Tseng Li-Wei, Chen Wei-Cheng
Department of Mechatronics Engineering, National Changhua University of Education, Changhua 50007, Taiwan.
Materials (Basel). 2025 Jun 16;18(12):2842. doi: 10.3390/ma18122842.
In this study, the effect of aging heat treatment on the superelastic properties and microstructure of [001]-oriented FeNiCoAlTiNb (at.%) single crystals was investigated using the cyclic superelastic strain test and a transmission electron microscope (TEM). The TEM results reveal that the average precipitate size is around 3-5 nm in the 600 °C/24 h samples, 6-8 nm in the 600 °C/48 h samples, and 10-12 nm in the 600 °C/72 h samples. The results indicate that precipitate size increases as aging time increases from 24 to 72 h. EDS analysis results show decreased Fe and increased Ni when the analyzed line crosses the precipitate region. The diffraction pattern results show that the precipitate has an L1 crystal structure. The thermo-magnetization curves of single crystals under the three aging conditions (600 °C/24 h, 600 °C/48 h, and 600 °C/72 h) show that the values of the transformation temperatures increased from 24 to 72 h. Magnetization was saturated at 140 emu/g under the magnetic field of 7 Tesla. When increasing the magnetic field from 0.05 to 7 Tesla, the transformation temperatures rose. The results indicate that magnetic fields can activate martensitic transformation. From the results of the superelasticity test at room temperature, [001]-oriented FeNiCoAlTiNb single crystals aged at 600 °C for 24, 48, and 72 h presented recoverable strains of 3%, 5.1%, and 2.6%, respectively. Digital image correlation (DIC) results of the aged samples show that two martensite variants were activated during the superelasticity test. The two variants form corresponding variant pairs (CVPs) and improve the recoverable strain of superelasticity. Although maximum recoverable strain was obtained for the 600 °C/48 h samples, the samples show poor cyclic stability at room temperature after applying the 6% strain. According to the DIC results, the retained martensite, which is pinned by dislocations, was observed after the test. The irrecoverable strain was attributed to the residual martensite. For the 600 °C/72 h samples, the large size of the precipitates poses an obstacle to dislocation transformation and formation. The dislocations increase the stress hysteresis width and stabilize the martensite, causing poor recoverability.
在本研究中，使用循环超弹性应变测试和透射电子显微镜（TEM）研究了时效热处理对[001]取向的FeNiCoAlTiNb（原子百分比）单晶的超弹性性能和微观结构的影响。TEM结果表明，600℃/24h样品中的析出相平均尺寸约为3-5nm，600℃/48h样品中的为6-8nm，600℃/72h样品中的为10-12nm。结果表明，随着时效时间从24小时增加到72小时，析出相尺寸增大。能谱分析结果表明，当分析线穿过析出相区域时，Fe含量降低而Ni含量增加。衍射图谱结果表明，析出相具有L1晶体结构。三种时效条件（600℃/24h、600℃/48h和600℃/72h）下单晶的热磁化曲线表明，转变温度值从24小时到72小时有所升高。在7特斯拉的磁场下，磁化强度在140emu/g时达到饱和。当磁场从0.05特斯拉增加到7特斯拉时，转变温度升高。结果表明，磁场可以激活马氏体转变。从室温下的超弹性测试结果来看，在600℃下时效24、48和72小时的[001]取向FeNiCoAlTiNb单晶的可恢复应变分别为3%、5.1%和2.6%。时效样品的数字图像相关（DIC）结果表明，在超弹性测试过程中激活了两个马氏体变体。这两个变体形成相应变体对（CVP）并提高了超弹性的可恢复应变。尽管600℃/48h样品获得了最大可恢复应变，但在施加6%应变后，这些样品在室温下表现出较差的循环稳定性。根据DIC结果，测试后观察到由位错钉扎的残余马氏体。不可恢复应变归因于残余马氏体。对于600℃/72h样品，析出相的大尺寸对位错转变和形成构成障碍。位错增加了应力滞后宽度并使马氏体稳定，导致可恢复性较差。
