Papers by Keyword: Age Hardening

Abstract: Microstructure characterization and hardening behavior of a new designed Ti-12.1Mo-1Fe alloy during solution and aging treatment was investigated in the present study by OM, Vickers hardness. The results showed that the beta transus temperature of new Ti-12.1Mo-1Fe alloy was about 780°C. Further observation of ω phase should be performed by TEM later. It is also found that ω phase played a more important role than α phase in hardening. The hardening due to ω phase can lead to a high hardness about 470 Hv but the coarse α phases result in a hardness below 300 Hv.

Abstract: The effect of tool traverse and rotation speeds on the microstructures and mechanical properties are quantified for welds between non-age-hardening Al5083 and age hardening Al2024 and compared to single alloy joints made from each of the two constituents. In this paper, we report the results of microstructural, mechanical property investigations of Al5186Al2024 friction stir welds produced using various rotations and traveling speeds of the tool to investigate the effects of the welding parameters on the joint strength. Metallographic studies by optical microscopy, electron probe microscopy, and the utilization of the X-ray diffraction technique have been conducted. It was found that the weld properties were dominated by the thermal input rather than the mechanical deformation by the tool. In particular the larger stresses under the weld tool on the AA5186 side compared to the AA2024 side are related to a transient reduction in yield stress due to dissolution of the hardening precipitates during welding prior to natural aging after welding.

Abstract: Grain boundary embrittlement and de-embrittlement observed in age hardening iron alloys were reviewed. Fe-Mn-Ni and Fe-Ni-Ti alloys show excellent hardening response during aging treatment. However these alloys all suffer grain boundary embrittlemnt and show no tensile ductility even after very short aging treatment. Precipitation of intermetallic phases, θ-MnNi in Fe-Mn-Ni alloys and η-Ni3Ti in Fe-Ni-Ti alloys, at grain or lath boundaries was suggested as the reason for the weakening of grain boundary strength. Grain boundary strength recovered when these precipitates transform to austenite after extended aging. Dislocation glide or dislocation climb did critical role in conversion of these grain boundary precipitates to austenite.

Abstract: Artificial aging behaviour of a 7017 Al-Zn-Mg alloy was studied by hardness measurement. The electrochemical behaviour of various alloy tempers, such as under-, peak-, and over-aged, have been evaluated by measuring variation of open circuit potential (OCP) with time and potentiodynamic polarization study in different environments. All the alloy tempers were tested in 3.5 wt. % NaCl solution at neutral (pH 7), at acidic (pH 1) and at alkaline (pH 12) conditions. It has been observed that the OCP values of the alloy tempers shifted toward noble direction with the increase of aging time in 3.5 wt. % NaCl solution. The polarization curves are more or less similar in shape exhibiting only active region in neutral (pH 7) and in acidic (pH 1) conditions, but an active-passive region at alkaline (pH 12) condition. The electrochemical parameters; E_corr, I_corr, passive potential range (E_p) and passive current (i_p), obtained from potentiodyanamic polarization curves for the alloy tempers in acid, neutral and alkaline solution; depend on the alloy tempers and the pH of the solution as well. Attempts have been made to explain the observed electrochemical behaviour of the alloy tempers, which is influenced by the microstructure, presence and distribution of second phase precipitates, accessed by DSC and XRD techniques.

Abstract: In this study, age-hardening behavior of Mg-Xmass%Al alloys (X=3, 4.5, 6, 7.5 and 9mass%) were investigated by Vickers hardness measurement and optical microscopic observation. Each alloy was solution-treated and then isothermal-aged at 473, 498 and 523K. In the case of aluminum content less than 6mass%, Mg-3mass%Al and Mg-4.5mass%Al alloys, occurred insignificant age hardening. In the case of aluminum content higher than 6mass%, Mg-6mass%Al, Mg-7.5mass%Al and Mg-9mass%Al alloys, occurred remarkable age hardening. For each aging temperature, with heighten aluminum content, increase the value of maximum hardness and shorten time to maximum hardness. Mean hardness of discontinuous precipitation during aging increased with lower aging temperature and higher aluminum content. Furthermore, over-aged microstructure of Mg-Al system alloys differed from aluminum content or aging temperature.

