Novel α + ß Zr Alloys with Enhanced Strength.

Low-alloyed zirconium alloys are widely used in nuclear applications due to their low neutron absorption cross-section. These alloys, however, suffer from limited strength. Well-established guidelines for the development of Ti alloys were applied to design new two-phase ternary Zr alloys with improved mechanical properties. Zr-4Sn-4Nb and Zr-8Sn-4Nb alloys have been manufactured by vacuum arc melting, thermo-mechanically processed by annealing, forging, and aging to various microstructural conditions and thoroughly characterized. Detailed Scanning electron microscopy (SEM) analysis showed that the microstructural response of the alloys is rather similar to alpha + beta Ti alloys. Duplex microstructure containing primary alpha phase particles surrounded by lamellar alpha + beta microstructure can be achieved by thermal processing. Mechanical properties strongly depend on the previous treatment. Ultimate tensile strength exceeding 700 MPa was achieved exceeding the strength of commercial Zr alloys for nuclear applications by more than 50%. Such an improvement in strength more than compensates for the increased neutron absorption cross-section. This study aims to exploit the potential of alpha + beta Zr alloys for nuclear applications.

The Effect of Hot Working on the Mechanical Properties of High Strength Biomedical Ti-Nb-Ta-Zr-O Alloy.

Beta titanium alloy Ti-35Nb-6Ta-7Zr-0.7O (wt%) was developed as a material intended for the manufacturing of a stem of a hip joint replacement. This alloy contains only biocompatible elements and possesses a very high yield strength already in the cast condition (900 MPa). However, the porosity, large grain size and chemical inhomogeneity reduce the fatigue performance below the limits required for utilization in the desired application. Two methods of hot working, die forging and hot rolling, were used for processing of this alloy. Microstructural evolution, tensile properties and fatigue performance of the hot worked material were investigated and compared to the cast material. Microstructural observations revealed that porosity is removed in all hot-worked conditions and the grain size is significantly reduced when the area reduction exceeds 70%. Static tensile properties were improved by both processing methods and ultimate tensile strength (UTS) of 1200 MPa was achieved. Fatigue results were more reproducible in the hot rolled material due to better microstructural homogeneity, but forging leads to an improved fatigue performance. Fatigue limit of 400 MPa was achieved in the die-forged condition after 70% of area reduction and in the hot rolled condition after 86% of area reduction.

Transformation Pathway upon Heating of Metastable ß Titanium Alloy Ti-15Mo Investigated by Neutron Diffraction.

A transformation pathway during thermal treatment of metastable ß Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases α , ß , and ω was investigated during linear heating with two heating rates of 1.9 ∘ C / min and 5 ∘ C / min and during aging at 450 ∘ C . The results showed that with a sufficient heating rate (5 ∘ C / min in this case), the ω phase dissolves before the α phase forms. On the other hand, for the slower heating rate of 1.9 ∘ C / min , a small temperature interval of the coexistence of the α and ω phases was detected. Volume fractions and lattice parameters of all phases were also determined.

Anomalous X-ray diffraction from ω nanoparticles in ß-Ti(Mo) single crystals.

Anomalous X-ray diffraction (AXRD) is a technique which makes use of effects occurring near the energy of an absorption edge of an element present in the studied sample. The intensity of the diffracted radiation exhibits an anomalous decrease when the primary beam energy matches the energy needed to excite an electron from an atomic orbital. The characteristics of this step are sensitive to the concentration of the `anomalous' element and its spatial distribution in the sample. In the present investigation, AXRD was employed to study ω particles in a metastable ß titanium alloy Ti-15Mo (in wt%). The experiments were done in an energy range around the Mo K edge at 20.0 keV, allowing investigation of the distribution of Mo in the material, which is rejected from the volume of ω particles during their diffusion-driven growth. This paper deals with diffuse scattering patterns around the (006)ß diffraction maximum. It was observed that different regions of the diffuse scattering exhibited different variations of diffracted intensity with the incident photon energy near the absorption edge. Numerical simulations of diffuse scattering patterns as well as of energy dependences of the scattered intensity were performed. It was found that the observed patterns and their dependence on the primary beam energy can be explained by taking into account (a) elastic deformation of the ß matrix arising from the presence of slightly misfitting ω particles and (b) the presence of a `cloud' of a higher Mo concentration around ω particles.

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.

Increasing strength of a biomedical Ti-Nb-Ta-Zr alloy by alloying with Fe, Si and O.

