Papers by Keyword: Reversion

Abstract: Aging phenomena of Al-Zn alloys have been much investigated and many studies have been made on the change of mechanical properties on aging. Appearance of saw-tooth yielding (serration) in the stress-strain curve of tensile deformation has been reported in several reports. In this study, the relationship between occurrence of serration and reversion heat treatment was studied by tensile test on Al-Zn alloys. Serration was observed for the furnace cooled specimen, but not for the as-quenched one and the shortly aged one, which result is difficult to be interpreted by the Cottrell theory. The stress amplitude in the serration increased with increasing the time of annealing for reversion and then stayed at a constant value which might be interpreted by the Cottrell theory. The stress amplitude in the serration decreased with increasing the annealing temperature, which is contrary to the prediction by the Cottrell theory. The specimen directly annealed without aging and the one annealed after aging agreed well with each other in the tensile strength and the stress amplitude in the serration, which suggests solute clusters as the cause of serration.

Abstract: Age-hardening of Al-Zn alloy after quenching develops inhomogeneously due to the effect of surface as a vacancy sink and grain boundary as an easy path. In this study, reversion of the age-hardened Al-Zn alloys, in which ellipsoidal GP zones were formed, was investigated by Vickers micro-hardness test. Ellipsoidal GP zones were reverted more quickly near the surface and grain boundary than in the interior, as spherical GP zones in Al-10%alloy did. It is considered that the surface and grain boundary plays a role of effective source for vacancies, in addition to the interior source such as dislocations, as in the case of the reversion of spherical GP zones.

Abstract: Rubber reinforcement in general depends on the type of filler used in the rubber mixture. Instead of carbon black, Silica filler has been widely accepted in tyre making due to its low rolling resistance property. In recent years, there is a trend in using higher curing temperature in order to improve productivity of vulcanisation line without drawbacks in the performance of tyres. In this work, effect of vulcanisation temperature based on the silica filled ENR curing behaviour was studied. Results indicate that time-dependant reversion behaviour of ENR was similar to that of unmodified Natural Rubber. The physical properties slowly deteriorated as the curing temperature approached 180°C which related to low crosslink density. Carbon Black filled ENR compound was used as a comparison.

Abstract: A novel one-dimensional metal-organic coordination polymer, [Zn (NH₃)₂(BDC)]_n (BDC=1,4-benzenedicaboxylate), has been synthesized by solvent evaporation method through self-assembly of Zn (II) salts with terephthalic acid ligand in ammonia aqueous solution. Single crystal X-ray diffraction analysis indicated that each Zn (II) was coordinated by two nitrogen donors from two NH₃ and two oxygen counter donors from terephthalic acid ligand. The adjacent zigzag chains are arranged in a parallel fashion and linked by interchain hydrogen bonding interaction and π-π stacking interactions into higher-dimensional framework. The compound has also been characterized by CHN elemental analyses, Single crystal X-ray diffraction analysis, powder X-ray diffraction (PXRD) analysis, Fourier transform infrared (FT-IR) spectra, Thermalgravimetric-differential scanning calorimetric (TG-DSC) and Solid-state nuclear magnetic resonance (NMR), etc. Results showed that the framework of compound was stable at the temperature up to 246°C. The desolvated product [Zn (BDC)]_n, which was obtained by removal of molecular NH₃ from [Zn (NH₃)₂(BDC)]_n, can be transferred to the different skeleton structures through coordinating different small hydrogen-bond-forming molecules.

Abstract: This study demonstrates applying local laser heat treatment to produce ultrafine-grained austenite (UFGA) structures in an AISI 301LN type commercial austenitic steel. Pieces of 50% cold-rolled sheets containing more than 90% strain-induced martensite were heated locally by a laser beam to various peak temperatures to obtain different degrees of martensite reversion to austenite. Mechanical properties and formability of grain-refined and coarse-grained structures were measured by tensile and Erichsen cup tests. In addition to standard Erichsen cup test, additional interrupted tests were carried out, where cups were first stretched close to the critical strain. Drawn cups were then heated locally by a laser beam to revitalize the structure and thereby enhance the formability in the following cupping test until failure. Results showed that local laser heat treatment is suitable for the reversion treatment to refine the austenite grain size. Various structures were produced: completely reverted microstructures (T > 700 °C) with grain sizes 0.9 - 2 µm in addition to partially reverted structure (T < 700 °C) containing nano- and ultrafine-grained austenite (0.6 µm) with some martensite. The grain refinement by local annealing improved the strength properties. The Erichsen cup tests showed that the formability was equal in the completely reverted ultrafine-grained structures to that of the coarse-grained sheets. It was demonstrated that the local laser treatment restored formability of the drawn cups, allowing stretching to be continued. The second forming step after the laser-treatment provided an enhancement of 19 and 14% in the cup depths in coarse-grained and ultrafine-grained steels, respectively, even though the laser-treatment parameters were not optimized yet.

