A fundamental understanding of Al-Ni-Mn alloys: microstructure, mechanical properties, solidification and minor alloying

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonThe development of aluminium alloys for applications at elevated temperatures has become an important topic for several decades. Since the application temperatures of Al Si based alloys are limited because of the deterioration of mechanical properties at high temperatures, the development of new aluminium alloys is increasingly attractive. The Al Ni Mn alloys are potentially good candidates because the eutectic temperature (640 ℃) is higher than that of Al Si systems (577 ℃), and the Al 3 Ni phase is stable up to 600 ℃. However, there is limited research in the existing literature s and the understanding of A l Ni based alloys is not sufficient to draw solid conclusions either from fundamental theory or from applicable evidence s . Therefore, the aim of present study is to fill the knowledge gap to enhance the understanding of Al Ni Mn alloys from the microstructural characteristics a nd mechanical properties at ambient and elevated temperatures, in association with the solidification process and the effect of minor elements in alloy systems. The effect of Mn on the microstructure and mechanical properties of hypoeutectic, eutectic and hypereutectic Al Ni alloys was thoroughly studied. The microstructural characteristics of Al 4Ni xMn, Al 6Ni xMn and Al 8Ni xMn (x=0, 2 ,3 ,4) alloys revealed that 2% Mn addition could transform the eutectic structure from α Al+Al 3 Ni to α Al+Al 9 (Ni, 2 For Al 6Ni and Al 8Ni alloys, Mn also transformed the primary phase from Al 3 Ni to Al 9 (Ni, Mn) 2 . The κ phase Al 15.6Mn 4.8Ni (at.%) at.%)) Ο phase Al 12.6Mn 7.0 Ni (at.%) at.%)) and Al 6 Mn were formed with increasing Mn content to the level of 3.0 and 4.0%. Moreover, the addition of Mn in Al Ni alloys improve d the tensile strength at both ambient temperature and 250 °C. The best tensile strength was obtained by the Al 6Ni 4Mn alloy. It offer ed yield strength of 164 MPa, ultimate tensile strength (UTS) of 256 MPa at 25 °C, and the yield strength of 134 MPa, UTS of 176 MPa at 250 °C. The significant progresses have also been made for the understanding in the microstructural evolution of Al 4Ni 2Mn alloy under different cooling rates. The eutectic structure was α-Al+Al 9 (Ni, 2 in PM C alloy and then became α Al+Al 9 (Ni, 2 +Ο and a mixture of α Al+Al 9 (Ni, 2 Ο+Al 3 Ni as the cooling rates decreased to 20 K/min and 0.5 K/min, respectively. The primary α Al phase was the only primary phase in the alloy processed by PMC . When the cooling rate decreased to 5 K/min and 0.2 K/min, the Ο phase and Al 6 Mn phase began to form, respectively. The evaluation of the thermal stability of the eutectic Al 9 (Ni, 2 phase in Al Ni Mn alloys was not foreseen in previous research es . The eutectic Al 9 (Ni, 2 phase showed excellent thermal stability at 250 °C. No coarsening of eutectic rods was observed after 2000 h isothermal holding at 250 °C. However, after 720 h at 300 °C or 20 h at 400 °C, the κ phase was observed at α Al Al 9 (Ni, 2 interphase boundaries and segmented eutectic Al 9 (Ni, 2 rods was clear in the microstructure, which are responsible for the deterioration of mechanical properties at elevated temperatures. The effect of Cu and Ce on the microstructure and mechanical properties were firstly studied in Al Ni Mn alloys. The Cu addition in the Al 5Ni 2Mn alloy could increase the strength under as cast condition. When Cu content was increased from 0 to 2.0 wt.% Cu, the yield strength increased from 109 MPa to 130 MPa and the UTS increased from 2 28 MPa to 248 MPa , respectively. However, T6 heat treatment was found to be able to provide very limited strengthening effect for the Cu addition. The main reason is attributed to the dissolution of Cu in the Al 9 (Ni, 2 phase. On the other hand, the Ce addition was found to introduce eutectic Al 11 Ce 3 phase when Ce content reached 0.5% and to form primary Al 10 Mn 2 Ce phase when Ce reached 5.0%. The morphology of the eutectic Al 9 (Ni, 2 phase was completely changed from rod like shape s to fibrous morphologies in the alloy with 2.0% Ce addition. It is interesting to find that the hardness of the Al 5Ni 2Mn alloy increased significantly with increasing the Ce content s but the tensile strength was not significantly improved. It is mainly due to the morphological change of the eutectic Al 9 (Ni, Mn) 2 phase from well aligned rods to fibrous structure s, which provides weaker strengthening effect of the eutectic Al 11 Ce 3 phase compared with the Al 9 (Ni, 2 phase