Peter Hodgson

Ultrafine ferrite in low carbon steel

In previous studies related to the austenite to ferrite transformation of controlled rolled steels it was shown that there is a limiting ferrite grain size of approximately 5 {micro}m regardless of the level of retained strain introduced into the austenite. This is at least partly due to ferrite coarsening during transformation. If, however, a 1 {micro}m grain size -- here called ultrafine ferrite (UFF) -- could be produced and retained then this would increase the yield strength by almost 350 MPa compared to a 5 {micro}m ferrite. Most recently, the current authors have reported the development of a new thermomechanical process which produces UFF in hot rolled steel strip. This grain refinement appears to be the result of a strain induced transformation reaction activated over a significant volume of the austenite. This requires high undercooling and high effective rolling strains. This paper describes how this combination of austenite conditioning, deformation conditions and temperature control can lead to the formation of significant volumes of UFF in low carbon steel.

Mg–Zr–Sr alloys as biodegradable implant materials

Novel Mg-Zr-Sr alloys have recently been developed for use as biodegradable implant materials. The Mg-Zr-Sr alloys were prepared by diluting Mg-Zr and Mg-Sr master alloys with pure Mg. The impact of Zr and Sr on the mechanical and biological properties has been thoroughly examined. The microstructures and mechanical properties of the alloys were characterized using optical microscopy, X-ray diffraction and compressive tests. The corrosion resistance was evaluated by electrochemical analysis and hydrogen evolution measurement. The in vitro biocompatibility was assessed using osteoblast-like SaOS2 cells and MTS and haemolysis tests. In vivo bone formation and biodegradability were studied in a rabbit model. The results indicated that both Zr and Sr are excellent candidates for Mg alloying elements in manufacturing biodegradable Mg alloy implants. Zr addition refined the grain size, improved the ductility, smoothed the grain boundaries and enhanced the corrosion resistance of Mg alloys. Sr addition led to an increase in compressive strength, better in vitro biocompatibility, and significantly higher bone formation in vivo. This study demonstrated that Mg-xZr-ySr alloys with x and y ≤5 wt.% would make excellent biodegradable implant materials for load-bearing applications.

Cytotoxicity of Titanium and Titanium Alloying Elements

It is commonly accepted that titanium and the titanium alloying elements of tantalum, niobium, zirconium, molybdenum, tin, and silicon are biocompatible. However, our research in the development of new titanium alloys for biomedical applications indicated that some titanium alloys containing molybdenum, niobium, and silicon produced by powder metallurgy show a certain degree of cytotoxicity. We hypothesized that the cytotoxicity is linked to the ion release from the metals. To prove this hypothesis, we assessed the cytotoxicity of titanium and titanium alloying elements in both forms of powder and bulk, using osteoblast-like SaOS2 cells. Results indicated that the metal powders of titanium, niobium, molybdenum, and silicon are cytotoxic, and the bulk metals of silicon and molybdenum also showed cytotoxicity. Meanwhile, we established that the safe ion concentrations (below which the ion concentration is non-toxic) are 8.5, 15.5, 172.0, and 37,000.0 µg/L for molybdenum, titanium, niobium, and silicon, respectively.

Facile synthesis of NiCo2O4 nanorod arrays on Cu conductive substrates as superior anode materials for high-rate Li-ion batteries

In this work, we report a mild and cost-effective solution method to directly grow Ni-substituted Co3O4 (ternary NiCo2O4) nanorod arrays on Cu substrates. Electrochemical impedance spectroscopy (EIS) measurements show that the values of the electrolyte resistance Re and charge-transfer resistance Rct of NiCo2O4 are 6.8 and 63.5 Ω, respectively, which are significantly lower than those of binary Co3O4 (10.4 and 122.4 Ω). This EIS characterization strongly confirms that the ternary NiCo2O4 anode has much higher electrical conductivity than that of the binary Co3O4 electrode materials, which should greatly enhance the lithium storage performances. Due to the well-aligned 1D nanorod microstructure and a higher electrical conductivity, these ternary NiCo2O4 nanorod arrays manifest high specific capacity, excellent cycling stability (a high reversible capacity of about 830 mA h g−1 was achieved after 30 cycles at 0.5 C) and high rate capability (787, 695, 512, 254, 127 mA h g−1 at 1 C, 2 C, 6 C 50 C and 110 C, respectively).

Formation of ultra-fine ferrite in hot rolled strip-potential mechanisms for grain refinement

A novel single-pass hot strip rolling process has been developed in which ultra-fine (<2 μm) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 μm. Two experiments were performed on strip that had been re-heated to 1250°C for 300 s and air-cooled to the rolling temperatures. The first involved hot rolling a sample of 0.09 wt.%C–1.68Mn–0.22Si–0.27Mo steel (steel A) at 800°C, which was just above the Ar3 of this sample, while the second involved hot rolling a sample of 0.11C–1.68Mn–0.22Si steel (steel B) at 675°C, which is just below the Ar3 temperature of the sample. After air cooling, the surface regions of strip of both steel A and B consisted of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure. In the case of steel B, a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section. Based on results from optical microscopy and scanning/transmission electron microscopy, as well as bulk X-ray texture analysis and microtextural analysis using Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra-fine ferrite most likely forms by a process of rapid intragranular nucleation during, or immediately after, deformation. This process of inducing intragranular nucleation of ferrite by deformation is referred to as strain-induced transformation.

