Josef Stráský

Surface treatment by electric discharge machining of Ti-6Al-4V alloy for potential application in orthopaedics.

This study investigated the properties of Ti-6Al-4V alloy after surface treatment by the electric discharge machining (EDM) process. The EDM process with high peak currents proved to induce surface macro-roughness and to cause chemical changes to the surface. Evaluations were made of the mechanical properties by means of tensile tests, and of surface roughness for different peak currents of the EDM process. The EDM process with peak current of 29 A was found to induce sufficient surface roughness, and to have a low adverse effect on tensile properties. The chemical changes were studied by scanning electron microscopy equipped with an energy dispersive X-ray analyser (EDX). The surface of the benchmark samples was obtained by plasma-spraying a titanium dioxide coating. An investigation of the biocompatibility of the surface-treated Ti-6Al-4V samples in cultures of human osteoblast-like MG 63 cells revealed that the samples modified by EDM provided better substrates for the adhesion, growth and viability of MG 63 cells than the TiO2 coated surface. Thus, EDM treatment can be considered as a promising surface modification to orthopaedic implants, in which good integration with the surrounding bone tissue is required.

The effect of microstructure on fatigue performance of Ti–6Al–4V alloy after EDM surface treatment for application in orthopaedics

Three different microstructures--equiaxed, bi-modal and coarse lamellar--are prepared from Ti-6Al-4V alloy. Electric discharge machining (EDM) with a high peak current (29 A) is performed in order to impose surface roughness and modify the chemical composition of the surface. Detailed scanning electron microscopy (SEM) investigation revealed a martensitic surface layer and subsurface heat affected zone (HAZ). EDX measurements showed carbon enriched remnants of the EDM process on the material surface. Rotating bending fatigue tests are undertaken for EDM processed samples for all three microstructures and also for electropolished-benchmark-samples. The fatigue performance is found to be rather poor and not particularly dependent on microstructure. The bi-modal microstructure shows a slightly superior high cycle fatigue performance. This performance can be further improved by a suitable heat treatment to an endurance limit of 200 MPa.

Newly developed Ti-Nb-Zr-Ta-Si-Fe biomedical beta titanium alloys with increased strength and enhanced biocompatibility.

Beta titanium alloys are promising materials for load-bearing orthopaedic implants due to their excellent corrosion resistance and biocompatibility, low elastic modulus and moderate strength. Metastable beta-Ti alloys can be hardened via precipitation of the alpha phase; however, this has an adverse effect on the elastic modulus. Small amounts of Fe (0-2 wt.%) and Si (0-1 wt.%) were added to Ti-35Nb-7Zr-6Ta (TNZT) biocompatible alloy to increase its strength in beta solution treated condition. Fe and Si additions were shown to cause a significant increase in tensile strength and also in the elastic modulus (from 65 GPa to 85 GPa). However, the elastic modulus of TNZT alloy with Fe and Si additions is still much lower than that of widely used Ti-6Al-4V alloy (115 GPa), and thus closer to that of the bone (10-30 GPa). Si decreases the elongation to failure, whereas Fe increases the uniform elongation thanks to increased work hardening. Primary human osteoblasts cultivated for 21 days on TNZT with 0.5Si+2Fe (wt.%) reached a significantly higher cell population density and significantly higher collagen I production than cells cultured on the standard Ti-6Al-4V alloy. In conclusion, the Ti-35Nb-7Zr-6Ta-2Fe-0.5Si alloy proves to be the best combination of elastic modulus, strength and also biological properties, which makes it a viable candidate for use in load-bearing implants.

Fatigue endurance of Ti-6Al-4V alloy with electro-eroded surface for improved bone in-growth.

Ti-6Al-4V hour-glass shaped rotating beam specimens with duplex microstructure were processed by electric discharge machining (EDM). A comparatively high peak current of 29A was utilized in order to increase surface roughness for improved osteointegration. High cycle fatigue (HCF) tests were performed in rotating beam loading (R=-1) on these EDM specimens and results were compared with electrolytically polished specimens serving as reference. As expected, the HCF performance of EDM specimens was inferior to the electrolytically polished specimens. A detailed study of fatigue crack nucleation and microcrack growth was carried out on failed specimens by SEM. The poor HCF strength of EDM specimens is explained by early crack nucleation due to the high notch sensitivity of Ti-6Al-4V. In addition, process-induced residual tensile stresses and microstructural effects may also account for the drastic loss in HCF performance relative to the electropolished baseline.

Ordered array of ω particles in β-Ti matrix studied by small-angle X-ray scattering

Abstract Nanosized particles of ω phase in a β -Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the 〈 1 0 0 〉 -directions in the β -Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.

