Characterization of Precipitates in an CuAlNi Shape-Memory Alloy After Aging Thermal Treatments

MOSQUERA PANIZO, M.; DE CASTRO BUBANI, F; GASTIEN, R.; ZELAYA, E.

Rio de Janeiro

Congreso; 28o Congresso Brasileiro de Microscopia e Microanálise da SBMM, 12 a 15 de julho de 2021, online; 2021

UFRJ, CSBMM

Shapememory alloys are considered smart materials because of theirparticular thermo-mechanical properties, which are related to a firstorder martensitic transformation (MT) between a parent β phase andone or more martensitic phases [1].Particularly, shape-memory CuAlNi alloys have a high temperaturedisordered body-centered cubic phase. After quenching, which consistsin annealing at 1203K for 1h followed by quenching in water mixedwith ice, an ordered β phase can be obtained and kept in ametastable state at room temperature. This β phase can undergomartensitic transformation either by cooling or by applyingmechanical stress. It is possible to modify the thermo-mechanicalbehavior and microstructure of these alloys with isothermaltreatments, which are called ageing treatments, at suitabletemperatures [2,3]. Transmission electron microscopy images are ableto reveal microstructural defects after these aging thermaltreatments, such as precipitation of stable phases (γ, α) anddislocations in the β phase matrix. The aim of the present work isto compare the microstructural evolution of two samples obtained fromthe same monocrystal, submitted to different ageing treatmentprocedures at 473K. Both samples were aged for 7.5h. In one of them,the ageing treatment was performed in 8 half-hour cycles, and theother in one single cycle of 7.5h. The microstructural evolution wasanalyzed by various transmission electron microscopy techniques. αand γ precipitates were observed in both samples and precipitatesizes were compared. The presence of α and γ precipitates in themetastable β matrix was corroborated by to the indexation ofselected area diffraction patterns consistent with the γ and αstructure. Also, γ precipitate size histograms of samples withdifferent heat treatments are compared. Results show that precipitatesize distribution is similar in the conditions studied. Scanningtransmission electron microscopy (STEM) images obtained with a shortcamera length show lighter contrast for the alpha precipitates, whichhave a characteristic cross shape. Moreover, an energy dispersiveX-ray spectroscopy (EDS) line scan shows an increase in Cu in thealpha precipitate. An annular dark field image reveals the presenceof the order domains around the alpha precipitates and the presenceof dislocations. The observation of this kind of dislocations areattributed to a relaxation of the system due to the presence of alphaphase precipitates in beta matrix.p { margin-bottom: 0.1in; direction: ltr; line-height: 115%; text-align: left; orphans: 2; widows: 2; background: transparent }