Activation Energy of Recovery Process in a Heat-Resistant Alloy Type RR58 with Zirconium

Influence of zirconium on activation energy of the recovery process of Al alloy type RR58 is presented in this paper. By following changes in micro-hardness as a function of time for different temperatures and at different zirconium contents, activation energy of the recovery process, which served as an indicator of the creep resistance of the alloys, was determined. A mathematical model, which enables to evaluate and predict required quality of alloys was also defined. Introduction On the basis of the literature data [1-6] and previously obtained results [7,8], it was concluded that the transition metals with relatively low solubility in solid solution (manganese, chrome, zirconium, as well as lanthanides), added to the aluminum alloys can be used for strengthening of grain and sub-grain boundaries, due to precipitation in the form of fine dispersed particles. In this way, the recovery resistance can be increased and creep velocity decreased, which leads to high values of hardness parameters at raised working temperatures. Since one of the heat-resistance criteria is yield creep, taking into account that difficulties in the recovery process mean at the same time difficulties in the creep process, the accent was put on the investigation of the recovery process. Calculated activation energy and relative recovery factor values served as the indicators of the difficulties in development of the recovery process, i.e. effect of zirconium on the creep resistance. In this paper, only the results relating to the activation energy of the alloys containing zirconium were presented. Experimental The initial material for the experiments was an Al-based heat-resistant alloy, type RR58 (AA2618), of a complex composition, produced by the standard melting process, while zirconium was added in the form of Al-Zr5 master alloy. Chemical composition of manufactured alloys is given in Table 1. 2nd International Conference on: 214 DEFORMATION PROCESSING AND STRUCTURE OF MATERIALS Table 1. Chemical composition of the produced alloys [mas%]. Alloy Cu Fe Ni Mg Si Zr Alloy 1 2,10 0,96 1,21 1,28 0,30 Alloy 2 2,15 0,91 1,20 1,26 0,29 0,0834 Alloy 3 2,10 0,93 1,20 1,25 0,28 0,1763 Alloy 4 2,10 0,91 1,18 1,24 0,28 0,244 Casting process was followed by homogenization at the temperature of 510-515 °C for 22 hours, and then by hot forging and cold rolling process. Recovery process was performed on all alloys at five selected temperatures, at a range of 150-350 °C, and for nine different time period. Results and discussion Activation energy of the recovery process. Activation energy of a recovery process in this paper was defined on the basis of analysis of the changes in micro-hardness with time, at different temperatures and with different zirconium contents [8]. Figure 1 shows time dependence of micro-hardness for the Alloy 1 (base alloy). Fig. 1. Changes in micro-hardness for Alloy 1. Quantitative definition of the dependance of the recovery rate on time and temperature is usually followed by a complex procedure. In some cases, however, a simplified analysis can be applied [9]. Certain physical or mechanical properties (micro-hardness in this case) during recovery process of the deformed metal was analysed. According to Eq. 1: t / K dt / X d = ∆ (1) decrease or increase of velocity of any characteristics (∆X) is inversely proportional to the recovery time (t) [10]. Parameter K in Eq.1 is constant for the given temperature and it depends on activation energy of the process (Q), according to the Arrhenius equation. If Xd indicates value of micro-hardness after deformation, i.e. prior to the recovery process, micro-hardness after recovery is denoted by X, then the total change occurring during recovery can be expressed as Xu = Xd – X. Parameter Xu is proportional to the volume error concentration C, occurring during deformation [9]. According to those statements, changes of the investigated parameters values with time can be given by: Activation Energy of Recovery Process in a Heat-Resistant Alloy Type RR58 with Zirconium 215