Control of Superelastic Behavior of NiTi Wires Aided by Thermomechanical Treatment with Reference to Three-Point Bending

The aim of this study is to investigate the effect of thermomechanical treatment on the superelastic behavior of a Ti-50.5 at.%Ni wire in terms of loading/unloading plateau, mechanical hysteresis, and permanent set to optimize these parameters for orthodontic applications. A new three-point bending fixture, oral cavity configuration three-point bending (OCTPB) test, was utilized to determine the superelastic property in clinical condition, and therefore, the tests were carried out at 37 °C. The results indicate that the thermomechanical treatment is crucial for thermal transformation and mechanically induced transformation characteristics of the wire. Annealing of thermomechanically treated specimens at 300 and 400 °C for 1/2 and 1 h leads to good superelasticity for orthodontic applications. However, the best superelasticity at body temperature is obtained after annealing at 300 °C for 1/2 h with regard to low and constant unloading force and minimum permanent set.     

Microstructural evolution and mechanical properties of CoCrFeNiMnTix high-entropy alloys

This study was conducted to investigate the effect of titanium addition on the microstructure and properties of an equitaomic CoCrFeNiMn high-entropy alloy. Homogenized microstructures of CoCrFeNiMnTi_x (x = 0.1 and 0.3) alloys consist of face-centered cubic phase; however, addition of more titanium led to formation of a (chromium, titanium)-rich σ phase in CoCrFeNiMnTi_0.4 alloy. The average electron hole number calculations indicate the higher possibility of σ phase formation by adding more titanium. Furthermore, addition of an atom like titanium with a larger atomic radius in comparison with other elements can affect stability of face-centered cubic structure. Chromium and manganese has a destabilizing influence on the single face-centered cubic phase and manganese may reject chromium to facilitate the formation of a (chromium, titanium)-rich phase in alloys containing more than 5.5 at.% titanium (x>0.3). The mechanical properties revealed an improvement in strength without losing the ductility drastically by adding titanium up to 5.5 at.% (x = 0.3). Nevertheless, the strength remarkably increased and ductility significantly decreased in CoCrFeNiMnTi_0.4 alloy due to formation of brittle σ phase in the microstructure.     

On the Stability of Reversely Formed Austenite and Related Mechanism of Transformation in an Fe-Ni-Mn Martensitic Steel Aided by Electron Backscattering Diffraction and Atom Probe Tomography

The stability of reversely formed austenite and related mechanism of transformation were investigated against temperature and time in an Fe-9.6Ni-7.1Mn (at. pct) martensitic steel during intercritical annealing at a dual-phase (α + γ) region. Dilatometry, electron backscattering diffraction (EBSD), atom probe tomography (APT), and X-ray diffraction (XRD) were used to characterize the mechanism of reverse transformation. It was found that under intercritical annealing at 853 K (580 °C), when the heating rate is 20 K/s (20 °C/s), reverse transformation takes place through a mixed diffusion control mechanism, i.e., controlled by bulk diffusion and diffusion along the interface, where Ni controls the diffusion as its diffusivity is lower than that of Mn in the martensite and austenite. Increasing the intercritical annealing to 873 K (600 °C) at an identical heating rate of 20 K/s (20 °C/s) showed that reverse transformation occurs through a sequential combination of both martensitic and diffusional mechanisms. The transition temperature from diffusional to martensitic transformation was obtained close to 858 K (585 °C). Experimental results revealed that the austenite formed by the diffusional mechanism at 853 K (580 °C) mainly remains untransformed after cooling to ambient temperature due to the enrichment with Ni and Mn. It was also found that the stability of the reversely formed austenite by martensitic mechanism at 873 K (600 °C) is related to grain refinement.     

