Materials Science Forum Vols. 706-709

Abstract: Titanium alloys were produced by blended elemental powder metallurgy (P/M) method. We focused on the effect of alloying elements (Fe, Mo, and Al) on the consolidation and mechanical properties of Ti compacts prepared by spark plasma sintering. The effects of amount of alloying elements and sintering temperature on the relative density and tensile properties of Ti compacts were investigated. The addition of β-stabilizing elements (Fe and Mo) significantly improved the densification of Ti compacts, where the relative density ratio of Ti-5 wt% Mo specimen became higher than 99.9 %, and Ti-5 wt% Fe specimen higher than 99.0 %. On the other hand, the addition of Al as α-stabilizing element led to improve the relative density of Ti-5 wt% Al compact with higher than 99.9 %. The tensile property for sintered Ti-5 wt% Mo compact had the highest elongation of 16 %. It will be discussed the microstructures and tensile property of the compacts.

Abstract: Titanium and its alloys have excellent specific strength and corrosion resistance. However, they have a high initial cost and high machining cost, and poor wear resistance. Therefore, both factors should be considered in order to reduce the cost and improve the wear resistance of titanium alloy parts. Powder metallurgy (PM) has been taken into account for lowering the their costs. Titanium alloys reinforced with ceramic particle improves their wear resistance and hardness. In this study, we focus on the microstructures, mechanical properties, and wear resistance of TiC or B₄C reinforced Ti-6Al-4V composites by using spark plasma sintering. The volume fraction of the reinforcement from 0 to 19.9 vol. %. The tensile strength of TiC/ and B₄C/Ti-6Al-4V had 1058MPa (14.9 vol. % TiC) and 1095MPa (1.7 vol. % B₄C), respectively. Vickers harnesses of TiC/ and B₄C/Ti-6Al-4V increased with increase in the volume fraction of the reinforcement. The wear resistance of B₄C/Ti-6Al-4V exhibited superior to that of TiC/Ti-6Al-4V.

Abstract: Derived from laser cladding, the Direct Metal Deposition (DMD) laser process, is based upon a laser beam – projected powder interaction, and allows manufacturing complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DMD parts usually necessitate post-machining steps. In this context, the focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish. Our experimental approach is based upon: (1) adequate modifications of the DMD conditions (gas shielding, laser conditions, coaxial or off-axis nozzles), (2) a characterization of laser-powder-melt-pool interactions using fast camera analysis, (3) a precise check of surface aspects using 3D profilometry, SEM, (4) preliminary thermo-convective simulations to understand melt-pool hydrodynamics. Most of the experimental tests were carried out on a Ti6Al4V titanium alloy, widely investigated already. Results confirm that surface degradation depends on two aspects: the sticking of non-melted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters.

Abstract: A simple mesoscale model was developed for discontinuous dynamic recrystallization. The material is described on a grain scale as a set of (variable) spherical grains. Each grain is characterized by two internal variables: its diameter and dislocation density (assumed homogeneous within the grain). Each grain is then considered in turn as an inclusion, embedded in a homogeneous equivalent matrix, the properties of which are obtained by averaging over all the grains. The model includes: (i) a grain boundary migration equation driving the evolution of grain size via the mobility of grain boundaries, which is coupled with (ii) a dislocation-density evolution equation, such as the Yoshie–Laasraoui–Jonas or Kocks–Mecking relationship, involving strain hardening and dynamic recovery, and (iii) an equation governing the total number of grains in the system due to the nucleation of new grains. The model can be used to predict transient and steady-state flow stresses, recrystallized fractions, and grain-size distributions. The effect of the distribution of grain-boundary mobilities has been investigated.

Abstract: The external structures of supersonic vehicles are exposed in several critical regions to temperatures that could reach 1000°C. This limit is higher than any safe operative service not only for titanium alloys but also for commercial nickel superalloys. The simplest way to improve titanium and nickel matrices temperature behavior (i.e: strength and fatigue resistance) is to introduce a strengthening phase in its matrix. These class of materials are known as: Metal Matrix Composites (MMC). It is possible reinforce both titanium and nickel superalloys by mean of high temperature resistant ceramic fibers.

