Papers by Keyword: Phase Transformation

Abstract: The influence of molybdenum, and molybdenum with niobium addition on the phase transformation behaviour of a developed low-carbon CrNiMnB ultrahigh-strength steels, was investigated. Gleeble 3800 thermomechanical simulator was employed to simulate the hot-rolling process and to get the dilatation curves. After austenitization at 1250 °C for the complete dissolution of carbides, specimens received 0.6 total strain (i.e., 0.2 at 1100 °C and 2 x 0.2 at 900 °C) followed by cooling at various cooling rates (CRs) in the range of 2-60 °C/s. The final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, and hardness measurements. Then the continuous cooling transformation diagrams were constructed based on the dilatation curves, microstructure, and hardness values. The electrolytic extraction method was used to assess the elements' distribution and the composition of the forming precipitates. The addition of Mo increased the hardenability, decreased the transformation temperatures, and promoted the formation of low-temperature transformation products i.e., martensite and bainite ferrite, at different CRs and inhibit the formation of polygonal ferrite. The formation of coarse precipitates neglected the effect of Mo+Nb addition, decreased the hardenability and expanded the region of BF formation to high CRs. The variation in the hardness with microstructural changes was discussed.

Abstract: This paper presents the simulation of beef freezing process by using ANSYS software. On the basis of simulation results, factors affecting the freezing time including air velocity and freezing temperature were determined. Within the air velocity range ω = (5÷15) m.s^-1, an increase in the velocity by ω=1 m.s^-1 led to a decrease in the freezing time by τ =(6,5÷2,0) %. When the freezing temperature was reduced by t_e = 1 K, the freezing time was reduced by τ = (3,2÷2,5) % in the freezing temperature range t_e = (–35÷–45) °C.

Abstract: Shape memory alloys (SMAs) are a type of smart material and have excellent engineering and medical applications. TiNi binary alloys possess remarkable shape recovery, mechanical properties, corrosion resistance, and excellent biocompatibility. By ternary elements addition just like Au, Pt, Pd, Hf, and Zr, increases transformation temperatures, leading to high-temperature shape memory alloys (more than 100°C) but other elements (Fe, Cu, Co, and Mo) form low-temperature shape memory alloys, (lower than 100°C). In the present work, it is reported that the effect of ternary element addition on microstructural properties, shape memory properties, mechanical properties, corrosion resistance, and biocompatibility of ternary shape memory alloys. Ag, Au, and Cu-based TiNi ternary alloys have excellent biocompatibility. The addition of ternary elements such as Ag and Nb increases corrosion resistance, Fe rises the hysteresis loop, Hf enhances thermal stability, and Mo raises workability.

Abstract: Solid solutions of PZT ceramic at Zr/Ti ratio of 0.56/0.44 having various content of softener (La⁺³) and hardener (Sc⁺³) ions according to chemical formula (Pb_1-y La_y) (Sc_x Zr_0.56-x Ti_0.44) O₃, [y= (0.0 and 0.02); x= (0.02, 0.04 and 0.06)], were prepared via conventional solid state reaction methods. Structural and microstructural characteristics were investigated systematically. The measurements of XRD diffraction spectra showed occurring of phase transformation after doping with Sc⁺³ ions in both, PZT and PLZT ceramics, through emerging of tetragonal phase to coexist with the rhombohedral phase. Their fraction varied depending on content of Sc⁺³ ions. SEM mages display a different influence of softener and hardener ions, at 0.02 % mole, on the grain size. Doping with La⁺³ causes reduction in grain size, whereas Sc⁺³ increase the grain size. However, further increase in Sc⁺³ content in both (PZT and PZT) ceramics causes sharp decreasing in grain size. The latter feature is preferable in terms of increasing in the energy storage values.

Abstract: Steel alloys with high Mn and low C, low Cr wt.%, were designed based on the composition system for traditional high toughness, creep resistance, and longevity for high-temperature applications. In terms of energy resource utilization during production and refining, CALPHAD strategical optimization is preferable for all steel alloys. Thermo-Calc software calculates the phase diagrams α-BCC (Ferrite), and M₂₃C₆ (carbide) phases. The vital temperatures which are highlighted in this work are Ac₃ (threshold temperature at which ferrite is fully transformed into austenite (α→γ)), and A₄ (the threshold temperature at which austenite is fully transformed into Delta ferrite (γ→δ)) are essential for phase transformations. JMatPro software is used to predict the mechanical properties of steel alloys. The interfacial energies with regards to alloying elements for M₂₃C₆ are calculated to be between ~0.272 J/m^-2 to ~0.328 J/m^-2 for α-BCC) matrix, while γ-FCC has interfacial energy ranges to be between ~0.132 J/m^-2 to ~0.168 J/m^-2. This paper focuses on investigating the effect of alloying elements on phase transformations, interfacial energy, coarsening rate of carbides, and many other mechanical properties such as toughness at high-temperature applications using CALPHAD strategies.