Plastic Deformation and Elastic Properties of Ti-Nb-Zr-Ta(-Fe-Si) Biomedical Alloys

Beta titanium alloys are promising biomedical material for their excellent biocompatibility and low elastic modulus. Moderate strength of those materials in beta-annealed condition can be increased by precipitation hardening of alpha phase, but this causes significant increase in elastic modulus. In this study, small additions of Fe and Si are used to increase strength of commercial Ti - 35Nb - 7Zr - 5Ta (TNZT) alloy. Alloys with iron content up to 2% and silicon content up to 1% were manufactured. Elastic properties were investigated by pulse-echo method and flow curves were determined from tensile tests. Modulus of elasticity is increases from initial 60 GPa to 80 GPa due to Fe and Si content. Strength and modulus of elasticity were then related to chemical composition. Yield stress is increased from 450 MPa to 700 MPa thanks to small Fe and Si additions. Fe causes solid solution strengthening exhibited by sharp yield point. (Ti,Zr)5Si3 intermetallic particles further increase strength via precipitation hardening. An alloy containing 0.5% Si and 2% Fe showed improved properties for biomedical use.

Investigation of Microhardness and Microstructure of AZ31 Alloy after High Pressure Torsion

Cast commercial magnesium alloy AZ31 was processed by high pressure torsion (HPT) at room temperature for 1, 3, 5 and 15 rotations (strain ranged from 1 to 7). Micro structure evolution with strain imposed by HPT was observed by light and electron microscopy. HPT was shown to be a very effective method of grain refinement. The initial coarse grain structure was refined by a factor of almost 200 already after one HPT turn (e ≈ 4). Mechanical properties were investigated by detailed 2-D microhardness measurements. HPT straining was found to introduce a radial inhomogeneity in the material which is manifested by a pronounced drop in the center and the maximum near the specimen periphery. With increasing strain due to HPT this inhomogeneity is continuously smeared out tending to saturate with increasing strain. Integrated 3-D meshes across the total surface of disks revealed the undulating character of microhardness variations. The strain imposed by HPT was shown to saturate with increasing number of HPT turns.

Evolution of Microstructure and Microhardness in Ti-15Mo β-Ti Alloy Prepared by High Pressure Torsion

The main aim of this study is to analyze the effect of the severe plastic deformation (SPD) on the mechanical properties and defect structure of metastable beta Ti alloys. Experiments were performed on two different β-Ti alloys: Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al which were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT). The increase of hardness with increasing equivalent strain was determined by microhardness mapping. Dislocation density was studied by advanced techniques of positron annihilation spectroscopy (PAS). Microhardness and dislocation density increases with increasing equivalent strain inserted by severe plastic deformation.

Effect of Boriding Time on Microstructure and Residual Stresses in Borided Highly Alloyed X210CR12 Steel

Boriding of highly alloyed steels done with the aim of increasing their wear resistance faces several issues connected with the microstructure of the base material and restraints during the diffusion of boron. The aim of the performed analyses was to ascertain whether significant increase of boriding time can enhance the surface hardness, contribute to creation of more compact microstructure and even lead to beneficial state of residual stresses in the borided layer. Using combination of X-ray diffraction and electro-chemical polishing, residual stress depth distributions in few tens of micrometres thick borided layers were obtained.

Structural characterization of semi-heusler/light metal composites prepared by spark plasma sintering

A composite of powders of semi-Heusler ferromagnetic shape memory and pure titanium was successfully prepared by spark plasma sintering at the temperature of 950 °C. Sintering resulted in the formation of small precipitates and intermetallic phases at the heterogeneous interfaces. Various complementary experimental methods were used to fully characterize the microstructure. Imaging methods including transmission and scanning electron microscopy with energy dispersive X-ray spectroscopy revealed a position and chemical composition of individual intermetallic phases and precipitates. The crystalline structure of the phases was examined by a joint refinement of X-ray and neutron diffraction patterns. It was found that Co38Ni33Al29 decomposes into the B2-(Co,Ni)Al matrix and A1-(Co,Ni,Al) particles during sintering, while Al, Co and Ni diffuse into Ti forming an eutectic two phase structure with C9-Ti2(Co,Ni) precipitates. Complicated interface intermetallic structure containing C9-Ti2(Co,Ni), B2-(Co,Ni)Ti and L21-(Co,Ni)(Al,Ti) was completely revealed. In addition, C9-Ti2(Co,Ni) and A1-(Co,Ni,Al) precipitates were investigated by an advanced method of small angle neutron scattering. This study proves that powder metallurgy followed by spark plasma sintering is an appropriate technique to prepare bulk composites from very dissimilar materials.