Kadri Can Atli

Mechanical properties of refractory high entropy alloys (RHEAs) at ultra-high temperatures (>1,100°C) are reviewed. Deformation behavior and strengthening mechanisms of select compositions are discussed. The limited number of studies portray remarkable mechanical properties of newly developed RHEA compositions at temperatures beyond the melting point of commercial Ni-based superalloys. Yet, the lack of quasi-static tensile deformation data and application relevant creep deformation data indicates RHEAs are still far from being reliable alternatives to Ni-based superalloys as high temperature structural materials. Future studies should concentrate on tensile deformation and creep of these new alloys systems at very high temperatures.

Laser powder bed fusion (L-PBF) additive manufacturing (AM) is an effective method of fabricating nickel–titanium (NiTi) shape memory alloys (SMAs) with complex geometries, unique functional properties, and tailored material compositions. However, with the increase of Ni content in NiTi powder feedstock, the ability to produce high-quality parts is notably reduced due to the emergence of macroscopic defects such as warpage, elevated edge/corner, delamination, and excessive surface roughness. This study explores the printability of a nickel-rich NiTi powder, where printability refers to the ability to fabricate macro-defect-free parts. Specifically, single track experiments were first conducted to select key processing parameter settings for cubic specimen fabrication. Machine learning classification techniques were implemented to predict the printable space. The reliability of the predicted printable space was verified by further cubic specimens fabrication, and the relationship between processing parameters and potential macro-defect modes was investigated. Results indicated that laser power was critical to the printability of high Ni content NiTi powder. In the low laser power setting (P < 100 W), the printable space was relatively wider with delamination as the main macro-defect mode. In the sub-high laser power condition (100 W ≤ P ≤ 200 W), the printable space was narrowed to a low hatch spacing region with macro-defects of warpage, elevated edge/corner, and delamination happened at different scanning speeds and hatch spacing combinations. The rough surface defect emerged when further increasing the laser power (P > 200 W), leading to a further narrowed printable space.

Shape memory alloys (SMAs) have been utilized as an alternative to conventional materials for seismic retrofit applications in the last two decades. The effectiveness of SMAs to provide further improvement in seismic behavior of structures was demonstrated satisfactorily. Three 2/3-scaled, one-bay and one-storey substandard RC frames were constructed to represent the seismically vulnerable buildings with several deficiencies. The first RC frame was tested as a reference specimen under quasi-static reversed cyclic loading, which caused flexural plastic hinges at the column ends. Then, the rest of the two RC frames were upgraded with superelastic copper–aluminum–manganese (CuAlMn) alloy bars and conventional steel bars with the aim of enhancing seismic performance of substandard RC frames. The upgrading materials were attached to the RC frames through a retrofitting mechanism to provide tension-only retrofitting bars. Hence, the RC frame retrofitted by conventional steel bars was exposed to residual displacements after they yielded. The SMA-upgraded frame showed flag shaped hysteresis curve due to its superelastic behavior and caused considerable reduction in the residual displacement of RC frames.

The work output capacity of the two-way shape memory effect (TWSME) in a Ni _50.3 Ti _29.7 Hf ₂₀ (at%) high-temperature shape memory alloy (HTSMA) was investigated and compared to that of binary Ni _49.9 Ti _50.1 (at%). TWSME was induced through a training procedure of 100 thermomechanical cycles under different tensile stresses. It was observed that TWSME in as-extruded and trained Ni _50.3 Ti _29.7 Hf ₂₀ could produce 0.7% strain against a compressive stress of 100 MPa, corresponding to a maximum work output of 0.08 J g ⁻¹ , compared to a maximum value of 0.06 J g ⁻¹ for binary NiTi. A peak aging heat treatment of 3 h at 550 °C, which previously has been shown to result in near-perfect functional stability in Ni _50.3 Ti _29.7 Hf ₂₀ during isobaric thermal cycling, did not improve the TWSME and actually resulted in a decrease in the magnitude and stability of the TWSME and its work output capacity. Nevertheless, the magnitude of TWSM behavior of Ni _50.3 Ti _29.7 Hf ₂₀ , in the absence of an aging heat treatment, renders it an attractive candidate for high-temperature TWSM actuation.

Shape memory alloy (SMA) pipe couplers use the shape memory effect to apply a contact pressure onto the surface of the pipes to be coupled. In the current research, an SMA pipe coupler is designed, fabricated and tested. The thermally induced contact pressure depends on several factors such as the dimensions and properties of the coupler-pipe system. An in-house developed NiTi alloy system is considered for the coupler. The coupling pressure is measured using strain gages mounted on the internal surface of an elastic steel ring. Thermal actuation response of the coupler is determined under both stressed and stress-free conditions. In addition, the state of anisotropy is investigated in the coupler by characterizing samples in the longitudinal and transverse directions. Unlike the commercially available NiTiNb couplers, the NiTi coupler demonstrates a multiple actuation response and higher contact pressure. The state of anisotropy is investigated in the couplers by characterizing samples in the longitudinal and transverse directions. The results show no change in transformation temperatures with respect to two perpendicular transverse and longitudinal directions, however, the material can undergo higher transformation strains in the longitudinal direction.