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58Ni39Ti-3Hf alloy system has been proposed as an alternative for superelastic 60NiTi used in load-bearing applications. This research was focused on the development of heat treatment procedures leading to 58Ni39Ti-3Hf parts with maximum possible hardness values. Meanwhile, the relations existing between the heat treatment – microstructure – hardness of 58Ni39Ti-3Hf as compared to baseline 60NiTi were also investigated. These were done by applying the solution and consequent aging treatments on the studied alloy systems at different temperatures and for different times. The microstructure and hardness of the treated samples were, in the next step, analyzed by applying electron probe micro-analyzer (EPMA), energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and macro hardness testing techniques. It was understood that applying solution treatments at temperatures above about 1000 °C resulted in the relatively full dissolution of Ni-rich phases such as Ni 3 Ti and Ni 3 Ti 2 , existing in the microstructure of as-received parts, in the NiTi matrix of 60NiTi. On the other hand, for 58Ni39Ti-3Hf, temperatures above about 900 °C appeared to be adequate to solve these Ni-rich phases in the NiTi matrix. Applying aging treatments had either no significant effect on the hardness of solution treated 60NiTi samples (~58 HRC) or reduced their values; after coarsening of Ni 4 Ti 3 particles formed in NiTi matrix and their transformation to Ni 3 Ti and Ni 3 Ti 2 phases. On the other hand, some of the applied aging treatments could increase the hardness values of the solution treated 58Ni39Ti-3Hf alloys (~54 HRC). The maximum value of hardness (~59 HRC) for the aged 58Ni39Ti-3Hf parts was obtained for the ones treated at 500 °C for just 0.25 h. This level of obtained hardness is comparable to that of solution treated 60NiTi. A delayed precipitation of Ni 3 Ti and/or Ni 3 Ti 2 phases was noted in aged 58Ni39Ti-3Hf alloy system as compared to 60NiTi. • The difference amongst the solvus temperature of 58Ni39Ti-3Hf as compared to 60NiTi was understood. • The peak hardness for 58Ni39Ti-3Hf (~59 HRC) was obtained by aging the solution treated samples at 500 °C for 0.25 h. • Relations existing between the heat treatment-microstructure-hardness of 58Ni39Ti-3Hf as compared to 60NiTi were defined. [ABSTRACT FROM AUTHOR]

Equiatomic and near-equiatomic NiTinol alloys, mainly due to the exhibition of adjustable mechanical and thermal shape memory properties, are widely exploited to produce different smart and functional devices. Laser powder bed fusion technique was used in this research to process dense NiTinol parts from a prealloyed near-equiatomic NiTi powder, using three different volumetric energy densities. In the next step, samples, processed using a specific set of printing parameters, were annealed using two different procedures. Effects of different used processing volumetric energy densities and post-processing heat treatments on the microstructural properties, phase composition, crystallographic information, phase transformation behavior and hardness of the processed samples were investigated. As-printed Parts with Ni contents ranging from 55.12 to 55.27 wt%, density higher than at least 96% of theoretical and a similar phase structure as that of used powder, dominated by austenitic NiTi, were obtained. Applying a hot isostatic processing treatment at 1100 °C for 4 h or annealing the samples in a pressureless sintering furnace, working under a vacuum environment at the same temperature and holding time, had significant effects on the properties of the printed parts. These treatments, in addition to increasing the density of the printed samples, their average grain size and reducing the residual stresses, shifted the phase transformations to lower temperatures. These results suggest that post-printing heat treatments affect and, as similar to NiTinol alloys having Ni contents above 55.7 wt%, can be used as a strategy to alter the phase transformation temperatures of printed less Ni-rich near-equiatomic NiTinol alloys and consequently obtain NiTinol parts suitable for different applications. • Post-processing heat treatments, other than affecting the microstructural characteristics, shifted down the phase transformation temperatures of as-printed parts having Ni-contents less than 55.7 wt% Ni. • These consequently changed the hardness values of treated parts as compared to the as-printed ones. • These results suggest that, the same as that for the Ni-rich NiTinol alloys having Ni-contents above 55.7 wt% Ni, applying post-processing heat treatments can be used as a strategy to alter the phase transformation temperature of less Ni-rich NiTinol parts and subsequently change the obtained properties. [ABSTRACT FROM AUTHOR]

The mechanical properties of high-entropy alloys (HEAs) strongly depend on their microstructural features. In this work, the phase formation and morphologies of Fe 26.7 Co 26.7 Ni 26.7 Si 8.9 B 11.0 HEAs fabricated under different undercooling levels by the drop tube technique together with subsequent quenching were investigated in detail. It was found when the droplet size decreases (i.e. the increase of the undercooling level), the grain refinement is easily achieved without any phase transformation for the present hypoeutectic HEA which is composed of the fcc dendrites and eutectic structure. After quenching under large undercooling levels, the liquid should be deeply undercooled into an asymmetric couple zone, leading to the primary formation of eutectic structure and the subsequent precipitation of the fcc dendrites. The direct product of rapid solidification after deep undercooling and subsequent recalescence may induce the decoupled eutectic growth, resulting in a transition from the regular into anomalous eutectic structures. Furthermore, the volume fractions of the eutectic structures and the fcc dendrites increase and decrease with decreasing droplet size, respectively. As a result, the Vickers hardness gradually increases with decreasing droplet size, which exhibits a linear relationship with the volume fraction of eutectic matrix. The present study may provide a simple way to tailor microstructures and mechanical properties of HEAs. • High-entropy droplet were fabricated by the drop tube technique. • Grain refinement is easily achieved during containerless solidification. • Deep undercooling induces a transition from regular into anomalous eutectic matrix. • Mechanical properties can be tailored by deeply undercooled solidification. [ABSTRACT FROM AUTHOR]