Experimental Study of Parameters Affecting the Nusselt Number of Generator Rotor and Stator

In this research, the parameters affecting the Nusselt number of a generator rotor and stator under varying heat transfer rate are experimentally studied. In spite of the stator having no grooves, the rotor has four large triangular grooves. The temperature and then heat transfer rate of the rotor and stator are experimentally measured in three longitudinal and two angular positions. First, the effect of axial Reynolds number and rotor rotational speed on the rotor and stator Nusselt number with constant heat transfer rate ratio is studied. The range of the axial Reynolds number and rotational speed used is from 4000 to 30,000 and from 300 to 1500 rpm, respectively. Next, the effect of stator to rotor heat transfer rate ratio on the Nusselt number at constant axial Reynolds number and rotational speed is investigated. Three experiments were conducted at three heat transfer rate ratios (3, 5, and 8), defined as the ratio of heat transfer rate of the stator to the rotor. The results show that the higher the heat transfer rate ratio, the lower is the stator mean Nusselt number and the higher the rotor mean Nusselt number.     

An experimental study on the effect of a novel nature-inspired 3D-serrated leading edge on the aerodynamic performance of a double delta wing in the transitional flow regime

Journal of Mechanical Science and Technology - {tiThis study examines the effect of nature-inspired leading edge 3D serration on the aerodynamic performance of a 65/35-degree double delta wing....     

Nano- and microvoid formation in ultrafine-grained martensitic Fe-Ni-Mn steel after severe cold rolling

Severe cold-rolling was applied on solution annealed Fe-Ni-Mn steel with fully lath martensite structure to obtain ultrafine-grained structure. Field emission scanning electron microscopy and high resolution transmission electron microscopy (HRTEM) were employed to investigate the microstructural evolution after severe cold-rolling. HRTEM images showed the typical deformed structure consisting of lamellar dislocation cell blocks. HRTEM study also revealed strain-induced reverse martensitic transformation (activated during grain refinement). It was assumed that severe plastic deformation route and related deformation mode were responsible for microstructural evolutions. X-ray diffraction (XRD) diagram revealed 7% (volume fraction) reverted austenite after final deformation pass. Moreover, HRTEM images revealed nano-void nucleation at the interface of severely deformed martensite and reverted austenite presumably due to high strain energy of misfit and molar volume difference between the austenite and the martensite. It seems that the coalescence of nano-voids could lead to the formation of microvoids in the microstructure.     

Control of austenite to martensite transformation through equal channel angular pressing aided by thermodynamic calculations

The martensite start temperature and driving force were calculated for the austenite to martensite transformation of a dual-phase bainitic ferrite–austenite steel at room temperature. The mechanical energy was estimated during the first and the second pass of equal channel angular pressing (ECAP). The applied mechanical energy during the first pass is not enough to induce the austenite to martensite transformation. However, the mechanical energy during the second pass can provide the required energy for the martensite formation. Microstructural observations by transmission electron microscopy and scanning electron microscopy of as-received and ECAPed samples confirm the formation of martensite during the second pass.     

Centrifugal compressor shape modification using a proposed inverse design method

This paper is concerned with a quasi-3D design method for the radial and axial diffusers of a centrifugal compressor on the meridional plane. The method integrates a novel inverse design algorithm, called ball-spine algorithm (BSA), and a quasi-3D analysis code. The Euler equation is solved on the meridional plane for a numerical domain, of which unknown boundaries (hub and shroud) are iteratively modified under the BSA until a prescribed pressure distribution is reached. In BSA, unknown walls are composed of a set of virtual balls that move freely along specified directions called spines. The difference between target and current pressure distributions causes the flexible boundary to deform at each modification step. In validating the quasi-3D analysis code, a full 3D Navier-Stokes code is used to analyze the existing and designed compressors numerically. Comparison of the quasi-3D analysis results with full 3D analysis results shows viable agreement. The 3D numerical analysis of the current compressor shows a huge total pressure loss on the 90° bend between the radial and axial diffusers. Geometric modification of the meridional plane causes the efficiency to improve by about 10%.     

Microstructure evolution through heavy compression aided by thermodynamic calculations

The induced martensite transformation in a dual-phase bainitic ferrite–austenite steel during heavy compression was studied by thermodynamic computations. Compression tests were conducted at temperatures of 298 and 573 K on the rectangle samples at the strain rate of 0.001 s⁻¹. The samples were deformed to 40 and 70% of their original thickness. It was found that 70% compression of the steel at room temperature resulted in transformation of retained austenite to martensite, which is in agreement with thermodynamic calculations. Additionally, heavy compression resulted in the formation of fine grains with high angle grain boundaries which confirms grain refinement.