Impact of Alloy Composition and Thermal Stabilization on Martensitic Phase Transformation Structures in CuAlMn Shape Memory Alloys

Alloys of CuAlMn are known as cheap, high strength shape memory alloys with an excellent damping capacity within their austenitic-martensitic phase transformation, compared to alloy systems like NiTi, CuZnAl or MnCu. But CuAlMn alloys have disadvantage due to generation of voids by a high shrinkage which further increases the existing proneness to stress cracks during rapid cooling. Alloying grain refining elements improves the stress crack resistance and enables a wide range of rapid quenching parameters which are needed to control the temperature of martensitic phase transformation. Additionally, the elements itself influence the in- or decreasing of the phase transformation temperature and the SMA effects. Furthermore, some of these elements can reduce the internal friction indirectly by decomposing areas of metastable martensite into its stabilized forms, where no transformation occurs. This thermic stability can be calculated by the concentration of valence electrons in a unit cell. The proneness to ageing is controlled by multistep heat treatments. Annealing and rapid quenching into the area of martensitic phase transformation maximize the generation of point defects. A high amount of point defects contradicts the negative effect of pinning. It also preserves the material from extreme brittleness. The influences of these effects are shown at single cantilever bending beams by elastic strain amplitude (ε = 12E-4) depending measurements of internal friction at natural frequency along the ageing at room temperature (293 K) up to 2500 h. The samples are annealed at 1123 K for 15 min (CuAl14Mn2) and 1100K for 30min (CuAl11Mn5) afterwards rapid quenched to 370 K with no further thermic stabilisation. The base alloy of CuAl14.1Mn2.0Ni1.9Fe0.4 had an internal friction measured as logarithmic Decrement (δ) of 0.155 and 0.11 after 2500 h of ageing at RT. The phase transformation is located between 284 K and 352 K, measured by DSC. The alloy of CuAl11.1Mn5.5Zn2.9Ni2.1 had a logarithmic decrement of 0.31 and diminish continuously to 0.12 after 2500 h of ageing at RT. The phase transformation is located between 287 K and 318 K.

CuAlMn合金是一类低成本、高强度的形状记忆合金（shape memory alloy, SMA），相较于NiTi、CuZnAl或MnCu等合金体系，其在奥氏体-马氏体相变区间内具备优异的阻尼性能。但CuAlMn合金存在显著缺陷：快速冷却过程中，高收缩率会产生孔洞，进一步加剧了材料的应力裂纹倾向。添加晶粒细化元素可提升抗应力裂纹性能，并拓宽可控马氏体相变温度所需的快速淬火工艺参数范围。此外，此类元素本身会对相变温度及形状记忆合金效应产生增、减调控作用。部分此类元素还可通过将亚稳马氏体区域分解为稳定形态（无相变发生），间接降低内摩擦。该热稳定性可通过晶胞内的价电子浓度进行计算。材料的时效倾向可通过多级热处理进行调控。将试样退火后快速淬火至马氏体相变区间，可最大化点缺陷的生成量。高浓度点缺陷可抵消钉扎效应的负面影响，同时避免材料发生过度脆化。上述各效应的影响可通过以下方式展示：采用弹性应变幅值为12×10^-4的单悬臂梁试样，在室温（293 K）下开展长达2500 h的时效过程中，对固有频率下的内摩擦进行测试。试样分别为CuAl14Mn2（1123 K退火15 min）与CuAl11Mn5（1100 K退火30 min），随后快速淬火至370 K，未进行后续热稳定化处理。基础合金CuAl14.1Mn2.0Ni1.9Fe0.4的对数衰减率（δ）经室温时效2500 h后分别为0.155与0.11；其相变区间为284 K至352 K，经差示扫描量热法（Differential Scanning Calorimetry, DSC）测试测得。另一合金CuAl11.1Mn5.5Zn2.9Ni2.1的初始对数衰减率为0.31，经室温时效2500 h后持续衰减至0.12，其相变区间为287 K至318 K。