Advanced Materials Research Vols. 89-91

Abstract: Hot rolled, laboratory-cast, TWIP steel samples (5.4 mm thick) of 22% Mn - 0.6% C (in mass-%) were cold rolled to different reductions (from 40 % to 70 %) and subsequently isothermally annealed for various times at temperatures ranging from 450º C to 1100º C. The evolution of recrystallization and grain growth was followed by control of the softening kinetics complemented by metallographic, OIM and microtexture observations. A map of the recovery, recrystallization and grain growth in the temperature-time space was obtained. In all instances, the grain size at the end of recrystallization was very fine, D ≤ 2 µm and larger grain sizes were the result of grain growth. A range of grain sizes 2 µm ≤ D ≤ 50 µm was covered by the grain growth experiments. A phenomenological grain growth equation that is useful for the annealing control of this steel was derived from the measurements.

Abstract: High-performance concrete (HPC) are advanced materials used in advances applications such as tunnels or nuclear power plant in which they can be accidentally submitted to severe stress or thermal conditions. The present study deals with the material response to thermal loading conditions. The main objective of this research is the characterization of the fracture toughness under Mode I at high temperature of high performance mortars by using notched specimens in three-point bending test in accordance with the RILEM recommendations. The mechanical loading is applied to the specimens while heated at various temperatures ranging from 25 to 900°C in isothermal conditions. The maximum applied load is found to be maximum at 300°C temperature and then to decrease sharply at higher temperatures. Analysis of SEM micrographs undertaken on the heated specimens after mechanical tests helps in the understanding of the material macroscopic behaviour. The evaluation of the material toughness during the hot testing is undertaken through analytical approach based on Fracture Mechanics. Lastly, the stress intensity factor as well as the energy of fracture evolves similarly versus temperature as the maximum applied load.

Abstract: Distraction Osteogenesis (DO) is a surgical technique used to reconstruct bone defects. In the maxillofacial specialty, it account for one of the best procedures to treat severe traumas as ballistic wounds. The evolution of forces acting during DO is known to be strongly linked with the clinical issue of the treatment. In this context, the aim of this study was to determine the time-dependent forces supported by a distraction device (DEOS, OBL, France), particularly indicated for severe traumas. Bone transport forces were evaluated for two patients undergoing respectively mandibular and malar DO following a gunshot wound. In order to evaluate the distraction forces, some fixing pins of the distraction device were equipped with strain gauges. Strain values were recorded by means of a data acquisition system (SCXI, National Instruments, USA) connected to a PC computer and executing a LabView program (National Instruments, USA). Records started about one minute before the DO activation and ended about one hour later. Eighteen measurements were done for both cases during the early phases of the treatment. An equilibrium analysis was achieved in order to determine the force and moment acting in bone regenerate from strains in the pins. This procedure was coded through a Fortran program, allowing to plot the evolution of DO force and moment. The present work confirmed the time-dependent feature of the bone regenerate mechanical behaviour. Important data have been obtained concerning forces and their evolution for both mandibular and malar DO.

Abstract: High-pressure torsion (HPT) was conducted on commercial grade pure titanium (99.4%) by applying pressures in a wide range from 1.2 to 40 GPa. When the microhardness was plotted against equivalent strain, the hardness saturates to a constant level at each applied pressure. Such a level at the saturation depends on the applied pressure: for up to the pressure of 4 GPa, the saturation level is independent of the pressure but, for the pressures above 4 GPa, the hardness gradually increases with pressure because of the formation of an  phase. Bending tests showed that an excellent ductility as well as high bending strength was achieved for the sample processed at 2 GPa. The bending ductility was reduced for the sample at 6 GPa because of the  phase formation.

Abstract: Four variants of AA7020 aluminum alloy having different Zr and Cr contents were investigated aiming at reaching high recrystallization resistance during and after hot deformation. Isothermal homogenization treatments were performed at temperatures of 390-550 °C for 2 to 48 hours. The uni-axial hot compression tests were conducted at 450 °C and strain rate of 10 s-1 at a strain of 0.6. Thereafter, the samples were annealed at 550 °C for 10 min. It was found that the samples with the highest Zr and Cr contents showed the lowest volume fraction of recrystallized grains which was attributed to the highest volume fraction of Zr- and Cr-containing dispersoids formed during homogenization. The optimum homogenization treatment to achieve highest recrystallization resistance for these samples was 470 °C for 24 hours.

Abstract: Thermal Barrier Coatings (TBCs) are used as insulators on hot section components to reduce operating temperatures in aircraft engines and industrial gas turbine. The TBC system consists of two layers: the ceramic top coat traditionally Yttria Stabilized Zirconia (YSZ) with a low conductivity, and the bond coat generally MCrAlY, M=Ni and/or Cr or Co or Pd or Pt modified aluminides. In the industry, two dry-route processes used to deposit TBCs give quite different microstructures of coatings. In one hand, coatings resulted by plasma spraying (PS) present a lamellar microstruture with a low thermal conductivity in the range from 0.7 to 0.9 Wm−1K−1. In the other hand, Electron Beam Physical Vapour Deposition (EBPVD) coatings with columnar microstruture coatings present the best mechanical performances but perpendicular orientation of the columns makes their thermal conductivity twice higher compared to PS coatings. The present study proposes the elaboration of zirconia coatings via the sol-gel route combined with dip-coating process. It is a versatile process able to produce either thin ceramic coatings or thick deposits. The main advantage of this method is to decrease the crystallization temperature, much lower than conventional processes. Moreover, the sol-gel process is a nondirectional deposition technique, which is very different to the physical methods described above. Doped zirconia have been chosen to constitute isolating multilayers coatings. Sol formulation, slurries stability but also dip-coating conditions have been optimized in order to obtain homogeneous layers on nickel based superalloys substrates.

