Properties Of Heat-treated Alloys Research Articles

Uncovering the kinetics of Li-rich clusters and monodisperse core–shell Al3(Zr, Sc) structures in Al–Li–Cu alloys

The size distribution and structural evolution of the precipitates are critical to the mechanical properties of heat treatable alloys. In Al alloys, many studies have reported that monodisperse core–shell Al3Li/Al3(Zr, Sc) structures in a Li-rich environment promote formation of δ′-Al3Li, T1-Al2CuLi, and θ′-Al2Cu precipitates. However, without quantitative measurements of the precipitate size distribution and tracking their crystal structure evolution, the kinetics of Li-rich clusters and excessive Li diffusion into monodisperse core–shell Al3(Zr, Sc) structures in Al alloys to prevent Li-cluster coarsening is still unknown. In this work, in-situ small angle neutron scattering (SANS) has been adopted to track the evolution of δ′-Al3Li precipitates. Upon quenching, Li-rich clusters remain at a smaller size and higher number density. Once aging starts, nucleation of δ′-Al3Li precipitates on top of Al3(Zr, Sc) is at the expense of dissolution of those Li-rich clusters below the critical size. From density functional theory (DFT) calculations, it has been found the δ′-Al3Li precipitate nucleation barrier on top of Al3(Zr, Sc) is negligible, as well as the nucleation barrier of θ′-Al2Cu on top of Al3(Zr, Sc) is much lower relative to that on α-Al. Besides, the substitutional energy for Li to replace Zr and Sc sites in Al3(Zr, Sc) is favorable. Combining SANS with high-resolution transmission electron microscopy (HRTEM), the effects of Al3(Zr, Sc) cores on preventing δ′-Al3Li size from coarsening by absorbing Li to form Al3(Li, Zr, Sc) have been uncovered. Simultaneously, the contributions of Al3(Li, Sc, Zr) particles and δ′-Al3Li to the strength has been quantified. This work opens a new engineering approach of precise control of precipitating kinetics at the presence of monodisperse precursors for industrially relevant structural materials.

Opportunities for aluminum-based nanocomposites

High performance aluminum alloys are conventionally made by heat treating alloys containing a variety of alloying elements in solid solution. Key performance attributes are controlled at the microstructural level by tailoring sizes and morphology of nano-sized second phases. This enabled the successful development of aluminum alloys having properties optimized in strength, damage tolerance and corrosion resistance. However, this process is naturally limited by the solubility of alloying elements in the aluminum matrix. In real world products, significant effort is deployed to achieve a homogeneous distribution of the alloying elements both at the macro and micro scales. Despite these efforts, heat treatable alloys can exhibit chemical gradients at grain boundaries, resulting in sub-optimized properties. Additionally, due to the very nature of the strengthening mechanisms, the properties of heat-treatable alloys are decreasing when exposed to elevated temperatures. To step outside the boundaries given by the solubility of alloying elements in the aluminum matrix, the extrinsic addition of nano-sized particles to the aluminum matrix is being evaluated.

Research on Sheet Metal of a New Ni-Based Superalloy Prepared by EB-PVD

The sheet metal of a new Ni-based superalloy has been prepared by Electron Beam Physical Vapor Deposition (EB-PVD) technology. The phases, the microstructures and mechanical properties of this alloy before and after heat treatment have been analyzed by X-ray diffractometer, transmission electron microscope, optical microscope, scanning electrical microscope and tensile equipment. Results showed that the size of γ' particles increases gradually and the morphologies of γ' particles changed from spherical shape into cubical shape when temperature increased from low to high. Compared with as-deposited alloy, mechanical properties of heat-treated alloy were improved obviously. It is feasible that superalloy of better properties can be prepared by EB-PVD technology.

Structure, Properties, and Fracture Mechanism of Cast Refractory Nickel Alloy

The temperatures of solidus and liquidus of alloy ZhS6U-VI are determined. The structure of specimens and parts from alloy ZhS6U-VI is studied in cast and homogenized states. Fractures of specimens after mechanical tests and of cast and heat treated blades tested for fatigue strength at room temperature are studied as well as fractures of specimens tested for long-term strength at a temperature of 975°C and a stress of 230 MPa. Regression analysis of the dependence of the properties of heat-treated alloy on the fluctuation of its chemical composition within the standardized range is preformed. Adequate “composition-property” mathematical models are created for describing the dependence of mechanical properties of alloy ZhS6U-VI on the content of the main components. Differences in the mechanical properties of specimens from different heats are explained.