Key Engineering Materials Vols. 345-346

Abstract: The mechanisms of creep and superplasticity occurring in conventional large-grained materials are now understood reasonably well. However, very recent advances in the processing of theoretically-dense metals with submicrometer grain sizes have provided the opportunity to extend the understanding of flow behavior to include materials where the grains are exceptionally small. Using processing through the application of severe plastic deformation, as in procedures such as equal-channel angular pressing, it is now feasible to fabricate relatively large samples having ultrafine grain sizes in the submicrometer or nanometer range. This paper examines these recent advances and gives examples of the flow behavior in ultrafine-grained aluminum alloys.

Abstract: A TiB particulate-reinforced Ti-22Al-27Nb (mol%) alloy, based on the orthorhombic intermetallic phase, was prepared using gas atomization powder metallurgy method. In the as-atomized condition, extremely fine TiB particulates of less than 1-μm diameter and 5-μm length were dispersed in the matrix. After annealing heat treatment (heat treated at 1423 K with subsequent furnace cooling), this composite exhibited a lamellar matrix microstructure and showed better creep properties than a composite produced using conventional ingot metallurgy method, with coarse TiB particulates of 5-μm diameter and 40-μm length. Coarsening of the matrix microstructure and growth of TiB particulates occurred after annealing heat treatment at higher temperature (ca. 1473 K). Creep-resistance improvement was also observed, which seemed to be mainly attribute to the effect of the matrix microstructure. From measurements of stress components and activation energy, all composites showed an identical creep mechanism: dislocation-controlled creep.

Abstract: It is known that Ti-6Al-4V alloy is one of the excellent candidates for aerospace structure due to their high specific strength.However, its higher cost and low formability relative to other materials tend to limit the wide usage of the material.The purpose of this study is to characterize the superplasticity of this alloy so to obtain materials and process parameters for superplastic forming and diffusion bonding for industrial application. High temperature tensile tests was carried out at the strain rate range of 10-4 to 10-2 s-1 and temperature range of 1123°C to 1223°C. According to the results of the experiment, the optimum diffusion bonding condition was obtained at 1148°C, applying pressure of 4MPa for 1 hour in argon gas environment, which condition is more practical than expensive vacuum condition. It is shown that at the optimum condition for diffusion bonding with parent metal, the oxide film becomes unstable and the oxygen is diffused into the bulk. At this condition, the mechanical and microstructural integrity at the bonding interface was observed in a sandwich structure and a heavy block of titanium part from massive diffusion bonding process.

Abstract: Recent researches have shown the premature breakdown of creep rupture strength in long term creep region of advanced high Cr ferritic steels. As safe operation of power plants becomes a serious problem we should be able to detect and predict the breakdown transition of creep rupture strength. Some methods for detecting the breakdown transition have been presented till now like the measurement of reduction of area after creep rupture and particle size of laves phase. However it will be more economic if we make use of non-destructive tests, for example, hardness testing. In this paper 3 types of ferritic steels with different Cr concentration have been studied. The results suggest that the hardness of aged structures is constant independently of exposure time in short term region, whereas the hardness breaks down in long term region. The boundary of breakdown in hardness coincides with that of breakdown in creep rupture strength.

Abstract: Influences of Ca addition on microstructures and mechanical properties at room and elevated temperatures have been investigated for Mg-1.5%Nd-1.0%RE-0.5%Zn-(0~1.0)%Ca casting alloys, on basis of experimental results from X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), tensile and creep tests. Microstructures of the alloys are characterized by dendritic α-(Mg) grains surrounded by Mg12Nd-Zn-(Ca) eutectic network phase. The average size of α grains decreases gradually with an increase in Ca content. At room temperature, yield strength (YS) is enhanced with increasing Ca content with a decrease in ultimate tensile strength (UTS) and elongation to fracture, whereas the Ca addition leads to greater YS and UTS at 175oC. The tensile creep strain and secondary creep rate, measured at 150 and 200oC under 100MPa for 100hrs, become lower with the increase in Ca content. The obtained tensile properties at elevated temperature demonstrate that the addition of Ca plays a role in improving high temperature mechanical properties including creep resistance for the Mg-Nd-RE-Zn-(Ca) alloys. In view of microstructural evolution, this would be attributed to the refined primary α grains and higher thermal stability of the Mg12Nd-Zn-Ca eutectic strengthening phase.

