−Zr−Sn−Y eutectic alloy was designed using a“cluster-plus-glue-atom”model,and then the alloy was prepared by laser additive manufacturing(LAM)on pure titanium substrate.The mechanical properties of the alloy were evaluated using micro-hardness and compression tester,and the elastic modulus was measured by nanoindenter.The results show that the alloy exhibits a high hardness of HV(788±10),a high strength of 2229 MPa,a failure strain of 14%,and a low elastic modulus of 87.5 GPa.The alloy also has good tribological,chemical,forming,and biological properties.The comprehensive performances of the Ti64.51Fe26.40Zr5.86Sn2.93Y0.30 alloy are superior to those of the Ti70.5Fe29.5 eutectic alloy and commercial Ti−6Al−4V alloy.All the above-mentioned qualities make the alloy a promising candidate as LAM biomaterial.

Exploration of Rare-Earth Element Sc to Enhance Microstructure, Mechanical Properties and Corrosion Resistance of Zr-8.8Si Biomedical Alloy

The main mechanism of rare-earth element Sc on the microstructure and properties of Zr-8.8Si biomaterial alloy was explored.The novel Zr-8.8Si-xSc(x=0,5,10 and 15 at.%)alloys were prepared by electric arc smelting with Ar protection.The microstructural and mechanical properties as well as electrochemical corrosion and tribological behaviors in artificial saliva solution of the Zr-8.8Si-xSc alloys were systematically studied.The results show that the Zr-8.8Si-xSc alloys only consist of two-phase α-Zr and Zr_(3)Si,Sc dissolve in α-Zr matrix to form Zr-Sc solid solution.The addition of Sc is conducive to refine microstructure,and reduce micro-pores of Zr-8.8Si alloy which lead to higher Young's modulus,compressive strength and micro-hardness.Among them,the highest value of Young's modulus is 31.5 GPa,still at a low level in biomedical alloy.The promotion of corrosion resistance can be attributed to the addition of Sc which can accelerate the formation of passive film,slow down the appearance of pitting and reduce the accumulation of corrosion products in surface.Under the condition of sliding wear test with artificial saliva solution,compared with Zr-8.8Si-0Sc alloy,the wear loss of samples with Sc is greatly decreased,and wear resistance is increased with increasing content of Sc.The experimental results indicate that the combination of good mechanical properties,corrosion resistance and tribological properties of Zr-8.8Si-(5,10 and 15 at.%)Sc alloys was much better than Zr-8.8Si-0Sc alloy.Among them,the comprehensive property of Zr-8.8Si-10Sc alloy is preferable in this work.

Parameter optimization of microwave sintering porous Ti-23%Nb shape memory alloys for biomedical applications

Porous Ti-23%Nb(mole fraction)shape memory alloys(SMAs)were prepared successfully by microwave sintering with excellent outer finishing(without space holder).The effects of microwave-sintering on the microstructure,phase composition,phase-transformation temperature,mechanical properties and shape-memory effect were investigated.The results show that the density and size of porosity vary based on the sintering time and temperature,in which the smallest size and the most uniform pore shape are exhibited with Ti-23%Nb SMA after being sintered at 900°C for 30 min.The microstructure of porous Ti-Nb SMA consists of predominantα',α,andβphases in needle-like and plate-like morphologies,and their volume fractions vary based on the sintering time and temperature.Theβphase represents the largest phase due to the higher content ofβstabilizer element with little intensities ofαandα'phases.The highest ultimate strength and its strain are indicated for the sample sintered at 900°C for 30 min,while the best superelasticity is for the sample sintered at 1200°C for 30 min.The low-elastic modulus enables these alloys to avoid the problem of“stress shielding”.Therefore,microwave heating can be employed to sinter Ti-alloys for biomedical applications and improve the mechanical properties of these alloys.

Deposition of DLC Coating on Biomedical TiNi Alloys by Plasma Based Ion Implantation to Improve Surface Properties

Diamond-like carbon （DLC） films were successfully deposited on Ti- 50.8 at% Ni using plasma based ion implantation （PBII） technique. The influence of the pulsed negative bias voltage applied to the substrate from 12 kV to 40 kV on the microstracture, nano-indentation hardness and Young＇ s modulus, the surface characteristics and corrosion resistant property as well as hemocompatibility were investigated. The experimental resalts showed that C 1 s peak depended heavily on the bias voltage. With the increase of bias voltage, the ratio of sp2 / sp3 first decreased, reaching a minimum value at 20 kV, and then increased. The DLC coating deposited at 20 kV showed the highest hardness and elastic modulus values as a result of lower sp2/sp3 ratio. The RMS values first decreased from 7.202nm（12 kV） to 5.279 nm（20 kV）, and then increased to 11.449 nm（30 kV） and 7.060 nm（ 40 kV）. The uncoated TiNi alloy showed severe pitting corrosion, due to the presence of Cl-ions in the solution. On the contrary, the DLC coated sample showed very little pitting corrosion and behaved better corrosion resistant property especially for the specimens deposited at 20 kV bias voltages. The platelet adhesion test show that the hemocompatibility of DLC coated TiNi alloy is much better than that of bare TiNi alloy, and the hemocompatibility performance of DLC coated TiNi alloy deposited at 20 kV is superior to that of other coated specimens.

