Structure Refinement And Properties Of A CuAlMn Shape Memory Alloy

As an important damping material, Cu based shape memory alloy（SMA） has the performance potential and price advantage. However, due to its coarse grains, intergranular fracture is prone to take place. To solve this problem, this thesis chose the Cu-11.9Al-2.5Mn（wt%）SMA as the research object and refined its grains by using the refining effect of the AlLaB inoculant. In the present study, the refining effect of the inoculant and the associated mechanisms were analyzed, and the examination of the micro-structures, as well as the researches on the hardness, the tensile property, the damping property and the tribological property of the refined CuAlMn SMA were also carried out. The results showed that:1. The Al LaB inoculants have obvious effect on refining the grains of the CuAlMn SMA. When the adding amount of the Al LaB inoculant reaches 2.5 wt% an optimum refining effect can be obtained. The average grain size can be reduced from 200 μm of the original alloy to about 40 μm. The grain size of the refined CuAlMn SMA would not dramatically increase during the betarising and quenching processes. For example, after quenching from 850 ℃ the grain size only increased to 67μm. It is believed that the LaB₆ phase in the AlLaB inoculant can act as nucleating centers of the heterogeneous nucleation during the solidification process of the CuAlMn SMA, leading to the dramatic increase of the grains in unit volume, i.e. the grains are refined efficiently. In the subsequent heat treatment processes, the grains is difficult to grow due to the pinning effect of the LaB₆ particles on the grain boundaries of the CuAlMn SMA.2. The optimum quenching process of the refined CuAlMn SMA is: After betarising at 850 ℃ for 20 min the specimen is quenched in oil. After quenching theαand γ2 phases in the as-cast alloy completely disappear, and the matrix of the alloy is mainly composed of self-accommodated M18 R matensites. By researching the effect of aging on the microstructures of the refined CuAlMn SMA, the optimum aging process is determined: aging at 350 ℃ for 10 min. After aging treatment, the average martensite size can also be refined.3. The mechanical properties of the CuAlMn SMA increase with the decrease of the average grain size. The hardness can increase from the original 176.9 HV to 209.9 HV, and the tensile strength can increase from the original 401.45 MPa to 588.60 MPa. Furthermore, the wear resistance of the refined alloy also increases.4. The damping capacity of the CuAlMn SMA increases after adding 1 wt% Al LaB inoculant. However, the further increase of the adding amount of the inoculant can lead to the decrease of the damping capacity. When the adding amount reaches 2.5 wt% the average grain size of the alloy is the smallest, but a worst damping capacity is obtained. It is believed that although the grain refinement can increase the amount of phase boundaries in unit volume, the mobility of phase boundaries and internal twin boundaries decrease, which results in the decrease of the damping capacity.