Abstract: The development of new creep resistant magnesium alloys has become a major issue in recent years. The alloys investigated in the present work are based on the binary system Mg-Sn. Sn as major alloying element was chosen due to its high solid solubility over a wide temperature range and due to the possible formation of Mg2Sn intermetallic precipitates with a high melting temperature of about 770°C. These characteristics suggest that a fairly large volume fraction of thermally stable Mg2Sn particles can be formed during solidification. This makes it possible that the Mg-Sn alloys can be developed as creep resistant magnesium alloys. In fact, previous investigations indicate that the Mg-Sn alloys have a comparable or even better creep property than AE42 alloy. The present work investigates the microstructure of Mg-Sn alloys with and without creep deformation using SEM and TEM technique. The effects of microstructural inhomogeneity on the creep response are presented. Based on the microstructural analysis, the mechanism responsible for improving the creep resistance will be discussed. It is shown that the grain boundary sliding is a dominant creep mechanism for the Mg-Sn binary alloy.

Abstract: For steady-state deformation caused by grain-boundary diffusion and viscous grain-boundary sliding, the creep rate of regular polyhedral grains is analyzed by the energy-balance method. For the microstructure, the grain-grain interaction increases the degree of symmetry of diffusional field, resulting in a decrease of the effective diffusion distance. Meanwhile, the viscous grain-boundary sliding is found to decrease the creep rate. The present analysis reveals that the grain-size exponent is dependent on the grain size and the grain-boundary viscosity: the exponent becomes unity for small grain sizes and/or high viscosity, while it is three for large grain sizes and/or low viscosity.

Abstract: The effect of boron on microstructure evolution and creep deformation behavior has been investigated for a tempered martensitic 9Cr-3W-3Co-0.2V-0.05Nb steel at 650oC. Creep tests were carried out at 650oC for up to about 6 x 104 h. The addition of boron retards the onset of acceleration creep at low stress and long time conditions, which results in lower minimum creep rate and longer time to rupture. The addition of boron also retards the Ostwald ripening of M23C6 carbides near prior austenite grain boundaries (PAGBs) during creep. The retardation of the onset of acceleration creep results from the retardation of the recovery of martensitic microstructure near PAGBs by pinning effects due to fine M23C6 carbides. The main effect due to boron is considered to occupy vacancies near growing M23C6 carbides, which makes it difficult to accommodate local volume change around the growing carbides. This reduces the rate of Ostwald ripening of M23C6 carbides.

Abstract: The effect of nanocrystalization on superplastic flow was examined in ZrO2-30vol%spinel composite. The nanocrystalization can increase the strain rate by one order of magnitude or lower the deforming temperature by about 100 K. Irrespective of the lowered flow stress, however, the tensile elongation to failure of nanocrystalline composite is lower than that of submicrom-grain composite. The limited tensile elongation in nanocrystalline composite can be ascribed mainly to accelerating cavity damage accumulation.

Abstract: A unified interpretation of super plastic flow (SPF) and cosmic micromechanics in spatially extended single and polycrystalline systems (SESPS) allows determined that the nature of the hyperbolic granular flow in SESPS is assisted by the movement of dislocations as the pattern of the inner dimension flow. Consequently in this work a mathematical model related with relativistic cosmology and quantum mechanics is used in order to obtain the activation energy for super plastic flow in SESPS. This correspondence law between SPF and cosmic micromechanics is important in the light of recent cosmological theories of the existence of dark matter and dark energy in the cosmic structure, because in this new interpretation of the universe the planets, stars, galaxies, clusters of galaxies, etc., are considered as precipitates on dislocations in the cosmic structure, which is formed in a nature way by the dark matter and dark energy, in a similar form of precipitates on dislocations in a SESPS of metals. Physically in this context the expansion process of the universe is highly dependent upon the volume fraction, size and distribution of precipitates on dislocations in the cosmic structure. Therefore, in this work the main results obtained in cosmic micromechanics and cosmic macromechanics are related with the Max Planck’s scale (MPE) and Edwin Hubble’s scale (EHS) respectively.