Effects of surface nanocrystallization on corrosion resistance of β-type titanium alloy

Surface mechanical attrition treatment (SMAT) was performed on biomedicalβ-type TiNbZrFe alloy for 60 min at room temperature to study the effect of surface nanocrystallization on the corrosion resistance of TiNbZrFe alloy in physiological environment. The surface nanostructure was characterized by TEM, and the electrochemical behaviors of the samples with nanocrystalline layer and coarse grain were comparatively investigated in 0.9% NaCl and 0.2% NaF solutions, respectively. The results indicate that nanocrystallines with the size of 10-30 nm are formed within the surface layer of 30 μm in depth. The nanocrystallized surface behaves higher impedance, more positive corrosion potential and lower corrosion current density in 0.9%NaCl and 0.2%NaF solutions as compared with the coarse grain surface. The improvement of the corrosion resistance is attributed to the rapid formation of stable and dense passive film on the nanocrystallized surface of TiNbZrFe alloy.

Microstructure and mechanical properties of Ti−Nb−Fe−Zr alloys with high strength and low elastic modulus

Zr was added to Ti−Nb−Fe alloys to develop low elastic modulus and high strengthβ-Ti alloys for biomedical applications.Ingots of Ti−12Nb−2Fe−(2,4,6,8,10)Zr(at.%)were prepared by arc melting and then subjected to homogenization,cold rolling,and solution treatments.The phases and microstructures of the alloys were analyzed by optical microscopy,X-ray diffraction,and transmission electron microscopy.The mechanical properties were measured by tensile tests.The results indicate that Zr and Fe cause a remarkable solid-solution strengthening effect on the alloys;thus,all the alloys show yield and ultimate tensile strengths higher than 510 MPa and 730 MPa,respectively.Zr plays a weak role in the deformation mechanism.Further,twinning occurs in all the deformed alloys and is beneficial to both strength and plasticity.Ti−12Nb−2Fe−(8,10)Zr alloys with metastableβphases show low elastic modulus,high tensile strength,and good plasticity and are suitable candidate materials for biomedical implants.

Mechanical and Corrosion Behavior of a Biomedical Mg–6Zn–0.5Zr Alloy Containing a Large Number of Twins

The strong texture of Mg alloys can lead to strong tension–compression yield asymmetry and corrosion anisotropy,and this will consequently affect the effectiveness of hard tissue implants.A biomedical Mg–6Zn–0.5Zr alloy containing a large number of{1012}primary twins and{1012}–{1012}secondary twins is successfully prepared by cross compression.The dual twin structure not only removes the tension–compression yield asymmetry completely,but effectively reduces the corrosion anisotropy without compromise of corrosion resistance.The difference between the largest corrosion rate and smallest one is~1.2 times compared to~1.6 times of the original materials.It is found that the reduced corrosion anisotropy is related to re-distribution of crystallographic orientations by twins.

Microstructure,texture evolution and mechanical properties of cold rolled Ti-32.5Nb-6.8Zr-2.7Sn biomedical beta titanium alloy

Ti-32.5 Nb-6.8 Zr-2.7 Sn（TNZS,wt%） alloy was produced by using vacuum arc melting method,followed by solution treatment and cold rolling with the area reductions of 50% and 90%.The effects of cold rolling on the microstructure,texture evolution and mechanical properties of the experimental alloy were investigated by optical microscopy,X-ray diffraction,transmission electron microscopy and universal material testing machine.The results showed that the grains of the alloy were elongated along rolling direction and stress-induced α＇＇ martensite was not detected in the deformed samples.The plastic deformation mechanisms of the alloy were related to {112} 111 type deformation twinning and dislocation slipping.Meanwhile,the transition from γ-fiber texture to α-fiber texture took place during cold rolling and a dominant {001} 110α-fiber texture was obtained after 90% cold deformation.With the increase of cold deformation degree,the strength increased owing to the increase of microstrain,dislocation density and grain refinement,and the elastic modulus decreased owing to the increase of dislocation density as well as an enhanced intensity of {001} 110α-fiber texture and a weakened intensity of {111} 112γ-fiber texture.The 90% cold rolled alloy exhibited a great potential to become a new candidate for biomedical applications,since it possesses low elastic modulus（47.1 GPa）,moderate strength（883 MPa） and high elastic admissible strain（1.87%）,which are superior than those of Ti-6 Al-4 V alloy.

