Papers by Keyword: Precipitation Hardening

Abstract: By means of Vickers hardness tester, optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and high resolution transmitted electron microscope (HRTEM), the bake softening and precipitation behaviors of AA5182 H19 sheet for can end stock at 205°C and 249°C were investigated. All specimens at both temperatures showed recovery and bake softening phenomenon, which meaning the dislocation density and HV decreased. However, the specimens baked at 205°C showed higher recovery impediment, because the bake softening curve departed from the dynamic laws when it had less amount of recovery than the specimens baked at 249°C. The hardness was higher for the specimen baked at 205°C compared with the specimen baked at 249°C, even both specimens had the same dislocation density measured by XRD. Further observations revealed that the precipitated particles in the specimens baked at 205°C distributed along the shear bands. The precipitates were needle shape with the length of 5-15 nm and the width of 5-10 atom layers, which occurred mostly in the area with higher dislocation density. These precipitates were guessed to be Al-Mg binary phases, which could contribute to the higher hardness of the specimens baked at 205°C.

Abstract: Aluminium is ranked after iron and steel in the metal market. Aluminium 6061 alloy has been selected by many designers and engineers for different kind of applications. Further, its strength can be improved by cold working, refinement of grains, precipitation and dispersion hardening. In this work, trials are conducted for the investigation of the effects due to age hardening parameters like, solutionizing time, temperature and time for aging on hardness of 6061 Aluminium alloy using Design of Experiments. The response is predicted by using linier regression model. From the results it can be perceived that, the ageing temperature and aging time have a substantial effect on the response whereas, solutionizing time does not have a significant effect. For a specific set of parameters the hardness is improved from 50 to 74 BHN, resulted in an increase in hardness by about 50%.

Abstract: CuAgZr alloy is a minor variation of CuAg alloy that is remarkably known for good combination of strength and electrical conductivity. Strengthening and conductivity enhancing of CuAgZr alloy is essentially proficient by the precipitation of Ag precipitates. The behavior of Ag precipitates at high temperature was investigated using in-situ transmission electron microscopy. These nanoscale Ag precipitates are formed in CuAgZr alloy during heating process with the average size of 5 nm. Growth of precipitates at higher temperature can be explained by the consumption of solute diffusing from smaller precipitates. Dislocation looping at high temperature would be the effects of a large lattice strain along matrix/precipitate interface that would retard the dislocation movement.

Abstract: Magnesium alloys have been widely applied in the automotive world cars or motorcycles and aircraft engines. This is because the weight of the magnesium itself is very lightweight and have high strength. And magnesium alloys have good thermal conductivity, high elastic modulus and good mechanical properties. Magnesium in engineering applications is usually in the mix with elements such as Al, Ag, Mn, Zn, Si, Zr and RE (rare element). Magnesium alloys with zinc are mostly found and used. This research has been carried out precipitation hardening treatment-Mg alloy-0.5Y 6Zn using variable temperature 150^oC, 175^oC and 200^oC with a holding time of 12 hours, 24 hours and 36 hours. The results show microstructure formed is αMg, MgZn, and Mg₃Zn₆Y (i-Phase). The formation of precipitates during the process of aging raise hardness values up to 75.8 BHN. Aging treatment reduces the thermal resistance of the alloy Mg-6Zn-0.5%Y.

Abstract: Recent investigations revealed major fluctuations in the material properties of selective laser melted AlSi10Mg, which corresponded with the varying precipitation-hardening state of the microstructure, caused by the differing dwell times at elevated temperatures. It was indicated that a subsequent heat treatment balances the age-hardening and results in a homogenized material strength. In order to further investigate this statement selective laser melted AlSi10Mg samples were subject to multiple post-heat-treatments. Subsequently, the surface hardness and tensile strength was determined and compared with the as-built results. The post-heat-treatment led to an arbitrary occurrence of rupture, indicating a successful homogenization, coupled with a remarkable improvement in ductility, but to the costs of a lowered tensile strength, which was highly dependent on the chosen heat-treatment procedure.