Abstract: A finite element approach is used to simulate the precipitation of Ni₃(Al,Ti) intermetallics in nickel-based superalloys containing a low volume fraction of spherical g’ precipitates, in which precipitation occurs following nucleation and growth mechanisms. Classical differential equations of nucleation and growth are implemented in the software Comsol (formerly Femlab), to compute the number of precipitates per unit volume and their mean size. Another originality of the model is the use of thermodynamic quantities coming from phase diagram computations (Thermo-Calc), like the temperature variation of the equilibrium g’ volume fraction, and the evolution of the concentration of g’ forming elements (Al, Ti) in the matrix with the volume fraction of precipitates. Once adjusted to experiment in the case of isothermal ageings, the model can be used to simulate precipitation during complex thermal histories. Finally, automatic heat treatment optimisation procedures are proposed and tested, which can reduce heat treatment times by a factor of more than five.

Abstract: In this study the effects of different aging heat treatments on the properties and microstructure of a high strength, high toughness metastable β Ti, BTi-6554 (Ti-6Cr-5Mo-5V-4Al), have been compared. An initial β phase solution treatment was followed by aging at moderate temperatures in the α/β dual phase zone by either step aging directly from the solution treatment temperature or by quenching to room temperature prior to the aging heat treatment. The differing heat treatment methods have significant effects on the microstructure and mechanical properties.

Abstract: The age hardening process for permanent mold samples of Al-7Si-0.3Mg cast alloy has been investigated by hardness measurement, differential scanning calorimetry (DSC), transmission electron microscope (TEM) and electron probe micro analyzer (EPMA). Age hardening results show that the age hardening response of Al-7Si-0.3Mg alloy is independent on cooling rate. There is a hardness value decrease about 10 HV after T4 treatment. Hardness value after as-cast aging at 150 °C for 20 h is just a little smaller than that after T6 treatment for permanent mold samples. The precipitation behaviors during T6 treatment and as cast aging treatment have been analyzed by DSC analyses. The hardness measurement results have been discussed by analyzing the precipitation behaviors and the Mg and Si concentration in α (Al).

Abstract: Effect of thermomechanical processing including equal-channel angular pressing (ECAP), solution treatment, water quenching and artificial aging on microstructure and mechanical properties of an Al-Cui-Mg-Ag-Sc alloy was examined. It was shown that ECAP provides extensive grain refinement. However, extensive grain growth occurs under subsequent solution treatment resulting in coarse grained structure. It was showed that ECAP following water quenching provides a minor increase in strength of the alloy. No effect of ECAP on the precipitation sequence under ageing was found. -phase precipitating under ageing condition is in dominant. As a result, the alloy exhibits high yield stress up to a temperature of 175oC. Conditions for attaining increased strength in the alloy by ECAP processing are discussed.

Abstract: The correlation of age hardening behavior and Si precipitation in α(Al) of Al-8wt%Si-0.35wt%Mg alloy has been investigated by micro hardness measurement, electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) analysis. The EPMA results show that Si concentration in the center of α (Al) dendrites is higher than that in the edge and the main concentration is about 1.5wt% for Al-8wt%Si-0.35wt%Mg alloy in as cast condition. After solution treatment at 530 °C for 8 h followed by water quenching (T4 treatment), hardness value decreases 9 HV, which is accompanied by the decrease of Si concentration in α (Al). Aging the as-cast sample and T4 treatment sample at 150 °C for 20 h, the main concentration of Mg and Si in α (Al) changes little. Hardness value after as-cast aging is only 3 HV lower than that after T6 treatment. Nanometer Si particles and β″ and/or β′ phases are found in aged samples. The higher hardness value for as-cast aging samples should contribute to the nanometer Si particles in α (Al).