Abstract: Much attention has been paid to unidirectionally solidified ceramic composites as candidates for high-temperature structural materials. We have recently developed potential ceramic eutectics, which are named Melt Growth Composites (MGC). The Al₂O₃/GAP(GdAlO₃) binary MGC has a novel microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal GAP phases interpenetrate without grain boundaries. Chain structure in the Al₂O₃/GAP binary system is formed due to the frequent branching of both phases resulted in the entangled structure. Therefore, the Al₂O₃/GAP binary MGC has excellent high-temperature characteristics in the air atmosphere at very high temperatures. In the paper, high temperature strength, thermal stability of microstructure and strength, and fracture toughness of the Al₂O₃/GAP binary MGC are reported.

Abstract: Titanium alloys are attractive for structural applications in the aerospace industry due to their high specific strength in comparison with other engineering materials. These properties are strongly related to the microstructure obtained during thermo-mechanical processes. The influence of the processing parameters on the microstructure is investigated to determine criteria for the control of the forming processes. Pre-forged specimens of alpha-beta Ti-6Al-4V alloy with elongated primary alpha grains are deformed below the beta transus temperature between 0.1 and 10/s of strain rate. Compression is carried out parallel and perpendicular to the preferential orientations of the primary alpha grains. The local strain within the compressed samples is determined by finite element methods and correlated to the microstructure observed there. The alpha content is affected by the temperature of deformation and the morphology of the alpha grains is influenced by the strain and strain rate. Specimens with previous primary alpha grains parallel to the compression axis show a rotation of the alpha grains which were oriented almost perpendicular to the load axis. EBSD measurements are used to determine the restoration mechanism involved during hot deformation. Continuous dynamic recrystallization in the alpha grains is revealed by increasing the cumulative crystallographic misorientation towards the grain boundary and the formation of new grains. This misorientation increases with increasing values of the Zener Hollomon parameter (Z). For lower values of Z restoration occurs mainly in the beta phase.

Abstract: This study investigates the effect of powder processing on powder flowability, compact ability, and the heat treatment of the resulting coatings on the mechanical properties of cold gas dynamic sprayed Ti-6Al-4V alloy. Nitrogen gas was used throughout the coating deposition process. Propellant gas temperature and pressure were attuned to maximize particle impact velocity. Three powder processing conditions were used in this study: as received (AR), low-energy ball milled (BM), and argon atmosphere heat treated (HT). Results showed coating porosities of around 6 to 7%, regardless of the feedstock powder used or the heat treatment performed. It was observed at 600 and 800°C anneals that a coating hardness reduction occurred, possibly due to static recovery and recrystallization, with minor sintering possibly occurring at the 800°C anneals. In addition, micro tensile tests showed an increase in cohesion strength at higher heat treatment temperatures.

Abstract: In this study, Zn-30Al-1Cu alloy was synthesized by the spray atomization and deposition technique. The microstructure and mechanical properties of the alloy were studied using optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile tests. It can be seen that the microstructure of spray-deposited Zn-30Al-1Cu alloy is composed of the Zn/Al eutectoids and few compounds. The Zn/Al eutectoids were shown lamellar, particle and Chinese script morphologies. The compound phases in the microstructure of the spray-deposited alloy were examined. The property test results indicate that the spray-deposited Zn-30Al-1Cu alloy displays superior tensile strength.

Abstract: In the present study, the fabrication of an Al-based metal matrix composite material obtained directly from the melting of the aluminium used beverage cans in a modified rheocasting process is presented. The analysed operational condition is the shear rate applied to the bath and its influence on the properties of the obtained samples. Additionally, samples were heat treated at two different times. The characterization of the phases obtained in Al-based MMC was made by means of metallography, scanning electron microscopy with energy dispersive spectroscopy and electron microprobe with wavelength dispersion spectroscopy. The results show that some constituents were formed during the fabrication process of the MMC, mainly Al₆(Fe, Mn), which are partially transformed during the heat treatment. Additionally, samples were evaluated using dynamic mechanical analysis, and the results suggest that the obtained MMC could have very good mechanical properties, similar or superior to the aluminium alloys commonly used for structural applications such as 6XXX family.