Abstract: Ultrasonic irradiation during the solidification of molten metals has an effect on grain refinement. However, the mechanism of grain refinement by ultrasonic vibration has not been fully understood yet, so that there exist difficulties to apply the ultrasonic grain refinement to industrial casting processes. In the present study, we propose the mechanism of ultrasonic grain refinement: the nucleation is based on the extremely high pressure generated by the collapse of acoustic cavitation in molten Al-Si alloys. The effect of ultrasonic irradiation into molten Al-Si alloy on the microstructure was firstly studied, that is, molten Al-12.6wt%Si alloy was rapidly cooled down from just above the eutectic temperature after the ultrasonic irradiation. The detailed microstructure observation exhibits that ultrasonic irradiation above the eutectic temperature causes crystalline -Al and silicon to nucleate. Through the measurement of silicon content in -Al nodules solidified with ultrasonic irradiation, the silicon content is higher than that in non-irradiated -Al nodules. It is known that the collapse of acoustic cavitation generates extremely high pressure. At the highly pressurized sites, the eutectic temperature rises and the crystallized -Al nodules contain higher amount of silicon compared with those solidified at ambient pressure. According to the fact that the -Al nodules crystallized above the eutectic temperature contain higher amount of silicon, the irradiated microstructure of -Al nodules is developed at the highly pressurized sites, that is, the collapse of acoustic cavitation induces nucleation and causes grain refinement.

Abstract: Coronary stents are metallic (316L stainless steel) devices employed during balloon angioplasty to reopen and prevent the re-obstruction of a diseased narrowed area within a coronary artery. To reduce restenosis rate, bare metal stent coating is a promising solution. The coating can act as an anticorrosive barrier against the aggressive properties of biological environment, improving the long-term safety of the device. The goal of this study is to develop a dry process to isolate metallic surface from the biological environment by depositing a thin plasma polymerized allylamine (CH2=CH-CH2-NH2) film on the metallic surface. Plasma polymerized allylamine films were deposited on flat electropolished 316L stainless steel samples in a low pressure plasma reactor (70 kHz). Chemical composition of the coatings has been analysed as a function of the discharge power and treatment time. Moreover, special attention has been paid on the stability of the coating after immersion during 24 hours in D.I. water. Finally, to mimic stent expansion conditions, a “small punch test” has been used to investigate the adhesive properties of the coating. Our results demonstrate that is possible to deposit a stable, cohesive and adhesive plasma polymerized allylamine thin film which can be used as a coating for cardiovascular stents

Abstract: The austenite-to-ferrite phase transformation, which is an inherent feature of low-alloyed ultra low carbon steels, has scarcely been investigated to control surface texture and microstructure evolution. This paper investigates the systematic evolution of texture and microstructure at the metal-vapour interface during interrupted annealing in vacuum. Interrupted annealing experiments were carried out on three ultra low carbon steel sheets alloyed with Mn, Al and Si. The texture and microstructures have been investigated by X-ray diffraction and SEM-EBSD techniques. These results reveal a very clear variation in the surface texture components as well as in the surface microstructure after BCC recrystallisation and double  transformation interrupted annealing. The recrystallisation texture consists mainly of a <111>//ND fibre, while the transformation texture at the surface exhibits a <100>// ND fibre in combination with components of the <110> //ND fibre. It has been revealed that the latter specific surface texture was present in a monolayer of outer surface grains which were in direct contact with the vapour atmosphere. This observed phenomenon could be explained by considering the role of surface energy anisotropy occurring during phase transformation annealing.

Abstract: Due to a nanolaminate structure, MAX phases are materials with an interesting set of properties. The present paper is focussed on the synthesis and characterization of Ti2AlC and Ti2AlN MAX phase coatings. They were deposited by dc magnetron sputtering from single elemental Ti, Al, and C targets (Ti-Al-C system); in addition to Ti and Al, nitrogen was used for the Ti-Al-N system. XRD analysis revealed the growth of cubic Ti3AlC and Ti3AlN perovskite phases in the coatings deposited at 540°C. After coating deposition an annealing treatment at 800, 1000 and 1200°C was carried out. The results indicate that annealing for 1 h in vacuum at 800°C enhances crystallization of the Ti2AlN and Ti2AlC MAX phases. It was also observed that annealing at temperatures higher than 1000°C enhances the decomposition of both phases, Ti2AlC and Ti2AlN, and gives rise to the formation of the carbide and nitride phases TiCx and TiNx, respectively.