Effects of aging on the stress-induced martensitic transformation and cyclic superelastic properties in Co-Ni-Ga shape memory alloy single crystals under compression

On the Impact of Additive Manufacturing Processes on the Microstructure and Magnetic Properties of Co–Ni–Ga Shape Memory Heusler Alloys

“…In literature, this nanometric secondary phase is reported to occur mainly in reannealed Co–Ni–Ga alloys at temperatures between 623 and 873 K. The γ’‐phase exhibits higher Co content than the surrounding β‐phase matrix and elongated or needle‐like shapes close to the equilibrium condition. [ 40,43,48,49 ] In present work, the formation of γ’‐phase can be most likely attributed to the elevated built temperature of 773 K during processing. Although the DED sample was processed using the same elevated built temperature, BSE images in Figure 3c,d do not reveal any needle‐like shaped precipitates.…”

“…The e/a ratio can be increased by substitution of Ga by Ni or Co by Ni or Ga by Co. [30,32,34,[36][37][38] T t and M sat are also affected by the formation of secondary phases (γ and γ'-phase), as their formation changes the chemical composition in the surrounding matrix (β-phase). [17,27,[39][40][41] The formation of γ'-phase is observed after annealing between 623 and 873 K, [39,42,43] while the formation of γ-phase is observed after annealing at higher temperatures between 973 and 1473 K. [27,40,44] In the present work, microstructure and magnetic properties of Co-Ni-Ga alloys processed by L-PBF and DED are compared with single-crystalline and polycrystalline cast material to determine the influence of the processing method and microstructure on the FOMST. For this, detailed microstructure analysis was carried out using high-resolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (TEM).…”

“…The e/a ratio can be increased by substitution of Ga by Ni or Co by Ni or Ga by Co. [ 30,32,34,36–38 ] T t and M sat are also affected by the formation of secondary phases (γ and γ’‐phase), as their formation changes the chemical composition in the surrounding matrix (β‐phase). [ 17,27,39–41 ] The formation of γ’‐phase is observed after annealing between 623 and 873 K, [ 39,42,43 ] while the formation of γ‐phase is observed after annealing at higher temperatures between 973 and 1473 K. [ 27,40,44 ]…”

Cyclic Superelastic Behavior of Iron-Based Fe-Ni-Co-Al-Ti-Nb Shape Memory Alloy

“…The degree in the cyclic functional degradation behavior, however, differs from temperature to temperature, i.e., increases with increasing test temperature (s. details below). Numerous studies over the last decades have proven that this cyclic evolution of the SE transformation characteristics is attributed to dislocation activities in the austenitic matrix (Ref [18][19][20]24). In order to accommodate the mismatch between the austenitic and martensitic crystal structure, dislocation arrangements are known to be formed at the interphase boundaries during stress-induced MT (Ref 25).…”

“…In general, cyclic deformation can cause a significant change of the SE stress-strain hysteresis, which can result in a complete loss of the SE material response. This phenomenon is attributed to microstructural mechanisms rationalized by dislocation-slip, i.e., the introduction and accumulation of dislocations during the repeated phase transformation ( Ref 1,[18][19][20]. However, in Fe-Ni-Co-Al-Ti-Nb SMAs cyclic functional properties have not been addressed so far.…”