Abstract: Dual phase (DP) structure formed by partial reversion from cold-deformed martensite was investigated to improve mechanical property of DP steel by grain refinement strengthening. A low carbon martensitic steel (0.15C-1.0Mn) was cold-rolled and then held just above A1 temperature to partially form austenite. In particular, the conditions of cold-rolling rate (0~60% reduction in thickness) and heating rate (0.083 and 100 K/s) were varied to understand their effects on the microstructural development of DP structure. Although the recrystallization has never occurred in undeformed martensite, cold-deformed martensite was more easily recrystallized before reversion with increasing rolling rate and lowering heating rate. Then, the matrix of DP structure was changed from tempered martensite to recrystallized ferrite, which had a large influence on the distribution of fresh martensite transformed from reversed austenite. The higher rolling and heating rates resulted in the finer DP structure, leading to a large improvement in strength level.

Abstract: The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

Abstract: It is well known that the ultra grain refinement can be achieved by sever cold rolling, followed by reversion treatment in metastable austenitic stainless steel plate. In this study, the cold rolling was replaced by cold drawing. This procedure was applied to a metastable austenitic steel (Fe-16Cr-10Ni alloy) thin wire, and then the microstructure development during cold drawing and annealing was investigated. The austenite phase transformed to martensite during the drawing. Vickers hardness of the wire markedly increased with increasing the drawing strain. When the drawing strain reached about 4.5, the wire exhibited martensite single structure and had high hardness of Hv4.4GPa. Annealing of the heavily drawn wire at around 900K for 0.6ks leads to the formation of reversed austenite with the diffusional reversion mechanism. As a result, ultra fine-grained austenitic single structure with the grain size of about 0.6μm was obtained. It was also found that the wire has an excellent combination of a strength and ductility.

Abstract: Heat-treatable Al-Mg-Si aluminum alloys for automotive body panel applications are shipped and formed in T4 temper while still formable and are subsequently given a bake cycle to increase strength by age hardening while the paint is cured. However, the hardening response during the rather short industrial paint-bake is impaired upon natural ageing after the solution treatment. It is thus essential to counteract the adverse effect of natural ageing on bake hardening. While this is often accomplished by an additional ageing cycle before the paint-bake treatment, reversion treatments ought to be used once substantial natural ageing has first taken place to improve the bake hardening response of a twin-roll cast 6016 sheet. The present work was undertaken to improve the paint bake response of a twin-roll cast 6016 sheet by employing reversion treatments before the paint-bake cycle. Reversion treatments between 200°C and 250°C was effective in improving the bake hardening response of the twin-roll cast 6016 sheet which, when processed without reversion, failed to meet in-service strength requirements with a rather poor bake hardening response of 35 MPa.

Abstract: A kinetic model has been developed to simulate the precipitate size distribution and the resulting yield strength during ageing of 7xxx alloys. The η phase is the only one considered. The kinetic model is mean field: precipitates of different sizes see each other through the average solid solution. Precipitates are assumed to be homogeneous in concentration and are allowed to change chemistry. Local equilibrium is assumed at the matrix-precipitate interface; the equilibrium concentrations are corrected by the curvature effect. Values of the equilibrium concentrations at the matrix-precipitate interface are solved by an iterative method: the resulting flux for each element must be compatible with equilibrium conditions and with the changing stoechiometry of the considered precipitate while maximizing the energy gained. The yield strength is derived from the precipitate size distribution through a mixture law combining the effect of each individual precipitate. The model can take into account non-isothermal treatments and can therefore simulate complicated multi-stage ageing treatment as well as a FSW weld. Results of the model are discussed and compared measurements.