Abstract: A chemically heterogeneous Fe-0.95C-1.8Si-1.5Mn-0.65Cr-0.34Mo-0.6Al-1.51Cowt.% steel was isothermally heat treated in the temperature range of 200-250 to produce nanostructured bainite. Observations using optical microscopy and field emission scanning electron microscopy revealed that the microstructures consisted of nanosized bainitic ferrite plates in a matrix of retained austenite, which was confirmed by X-ray diffraction analysis. The influence of chemical heterogeneity on the development of nanobainite structure was also examined. It was found that the ferritic constituent nucleates preferentially in the substitutional-solute-lean regions, leaving coarse blocks of untransformed austenite in the substitutional-solute-rich regions. A yield strength of 165511MPa and an ultimate tensile strength of 194121MPa were obtained after isothermal transformation at 250 for 16h while the ductility of the material was 40.8%. This low ductility was attributed to the instability of large regions of austenite retained untransformed in the substitutional-solute-rich regions. An unexpectedly high hardness of 800HV30 was obtained following transformation at 200 for 44h.

Abstract: Hot forging is a complex process involving the mutual influence of numerous thermo-mechanical-metallurgical material phenomena. In particular, the strains of transformation-induced plasticity (TRIP) have a significant influence on the distortions and residual stresses of the components. The TRIP strains refer to the anisotropic strains depending on the orientation and significance of the stress conditions during cooling superimposed to the phase transformation. With the use of numerical models, the impact of this effect can be investigated in order to ensure the production of high quality components. However, an experimental determination of the characteristic values of TRIP is challenging, which is why only few corresponding data are available in the literature. Therefore, this paper presents an experimental and numerical methodology as well as the results of studies on the interaction between stresses and phase transformations in the materials AISI 4140 and AISI 52100. The investigations of the TRIP strains are carried out using hollow specimens, which are thermo-mechanically treated in the physical forming simulator Gleeble 3800-GTC. The specimens are austenitised, quenched to test temperature and held there while diffusion controlled phase transformation takes place. The extent of TRIP as a result of different superimposed tensile or compressive loads is determined by means of dilatometry. In addition, the extent of TRIP for diffusionless martensitic phase transformations was investigated by continuous cooling tests under tensile and compressive loads. It was found that the transformation plasticity varies depending on the material, the phase type, the temperature and the tensile or compressive stresses. Subsequently, simulations of the physical experiments using the FE software Simufact.Forming verified the determined phase specific values of TRIP.

Abstract: The effect of Zn/Mg ratio on the as-cast microstructure and its evolution during homogenization of Al-Zn-Mg-Cu alloys was investigated by optical microscopy (OM), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and X-ray diffraction (XRD). Experimental results showed that serious dendritic segregation existed in the as-cast microstructures while the second phases were mainly AlZnMgCu phase and Al₂Cu phase. With the Zn/Mg ratio increasing from 1.5 to 2.0, the area fraction of AlZnMgCu phase decreased from 2.85% to 2.53%, which was attributed to the content of Mg element. Non-equilibrium eutectic phases dissolved into the matrix during homogenization and phase transformation from AlZnMgCu phase to Al₂CuMg phase (S phase) was observed in low-Zn/Mg ratio alloy and mid-Zn/Mg ratio alloy. In the high Zn/Mg ratio alloy, the eutectic AlZnMgCu phase directly dissolved into the matrix during the homogenization, and no transformation from AlZnMgCu phase to S phase was found. A higher number of S phases appeared in low-Zn/Mg ratio alloy during homogenization treatment compared with mid-Zn/Mg ratio alloy with a regime of 465°C/24h. It could be inferred that low-Zn/Mg ratio alloys had a stronger phase transformation tendency from AlZnMgCu phase to S phase. Increasing the homogenization treatment temperature could impair the transition tendency from AlZnMgCu phase to S phase.

Abstract: The diagrams of isothermal transformation based on kinetic curves R = (τ) for retained austenite in high-strength alloy low-carbon at overcooling have been built. It is shown that the temperature of quenching influences the stability at overcooling and resistance to isothermal transformation of austenite at sub-zero temperatures.

Abstract: Due to its to its optical, thermal, photocatalytic and electrophysical properties, nanocrystalline titanium oxide (TiO₂) is widely used in various fields. In the present work, a series of pure and Gd-modified (0.5, 5, 50 mol%) TiO₂ nanocrystalline powders were prepared by a novel synthesis approach – extraction-pyrolytic method (EPM). Metal containing extracts on the basis of valeric acid were used as precursors. Thermal behavior of produced individual and mixed precursors were investigated by thermogravimetric analysis (TGA) and high temperature differential scanning colometry (HDSC). Phase composition of pure and Gd-modified TiO₂ powders were studied as a function of pyrolysis temperature (450^o -850°C ) and gadolinium content by X-ray diffraction (XRD) method. Photocatalytic activity of produced powders was studied by photocatalytic degradation of methylene blue (MB) under UV/VIS light irradiation.