In-situ synthesis of high-quality pseudoelastic NiTi alloys with intrinsic Ni4Ti3 phase precipitation using laser DED

Microstructure and Mechanical Properties of NiTi Shape Memory Alloys by Laser Engineered Net Shaping

“…Because Ni 4 Ti 3 precipitates preferentially nucleate near grain boundaries or similar crystal defects, the Ni 4 Ti 3 precipitates will decrease due to the reduced number of grain boundaries. [31] The fracture morphologies after tensile tests were displayed in Figure 4a,c,e, and the surface of the fracture was obviously river pattern and tear-ridge pattern and accompanied by the dimples, which indicated the failure with a mixed fracture form of brittle fracture and ductile fracture. The fracture form has a certain influence on the elongation of NiTi alloys, and brittle fracture indicates that the strain of NiTi alloys is smaller and the elongation is lower.…”

“…[8,26,30] Meanwhile, the increase of laser power will reduce the cooling rate, leading to the increase of grain size and the decrease of grain boundary. [31] The laser power density is proportional to the laser printing power. The larger grain sizes tend to increase with increasing laser printing power.…”

“…During LENS fabrication, the laser melts the current powder layer when remelting the previously solidified powder layer, which results in NiTi remelting and cyclic heating/cooling process, where coherent metastable Ni 4 Ti 3 particles are formed. [ 31,32 ] The energy which is needed in the process of precipitation and growth generally comes from the reduction of chemical‐free energy, relaxation of elastic energy, reduction of interfacial energy, and external energy. Ke et al [ 33 ] study the generation mechanism of autocatalytic effect of Ni 4 Ti 3 precipitates by using the phase‐field model, and found that in the initial time of simulation, to alleviate the morphological mismatch between spherical Ni 4 Ti 3 precipitates and NiTi matrix phase, the chemical‐free energy of the system gradually increased, and the elastic energy and interface energy gradually decreased.…”

“…In addition, Zhang et al [ 31 ] also study the phase composition of NiTi using laser DED at different powers and they found that the increase in laser power makes the NiTi 2 phase and Ni 4 Ti 3 phase occur. In addition, increasing the laser power will reduce the cooling rate, which will lead to the increase of grain size.…”

“…Because Ni 4 Ti 3 precipitates preferentially nucleate near grain boundaries or similar crystal defects, the Ni 4 Ti 3 precipitates will decrease due to the reduced number of grain boundaries. [ 31 ]…”