Search

The effects of aging temperature (25–220 °C) on precipitation behavior and the resulting alloy strength were characterized for the Al–6Mg–9Si–10Cu–10Zn–3Ni (wt%) natural composite. At an aging temperature of 25 °C, spherical and ellipsoidal Guinier–Preston (GP) zones (Zn clusters) are formed within the Al matrix. The fastest kinetics of Zn clustering is exhibited at 70 °C, which is caused by the combined effects of accelerated Zn diffusion and decreased phase thermal stability with increasing aging temperature. Thus, the Zn clusters and precipitates are formed prior to those of Cu (GPI and θ'') at 70 °C. As the precipitation kinetics of θ accelerates and Zn decelerates at above 70 °C, the precipitation sequence changes at 170 °C; clusters and precipitates of Cu are formed prior to those of Zn at this temperature. At an aging temperature of 220 °C (above the solvus of hcp Zn), θ'-Al 2 Cu and Q′-Al 4 CuMg 6 Si 6 phases are mainly precipitated instead of Zn clusters and precipitates. The matrix strengthening induced by the fine clusters and precipitates that form below 70 °C results in a significant improvement in the strength of the composite. However, high-temperature aging has only a small strengthening effect due to the lower matrix hardening caused by precipitate coarsening and the softening caused by the decreased volume fraction and spheroidization of the secondary phases. [ABSTRACT FROM AUTHOR]

The precipitation behavior of the multicomponent Al–6Mg–9Si–10Cu–10Zn–3Ni (wt%) alloy was investigated during artificial aging at 120 °C through complementary experimental methods including hardness and electrical resistivity measurements, compression testing, differential scanning calorimetry, and transmission electron microscopy. The solution treatment causes the dissolution of the preexisting θ-Al 2 Cu and hcp Zn phases, thereby increasing the concentrations of Zn and Cu species in the Al matrix. During aging, two resistivity peaks (I R , II R ) become visible followed by the appearance of the related hardness peaks (I H , II H ). Spherical Guinier-Preston (GP) zones are formed first and then subsequently transformed into ellipsoidal GP zones. The former process results in the appearance of the I R peak, while the latter process produces the I H peak. During transformation of the ellipsoidal GP zones to hcp Zn, GPI zones are additionally formed, leading to the appearance of II R . The following transformation of the GPI zones into the θ''-Al 3 Cu phase leads to the appearance of the II H peak. Both the ellipsoidal GP zones and hcp Zn phases contain large quantities of Cu atoms, while the θ''-Al 3 Cu phase does not have any Zn atoms. Based on these insights into the structural and chemical compositional evolution of complex precipitates, the strengthening mechanism of the multicomponent alloy during aging has been established. [ABSTRACT FROM AUTHOR]

The effects of ultrasonic melt treatment (UST) and solution treatment on the microstructure and mechanical properties of a low-density multicomponent Al 70 Mg 10 Si 10 Cu 5 Zn 5 alloy at room and elevated temperatures were investigated. The as-cast Al 70 Mg 10 Si 10 Cu 5 Zn 5 alloy was composed of an fcc Al solid-solution, coarse secondary phases (5–30 μm), fine Cu-containing precipitates (<1 μm), and Zn clusters (<20 nm). The UST significantly reduced the porosity and size of primary Mg 2 Si particles and grains, resulting in an improvement in the strength and ductility of the as-cast alloy at 25 and 350 °C. The solution treatment at 440 °C caused the dissolution of hcp Zn and fine Cu-containing precipitates. It also resulted in the formation of Zn and Cu clusters in the supersaturated Al solid-solution at 25 °C, which led to an improved room-temperature strength. A phase transformation from Mg 2 Si to Q-Al 5 Cu 2 Mg 8 Si 6 occurred during the solution treatment along with the spheroidization of coarse secondary phases and grain growth, which deteriorated the ultimate compressive strengths at 25 and 350 °C and improved the ductility. Unlike the commercial Al alloys, the Al 70 Mg 10 Si 10 Cu 5 Zn 5 alloy showed excellent strengths at temperatures below 200 °C. This is attributed to the combined strengthening by coarse secondary phases, fine precipitates, and clusters. [ABSTRACT FROM AUTHOR]

Cr and Mo are known to be effective steel alloying elements for high-temperature applications, and they affect steel mechanical properties via carbide formation. Mechanical properties of Cr-Mo steels affected by the carbide evolution were investigated by performing heat treatments, and the treatment duration was determined by the Larson-Miller parameter fitting. The hardness value decreased after heat treatment at 690 °C for 10 h with the decomposition of the bainitic microstructure. The quantified chemical composition results for the carbides were plotted on the Fe-Cr-Mo ternary and Fe-Cr binary composition diagrams to classify the types of carbides formed. Most of the carbides were identified as Fe-rich M 3 C-type carbides at the initial stage of heat treatment. The Fe fraction in the carbides decreased with time while the Cr and Mo fractions increased. The Cr- and Mo-rich carbides were identified as M 23 C 6 - and M 2 C-type, respectively. An increase in the Cr and Mo fractions in the carbides resulted in the decrease in the alloying element concentration in the matrix, which in turn resulted in the deterioration of the solid solution strengthening. Further, it was determined that coarse carbides, observed on the grain boundaries after extended heat treatment, deteriorated the mechanical properties of the alloy. [ABSTRACT FROM AUTHOR]

We investigated the microstructural evolution and precipitation behavior of a multicomponent Al 83 Zn 5 Cu 5 Mg 5 Li 2 alloy using transmission electron microscopy (TEM) and atom-probe tomography (APT) during solution treatment, cooling, and natural aging. The as-cast alloy consisted of high volume fractions of V-Al 5 Cu 6 Mg 2 and η -Mg(Zn,Cu,Al) phases, which were partially dissolved during solution treatment at 450 °C. Slow furnace cooling (FC) after solution treatment resulted in the formation of polygonal V-phase, elongated η -phase, and platelet Y-phase precipitates. The Y-phase precipitates consisted of one Mg-rich central layer and three adjacent Zn/Cu-rich layers, both lying on the {111} Al planes. Instead of Y-phase precipitates, air cooling (AC) induced the formation of S- and Zn-phase precipitates with well-developed orientation relationships. No precipitation occurred during water quenching (WQ). The microhardness increased significantly during the natural aging of the AC and WQ alloys, whereas no hardening was observed in the FC alloy. TEM and APT analyses demonstrated that the natural aging induced the formation of fine solute clusters with a chemical composition of 52.3Al–21.3Li–18.3Zn–4.9Mg–3.1Cu (at%), contributing to the natural age-hardening of AC and FC alloys. The solute clusters were fully coherent with the Al matrix and exhibited an interfacial region rich in Zn and two central regions rich in Li and Zn. • Al 83 Zn 5 Cu 5 Mg 5 Li 2 cast alloy consists of highly interconnected V and η phases. • Cooling rate-dependent precipitation of V-, η-, Y-, S-, and Zn-phases is confirmed. • Y-phase comprises a Mg-rich layer and adjacent Zn/Cu-rich layers lying on {111} Al. • Li- and Zn-rich coherent solute clusters contribute to natural age-hardening. [ABSTRACT FROM AUTHOR]