Abstract: The low cycle fatigue (LCF) resistance of a precipitation hardened Cu-Ni-Si alloy has been investigated. Fully reversed strain controlled LCF tests were performed in air at room temperature at a total strain variation Δε_t included between 0.6 and 1.5% and at a strain rate equal to 4.10^-3 s^-1. The cyclic accommodation of the alloy is composed of a hardening step (for Δε_t > 0.8 %) followed by a continuous softening until the specimen fracture, except at Δε_t = 0.6 % where only a slight softening is observed. For all tested strain variations, the microstructure study of the fatigued alloy by transmission electron microscope (TEM) revealed some grains with a high density of isolated short dislocations pinned between two nanoprecipitates. For Δε_t ≥ 0.8 %, some grains present also deformation bands observed by TEM. The strain localisation into these bands implies an important shearing of the precipitates. It leads to their dissolution into the copper matrix and forms precipitate-free bands. The resulting macroscale cyclic accommodation depends on the ratio between the two mechanisms of plasticity accommodation at the microscale.

Abstract: This report presents a comparative assessment of the suitability of two existing physical models for predicting yield strength of artificially aged AA6063 Al-alloy. One model is based on the modified Orowan mechanism of dislocation by-pass for non-shearable rod shaped precipitates rejuvenated by Zhu and Stark, and the other model is based on classical dislocation-particle interaction that incorporates both cutting and by-pass mechanisms for spherical shaped precipitates developed by Deschamps and Brechet. Using these models, simulation of yield strength values have been performed considering nucleation-growth as well as nucleation-growth-coarsening of precipitates during the entire period of ageing. Comparison of experimental and simulated results reveals that the model by Deschamps and Brechet predicts yield strength more accurately when nucleation-growth-coarsening of precipitates are incorporated.

Abstract: Deformation behavior of Fe-15Al-18Co-3Ti (at.%) single crystals containing the Co₂AlTi precipitates was examined. In the single crystals furnace-cooled (FC) from 1373 K to room temperature, coarse Co₂AlTi phase with the L2₁ structure was precipitated in the bcc matrix. The L2₁ phase showed a cuboidal shape with a misfit strain of 0.59%. It is also noted that large amount of Fe substituted for Co in the Co₂AlTi precipitates. The FC single crystals exhibited high yield stress above 600 MPa up to 823 K while further increase in temperature resulted in a decrease in yield stress. In the FC crystals, 1/2<111> dislocations in the bcc matrix bypassed the coarse L2₁ precipitates due to their large misfit strain, resulting in high strength. In contrast, the fine L2₁ precipitates about 30 nm in diameter were observed in the crystals after solutionization and annealing at 823 K. The crystals with the fine L2₁ precipitates demonstrated high yield stress above 1400 MPa at room temperature. Paired 1/2<111> dislocations cut the fine L2₁ precipitates, which led to high strength. The dependence of the yield stress on the precipitate size was also discussed.

Abstract: Complicating factors that have stymied understanding of uranium-niobium’s aging response are briefly reviewed, including (1) niobium inhomogeneity, (2) machining damage effects on tensile properties, (3) early-time transients of ductility increase, and (4) the variety of phase transformations. A simple Logistic-Arrhenius model was applied to predict yield and ultimate tensile strengths and tensile elongation of U-4Nb as a function of thermal age. Fits to each model yielded an apparent activation energy that was compared with phase transformation mechanisms.

Abstract: Endeavours were undertaken to assess metallographically AlMgSi alloys, (containing 0.58 wt% of Mg, and 0.58 wt% or 6.5 wt% of Si) in their primary states as well as after applying external loads in the Static Compression Test. During the first stage of the study the primary microstructures of the investigated samples were assessed. Samples originated from two technological processes: after strain hardening, in a form of rods (φ 9.5 mm and φ 4.0 mm) gravity casting, rolling, homogenisation and precipitation hardening; and after gravity casting in sand moulds of shaped plates 100 mm x 100 mm x 4 mm of a primary structure and after the precipitation hardening. During the second stage of investigations, microstructures of analysed samples (of various technological history) were compared - after applied external loads - in order to obtain information in what way such diversified primary structure and which of its stereological parameters (amount, shape, size and placement) are essential in initiation of microcracks under static loads.