Roles of microalloying elements on the cluster formation in the initial stage of phase decomposition of Al-based alloys
- PDF / 1,162,832 Bytes
- 11 Pages / 606.24 x 786 pts Page_size
- 45 Downloads / 187 Views
10/30/03
12:04 PM
Page 2745
Roles of Microalloying Elements on the Cluster Formation in the Initial Stage of Phase Decomposition of Al-Based Alloys TATSUO SATO, SHOICHI HIROSAWA, KIYOSHIGE HIROSE, and TAKAHARU MAEGUCHI In age-hardenable aluminum alloys, nanoscale clusters composed of solute atoms and vacancies are formed and play very important roles in microstructure control and alloy properties. The microalloying elements in the aluminum alloys affect the cluster formation and resultantly precipitate microstructures. The fundamental effects of microalloying elements on Al-Cu, Al-Zn, Al-Li-Cu, and Al-Mg-Si alloys are demonstrated through changes in the electrical resistivity, hardness, differential scanning calorimetry (DSC), and microstructural evolution. In order to understand the complicated effects of microalloying elements, a Monte Carlo simulation model was developed and performed. In the simulation, it was found that some microalloying elements preferentially trap quenched-in excess vacancies to retard cluster formation in the initial stage of aging. The complex clusters of solute/microalloying element/vacancy were found to be formed in the subsequent stage and act as effective nucleation sites for clusters and Guinier–Preston (GP) zones. The roles of microalloying elements are well understood in terms of the ordering parameters and are well predicted based on the ordering parameter maps (OP maps) proposed in this work.
I. INTRODUCTION
ALUMINUM alloys with high mechanical strength have become extremely attractive as structural materials to reduce weight in various applications. A number of fabrication techniques have been developed to increase the mechanical strength of alloys. Among them, precipitation strengthening is one of the most essential techniques to increase the mechanical strength in aluminum alloys. Small additions of various alloying elements are of prime importance to modify microstructures and to improve mechanical properties of alloys.[1] Such microalloying effects have been widely applied to various alloy systems. In Al-Cu alloys, for example, trace elements of Sn, In, and Cd suppress low-temperature aging and enhance both the rate and extent of age hardening at elevated temperatures due to the finer and more uniform dispersion of the Al2Cu phase. The enhanced formation of the T1 phase through the addition of Mg and Ag to Al-Li-Cu alloys is also a typical example of the improved properties by microalloying elements. In Al-Zn alloys, trace additions of Mg are known to suppress the formation of Guinier–Preston (GP) zones, whereas alloying with Ag accelerates the formation of GP zones.[2] The effects of such microalloying elements are explained based on the interaction between Mg, vacancies and Ag-rich clusters. The mechanism, however, is still controversial. Microalloying elements in aluminum alloys alter properties by changing the morphology, chemistry, structure, spatial distribution, and size of precipitates. In Al-Cu alloys, solute atom clusters are formed in the very early stage
Data Loading...