Designation and development of biomedical Ti alloys with finer biomechanical compatibility in long-term surgical implants

Developing the new titanium alloys with excellent biomechanical compatibility has been an important research direction of surgical implants materials. Present paper summarizes the international researches and developments of biomedical titanium alloys. Aiming at increasing the biomechanical compatibility, it also introduces the exploration and improvement of alloy designing, mechanical processing, microstructure and phase transformation, and finally outlines the directions for scientific research on the biomedical titanium alloys in the future.

Effects of Anodic Voltages on Microstructure and Properties of Plasma Electrolytic Oxidation Coatings on Biomedical NiTi Alloy

Plasma electrolytic oxidation （PEO） coatings, formed under various anodic voltages （320-440 V） on biomedical NiTi alloy, are mainly composed of γ-AI203 crystal phase. The evolution of discharging sparks during the PEO process under different anodic voltages was observed. The surface and cross-sectional morphologies, composition, bonding strength, wear resistance and corrosion resistance of the coatings were investigated by scanning electron microscopy （SEM）, thin-film X-ray diffraction （TF-XRD）, energy dispersive X-ray spectrometry （EDS）, surface roughness, direct pull-off test, ball-on-disk friction and wear test and potentiodynamic polarization test, respectively. The results showed that the evolution of discharging sparks during the PEO process directly influenced the microstructure of the PEO coatings and further influences the properties. When the anodic voltage increased from 320 V to 400 V, the corrosion resistance and wear resistance of the coatings slowly increased, and all the bonding strength was higher than 60 MPa; further increasing the anodic voltages, especially up to 440 V, although the thickness and γ-AI203 crystallinity of the coatings further increased, the microstructure and properties of the coatings were obviously deteriorated.

Biomedical titanium alloys and their additive manufacturing

Titanium and its alloys have been widely used for biomedical applications due to their better biomechanical and biochemical compatibility than other metallic materials such as stainless steels and Co-based alloys.A brief review on the development of the b-type titanium alloys with high strength and low elastic modulus is given and the use of additive manufacturing technologies to produce porous titanium alloy parts,using Ti-6Al-4V as a reference,and its potential in fabricating biomedica replacements are discussed in this paper.

Microstructures and Hardness Properties for β-Phase Ti-24Nb-4Zr-7.9Sn Alloy Fabricated by Electron Beam Melting

Atomized, pre-alloyed Ti-24Nb-4Zr-7.9Sn （wt%） powder was used to fabricate solid, prototype components by electron beam melting （EBM）. Vickers microindentation hardness values were observed to average 2 GPa for the precursor powder and 2.5 GPa for the solid, EBM-fabricated products. The powder and solid product microstructures were examined by optical and electron microscopy. X-ray diffraction analyses showed that they had bcc β-phase microstructure. However, it was found by transmission electron microscopy that the EBM-fabricated product had plate morphology with space -100-200 nm. Although the corresponding selected area diffraction patterns can be indexed by β-phase plus α＂-martensite with orthorhombic crystal structure, the dark-field analyses failed to observe the α＂-martensite. Such phenomenon was also found in deformed gum metals and explained by stress-induced diffusion scattering due to phonon softening.

Design and fabrication of a low modulus β-type Ti-Nb-Zr alloy by controlling martensitic transformation

In this paper, high density of dislocations, grain boundaries and nanometer-scale α precipitates were intro- duced to a metastable Ti-36Nb-5Zr alloy （wt%） through a thermo-mechanical approach including severe cold rolling and short-time annealing treatment. The martensitic trans- formation was retarded, and the β phase with low content of β stabilizers was retained at room temperature after the thermo-mechanical treatment. As a result, both low mod- ulus （57 GPa） and high strength （950 MPa） are obtained. The results indicate that it is a feasible strategy to control martensitic transformation start temperature through microstructure optimization instead of composition design, with the aim of fabricating low modulus β-type Ti alloy.

Improving in-vitro biocorrosion resistance of Mg-Zn-Mn-Ca alloy in Hank's solution through addition of cerium

Two kinds of Mg-Zn-Mn-Ca alloys with and without cerium were designed and fabricated. In-vitro degradation tests and electrochemical evaluations were carried out to compare their biocorrosion behavior in Hank＇s solution at 37 oC. After adding cerium, the continuous network distributed Ca2Mg6Zn3 phases in Mg-2Zn-0.5Mn-1Ca alloy（Alloy I） were separated due to the emerging non-continuously distributed Mg2 Ca phase and Mg12 Ce Zn phase. This change led to corrosion acceleration of Mg matrix at the initial stage but also sped up the formation of compact corrosion products for Mg-2Zn-0.5Mn-1Ca-1.5Ce alloy（Alloy II）, and therefore enhanced its biocorrosion resistance. Cerium containing Alloy II has the potential to be used as future biomaterials.