The influence of surface energy of titanium-zirconium alloy on osteoblast cell functions in vitro

The success of an implant used for bone regeneration and repair is determined by the events that take place at the cell-material interface. An understanding of these interactions in vitro gives insights into the formulation of ideal conditions for their effective functioning in vivo. Thus, it is not only important to understand the physico-chemical properties of the materials but, also necessary to assess the cellular responses to them to determine their long-term stability and efficacy as implants. In the present study, we have compared the physico-chemical and biological properties of titanium (Ti) and two Ti-based alloys, namely: Ti- Zirconium (TiZr) and Ti-Niobium (TiNb). The morphology, chemical analysis, surface roughness, and contact angle measurements of the alloys were assessed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), profilometer, and contact angle goniometer, respectively whereas the biological properties of the materials were evaluated by measuring the adhesion, proliferation, and differentiation of MC3T3-E1 osteoblast cells on the surfaces of these alloys. Our results indicate that the biological properties of osteoblasts were better on TiZr surface than on TiNb surface. Furthermore, the surface energy and substrate composition influenced the superior biological activity of the TiZr alloy.

Analysis and characterisation of ultra-fine ferrite produced during a new steel strip rolling process

The designing of processing routes that minimize the final ferrite grain size is essential for the development of high strength steels with improved toughness and ductility. In this paper, a novel procedure for producing ultra-fine ferrite is investigated. This method is attractive in terms of its relative simplicity and ability to refine the ferrite grain size in relatively low cost steels. In an earlier paper, it was stated that the high level of ferrite grain refinement occurring during this process was likely to be the result of a strain-induced transformation mechanism. Thus, it has been termed the SITR (strain-induced transformation rolling) process. In the present paper, detailed characterization of the fine ferrite produced using this technique has been performed with the aim of providing a deeper insight into the important factors giving rise to its generation.

Influence of calcium ion deposition on apatite-inducing ability of porous titanium for biomedical applications

In the present study, the influence of calcium ion deposition on the apatite-inducing ability of porous titanium (Ti) was investigated in a modified simulated body fluid (m-SBF). Calcium hydroxide (Ca(OH)(2)) solutions with five degrees of saturation were used to hydrothermally deposit Ca ions on porous Ti with a porosity of 80%. Apatite-inducing ability of the Ca-ion-deposited porous Ti was evaluated by soaking them in m-SBF for up to 14 days. Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) confirmed that a thin layer of calcium titanate (CaTiO(3))/calcium oxide (CaO) mixture with a nanostructured porous network was produced on porous Ti substrates after hydrothermal treatment at 200 degrees C for 8 h. X-ray photoelectron spectroscopy results demonstrated that the content of the Ca ions deposited on Ti and the thickness of the CaTiO(3)/CaO layer increased with increasing saturation degree of the Ca(OH)(2) solution. The thickest (over 10 nm) CaTiO(3)/CaO layer with the highest Ca content was achieved on the Ti treated in an oversaturated Ca(OH)(2) solution (0.2 M). SEM, XRD, transmission electron microscopy and Fourier transformed infrared spectroscopy analysis indicated that the porous Ti samples deposited with the highest content of Ca ions exhibited the best apatite-inducing ability, producing a dense and complete carbonated apatite coating after a 14 day soaking in m-SBF. The present study illustrated the validity of using Ca ion deposition as a pre-treatment to endow desirable apatite-inducing ability of porous Ti for bone tissue engineering applications.

Softening and microstructural change following the dynamic recrystallization of austenite

To characterize the dynamic recrystallization behavior of austenite, continuous-torsion tests were carried out on a Mo steel over the temperature range 950 ‡C to {dy1000} ‡C, and at strain rates of 0.02, 0.2, and 2 s-1. Interrupted-torsion tests also were performed to study the characteristics of postdynamic recrystallization. Quenches were performed after increasing holding times to follow the development of the postdynamic microstructure. Finally, torsion simulations were carried out to assess the importance of metadynamic recrystallization in hot-strip mills. The postdynamic microstructure shows that the growth of dynamically recrystallized grains is the first change that takes place. Then metadynamically recrystallized grains appear and contribute to the softening of the material. The rate of metadynamic recrystallization and the meta-dynamically recrystallized grain size depend on strain rate and temperature and are relatively independent of strain, in contrast to the observations for static recrystallization. True dynamic recrystallization-controlled rolling (DRCR) is shown to require such short interpass times that it does not occur in isolation in hot-strip mills. As these schedules involve 20 to 80 pct softening by metadynamic recrystallization, a new concept known as metadynamic recrystallization-controlled rolling (MDRCR) is introduced to describe this type of situation.

A study of pore closure and welding in hot rolling process

Abstract The design of processing conditions to eliminate porosity in steel during hot rolling has become more critical with the advent of continuously cast feed stock. To predict appropriate process parameters, experiments were performed in which holes were drilled in steel slabs, at different depth below the surface perpendicular to the rolling direction. After hot rolling, the state of the deformed holes was examined by optical microscopy. The fracture surfaces of tensile specimens notched in the plane of the closed holes were examined under a scanning electron microscope to investigate the bonding ( welding ) between the surfaces of the closed holes because this bonding is the most important factor indetermining the transverse mechanical properties of the rolled product. The deformation of such holes in the roll gap was modelled as an clastic / plastic plane strain problem using the FE software ABAQUS to investigate the strain and stress around holes and to analyze the conditions required to promote pore closure and welding between the closed surfaces. Experiment results showed that the rate of pore closure was affected by the parameters of rolling process and the position of holes relative to the rolling contact surfaces. FE simulation of pore closure showed a good agreement with experimental results and showed that a certain level of hydrostatic pressure resulted in the closure of pore and that the holding period of the pressure in the compressive state determined the degree of welding of surface of pores; shear is favourable to this welding.