Low-modulus biomedical beta titanium alloys often suffer from low strength which limits their use as load-bearing orthopaedic implants. In this study, twelve different Ti-Nb-Zr-Ta based alloys alloyed with Fe, Si and O additions were prepared by arc melting and hot forging. The lowest elastic modulus (65GPa) was achieved in the benchmark TNTZ alloy consisting only of pure ß phase with low stability due to the 'proximity' to the ß to α'' martensitic transformation. Alloying by Fe and O significantly increased elastic modulus, which correlates with the electrons per atom ratio (e/a). Sufficient amount of Fe/O leads to increased yield stress, increased elongation to fracture and also to work hardening during deformation. A 20% increase in strength and a 20% decrease in the elastic modulus when compared to the common Ti-6Al-4V alloy was achieved in TNTZ-Fe-Si-O alloys, which proved to be suitable for biomedical use due to their favorable mechanical properties.

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.

Work-hardening induced tensile ductility of bulk metallic glasses via high-pressure torsion.

The mechanical properties of engineering materials are key for ensuring safety and reliability. However, the plastic deformation of BMGs is confined to narrow regions in shear bands, which usually result in limited ductilities and catastrophic failures at low homologous temperatures. The quasi-brittle failure and lack of tensile ductility undercut the potential applications of BMGs. In this report, we present clear tensile ductility in a Zr-based BMG via a high-pressure torsion (HPT) process. Enhanced tensile ductility and work-hardening behavior after the HPT process were investigated, focusing on the microstructure, particularly the changed free volume, which affects deformation mechanisms (i.e., initiation, propagation, and obstruction of shear bands). Our results provide insights into the basic functions of hydrostatic pressure and shear strain in the microstructure and mechanical properties of HPT-processed BMGs.

Innovative surface modification of Ti-6Al-4V alloy with a positive effect on osteoblast proliferation and fatigue performance.

A novel approach of surface treatment of orthopaedic implants combining electric discharge machining (EDM), chemical milling (etching) and shot peening is presented in this study. Each of the three techniques have been used or proposed to be used as a favourable surface treatment of biomedical titanium alloys. But to our knowledge, the three techniques have not yet been used in combination. Surface morphology and chemistry were studied by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. Fatigue life of the material was determined and finally several in-vitro biocompatibility tests have been performed. EDM and subsequent chemical milling leads to a significant improvement of osteoblast proliferation and viability thanks to favourable surface morphology and increased oxygen content on the surface. Subsequent shot-peening significantly improves the fatigue endurance of the material. Material after proposed combined surface treatment possesses favourable mechanical properties and enhanced osteoblast proliferation. EDM treatment and EDM with shot peening also supported early osteogenic cell differentiation, manifested by a higher expression of collagen type I. The combined surface treatment is therefore promising for a range of applications in orthopaedics.

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.

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.

Component position following total hip arthroplasty through a miniinvasive posterolateral approach.

Implant malposition and poor fixation are potential risks of compromising long-term results after total hip arthroplasty performed with a minimally invasive technique. Between September 2000 and February 2002, 120 cemented primary total hip arthroplasties were performed at the authors' institutions in patients with primary osteoarthritis of the hip and with BMI lower than 35. In 60 of these cases selected at random, a posterolateral incision no longer than 10 cm was used. The other 60 THA's were performed through a standard posterolateral approach. The inclination and anteversion of the cup and the position of the femoral stem were assessed on radiographs and statistically evaluated. In the miniinvasive group, the average inclination angle of the cup was 42.3 degrees (range: 36 to 52 degrees) and the anteversion angle 13.6 degrees (range: 6 to 29 degrees). The coronal alignment of the femoral component was within 3 degrees of neutral in 54 cases (90.0%). Following conventional implantation in the other group, the average cup inclination angle was 42.4 degrees (range: 35 to 50 degrees) and the anteversion angle 13.6 degrees (range: 8 to 24 degrees. A total of 53 stems (88.3 %) were implanted optimally. Statistical analysis found no significant difference between the two groups regarding components position. These findings suggest that using a smaller posterolateral incision as was done in this study does not introduce a potential risk of compromising long-term results.

Mosaicplasty for the treatment of femoral head defect after incorrect resorbable screw insertion.

Articular cartilage lesions and osteochondral defects remain a difficult problem for the patient and physician. A variety of procedures and treatments have been proposed to lessen symptoms and restore the articular surface. The knee joint has been the focus of the vast majority of these cartilage restoration procedures. Osteochondral defects of the proximal femur are significantly less common, and their management remains poorly defined. This article reports the case of a young man with a deep osteochondral defect of the femoral head caused by penetrated resorbable screw after internal fixation of a displaced large single fragment of the posterior acetabular rim and subsequent treatment using mosaicplasty.