Key Engineering Materials Vols. 340-341

Abstract: Ti3Al based alloys have been widely reported for their admirable superplasticity in the temperature range of 900-1000oC. However, the superplastic behavior of temperature lower than 900oC was seldom reported. Dual phase (α2+β) Ti3Al-10Nb alloy has shown superior superplastic elongation of 1500% at 960oC and 2x10-4 s-1. In this paper, it aims to investigate the superplastic behavior at lower temperature (700-900oC). The relationship of texture characteristics, phase transformation phenomena, and deformation mechanism at lower temperature (below 900oC) are studied. The optimum low-temperature superplastic condition with an elongation of 333% was occurred at 850oC and 5x10-4 s-1. With abundant hexagonal α2’ laths formed inside the β grains, the major accommodation process via dislocation slip across the β grains is impeded. It leads to premature failure and lower tensile elongations at lower temperature. Moreover, with the minor operating of grain rotations and grain boundary sliding, the texture intensity decreases significantly at temperature 850oC.

Abstract: This paper was designed to assess the adhesive properties of hard coatings made by physical vapor depositions on various substrates (AISI D2, AISI H-13 and M2) with and without an intermediate nitrided layer. An estimation of adhesion was carried out using the scratch test, where adhesion is measured by the critical load (Lc). This value was determined as the normal force affecting the indenter and causing the coating detachment as well as the acoustic emission signal containing the information on the extent of coating damage. The scratch track after the scratch test was also examined with an optical microscope to observe the failure modes of each coating. Hard coatings TiN, CrN and TiAlN were chosen for this study. Results of the test showed that harder substrates and coatings give higher values of critical loads.

Abstract: Addition of silicon promotes the development of a stable phase Ti5Si3 through thermomechanical treatment in TiAl-based alloys develop, while carbon seems to be a promising element for generating both solid solution effect and dispersion strengthening by precipitation of TiC or Ti2AlC during subsequent processing of such alloys. In this study, elemental powder mixture of 58Ti-30Al-6Si-6C (at%) was selected to find the structural changes during mechanical alloying (MA) process in order to understand the mechanism of alloying as well as study the subsequent thermal stability and evolution of phases. The results obtained would be useful towards in situ synthesis of titanium aluminide composites that are targeted to serve applications involving high tempearture. Such route will enable precipitation of harder Ti5Si3 and TiC particles within a matrix of TiAl through hot isostatic pressing (HIP). Firstly, MA of the chosen powder mix was performed up to 40 hours. The structural evolution of MA powders and/or annealed powders was characterized by X-ray diffraction (XRD). The stability of milled powders was investigated by differential scanning calorimetry (DSC) to study the phase transformations. Then, 10% of MA powder or the so called ‘precursor’ was incorporated into Ti-45Al-2Cr-4Nb-1.5Mn (at%) baseline matrix or baseline elemental powder, and was packed into stainless steel tubes after thorough blending. The sealed tubes were subjected to HIPing under a pressure of 150 MPa at 1100°C for 4 hours, followed by furnace cooling to room temperature. After HIPing, the resulted composites were annealed at 1150°C for 4 hours. The composites were evaluated for their mechanical properties and deformability at room and elevated temperatures. The results indicate that the synthesized composites have indicated extensive workability at a temperature of 800 oC as well as good mechanical proporties up to this temperature.

Abstract: This paper aims to investigate high temperature behavior of ZrO2 particle-dispersed Ni composites. The tension-compression tests under constant stress rates as well as creep tests including stress-dip tests were performed using a high-temperature material testing set up, which consists of an electric-hydraulic fatigue testing machine, electric furnace and extensometer with a laser sensor. ZrO2 particle-dispersed Ni composites were fabricated by using the powder metallurgical methods. The results obtained from the experimental study show that ZrO2 particles remarkably strengthen the composite and there exists a reasonable correlation between the tensioncompression stress-strain relation and the creep behavior. In addition, the creep behavior has been examined based on the micromechanical concept, which takes into account the diffusional mass flow at the interface between the particles and matrix. Some numerical analysis based on this concept demonstrates that even a little amount of ZrO2 particles can effectively increase the creep resistance of the composites