Corrosion behavior of a novel Mg-13Li-X alloy with different grain sizes by rapid solidification rate

As degradable biomaterials,the higher degradation rate of Mg-Li alloys in the physiological environment is the main challenge for the implant applications.In order to try and overcome this limitation,the present work was dedicated to studying the corrosion behavior of a novel Mg-13Li-X alloy fabricated by a rapid solidification process(RSP).The special Mg-13Li-X alloy was immersed in Hank’s balanced salt solution(HBSS),and the influence of immersion time on corrosion rate was analyzed.X-ray diffraction(XRD)and scanning electron microscopy(SEM),complemented with electrochemical techniques such as potentiodynamic polarization curves and electrochemical impedance spectroscopy,were applied.Microstructural characterization indicates that the mean grain sizes of RSP Mg-13Li-X alloy are 4.2,8.2 and 12.7μm with the solidification rate decreasing.By contrast,the conventional as-cast Mg-13Li-X alloy has an average grain size of about 150μm.The results of electrochemical test indicate that the sample with 4.2μm in grain size has the most positive corrosion potential(E_(corr))of−1.354 V and the minimum corrosion current(I_(corr))of 5.830×10^(−7)A·cm^(−2)after immersion for 2 h in HBSS.Therefore,the finest grain size can improve the polarization resistance of the alloy,reduce its corrosion current density and increase its corrosion resistance.However,because the weak layer of the corrosion product which consists of Mg(OH)_(2) does not afford strong protection,the corrosion resistance becomes worse after immersion for longer periods.

Hot deformation behavior of an antibacterial Co–29Cr–6Mo–1.8Cu alloy and its effect on mechanical property and corrosion resistance

In order to optimize the deformation processing, the hot deformation behavior of Co-Cr-Mo-Cu （here- after named as Co-Cu） alloy was studied in this paper at a deformation temperature range of 950-1150 ℃ and a strain rate range of 0.008-5 s^-1. Based on the true stress-true strain curves, a constitutive equation in hyperbolic sin function was established and a hot processing map was drawn. It was found that the flow stress of the Co-Cu alloy increased with the increase of the strain rate and decreased with the increase of the deforming temperature. The hot processing map indicated that there were two unstable regions and one well-processing region. The microstructure, the hardness distribution and the electro- chemical properties of the hot deformed sample were investigated in order to reveal the influence of the hot deformation. Microstructure observation indicated that the grain size increased with the increase of the deformation temperature but decreased with the increase of the strain rate. High temperature and low strain rate promoted the crystallization process but increased the grain size, which results in a reduction in the hardness. The hot deformation at high temperature （1100-1150 ℃） would reduce the corrosion resistance slightly. The final optimized deformation process was： a deformation temperature from 1050to 1100 ℃, and a strain rate from 0.008 to 0.2 s^-1, where a completely recrystallized and homogeneously distributed microstructure would be obtained.

Mixture of Oxides with Different Valence States in Nanotubes

Oxide nanotubes with different diameters and lengths were fabricated on the biomedical Ti2448 alloy by anodic oxidation in neutral electrolyte. Similar to oxide nanotubes fabricated on pure titanium and its alloys, the as-grown nanotubes on Ti2448 also exhibit gradually changing chemical distribution along the direction of tube growth. Furthermore, several kinds of oxides with different valence states （MxOy） are formed simultaneously for each alloying element M, while their volume fractions vary gradually along the tube-growth direction. The findings of this study would provide insight into the effect of valence states on the desired nanotube properties and help develop ways to enhance the properties of the preferred oxide.

Effects of Micro-arc Oxidation Coating on Corrosion Behavior of Mg-Y-Zn in Simulated Body Fluid

The application of magnesium and its alloy as high degradation rate in physiological environment. This degradable biomaterials is mainly confined due to its research focused on the effects of micro-arc oxidation （MAO） on biodegradable behavior of Mg-Y-Zn magnesium alloy in a simulated body fluid （SBF）. The corrosion rate of alloys was gauged by means of hydrogen evolution volume measurement and mass-loss method. Scanning electron microscope （SEM） was utilized to observe the surface of the magnesium alloy and the cross-section of oxidation coating layer before and after corrosion. The Mg-Y-Zn alloy with thicker oxidation coating exhibited greater corrosion resistance during the immersion test for 240 h.