Abstract: Carbon nanotubes (CNTs) have been attracting attention because of their prominent mechanical and electronic properties. In this study, we investigate the deformation of a single-walled carbon nanotube (SWCNT) with a bend junction using atomistic modeling with Brenner potential to analyze strain concentration caused by macroscopic tube shape and microscopic interatomic bond structure. The simulation model consists of (8,8) and (14,0) CNTs connected with a flexion angle of 30 degrees. For geometric reasons five and seven-membered rings are introduced at the inside and outside of the bend junction. After the structure under no external load is determined, tensile load is applied to the model. Then, we analyze the strain concentration at the bend junction, and high tensile strain is observed at the inside of the bend junction. The strain at the seven-membered ring at the inside of the bend junction has higher strain compared to the neighboring rings due to the microscopic effect.

Abstract: High temperature deformation behavior of AZ31 and AZ91 magnesium alloys was examined by compression tests over a wide strain rate range from 10–3 to 103 s–1 with emphasis on the behavior at high strain rates. The dominant deformation mechanism in the low strain rate range below 10–1 s–1 was suggested to be climb-controlled dislocation creep. On the other hand, experimental results indicated that the deformation at a high strain rate of ~103 s–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures. The solid-solution strengthening was operative for high temperature deformation at ~103 s–1.

Abstract: Bulk glassy metal is an alloy with the vitreous amorphous structure. Because of various excellent properties, this material is expected to use as an alternative structural material for several engineering applications very well. Although bulk glassy metal is very little deformed plastically in the room temperature, it shows the huge super-plastic behavior over the high temperature. However, there is not many reports mentioned about the mechanical properties of bulk glassy metal after plastic deformation under high temperature condition. From the above point of view, in this study, we have investigated the lower bound of temperature at which Zr55Cu30Al10Ni5 bulk glassy metal can be plastically deformed in uniaxial tensile load. Furthermore, it is focused on the strength property of bulk glassy metal in the room temperature after deformed under various high-temperature conditions. In the experimental result, when this material was heated at temperature of 685[K] or higher, this material crystallized and the mechanical strength in room temperature drastically decreased to 200[MPa], although this material as cast had the strength over 1500[MPa]. However, this material showed sufficiently the plastic deformation at temperatures of 643[K] and the strength in room temperature after cooling was equal to as cast. It is supposed that the strength depend on its atomic structure, i.e., amorphous or crystalline, and the change of its structure is affected strongly by heating process.

Abstract: The yield locus of type AZ31 magnesium alloy sheet was obtained by performing biaxial tensile tests, using cruciform specimens, at temperatures of 100, 150, 200, 250 and 300 P o PC at strain rates of 10P -2 P, 10P -3 P and 10P -4 PsP -1 P. Based on the experimental results, the effects of strain-rate and temperature on the yield locus was discussed. The size of yield locus drastically decreased with increasing temperature and decreased with decreasing strain-rate. Neither von Mises’s criterion or Hill’s can well predict the shape of the yield locus of this sheet metal. Instead of these quadratic yield functions, the yield criterion of Logan-Hosford or Barlat is a better choice for the accurate description of biaxial stress-strain responses at high temperature.

Abstract: Based on Mori-Tanaka’s concept of average stress in the matrix and Eshelby’s equivalent inclusions theory, the stress or strain of the matrix, the reinforced particles and the composite are derived under a prescribed traction boundary conditions. The plastic strains and strains due to thermal mismatch between matrix and reinforced phase are considered as eigenstrains. The matrix and composite are postulated isotropic and the matrix satisfies isotropic hardening law. The interface debonding is decided by the tensile strength of the particles whose debonding probability is described by Weibull distribution function. Then the overall elastoplastic constitutive relation of spherical particle-reinforced metal matrix composite is derived by secant modulus method considering the interface debonding. The theoretical uniaxial stress-strain bebavior of the composite agrees well with the experimental curves.