The present work presents a theoretical and experimental study regarding the microstructure, phase transformations and mechanical properties of advanced high-strength steels (AHSS) of third generation produced by thermal cycles similar than those used in a continuous annealing and galvanizing (CAG) process. The evolution of microstructure and phase transformations were discussed from the behavior of intercritical continuous cooling transformation diagrams calculated with the software JMatPro, and further characterization of the steel by scanning electron microscopy, optical microscopy and dilatometry. Mechanical properties were estimated with a mathematical model obtained as a function of the alloying elements concentrations by multiple linear regression, and then compared to the experimental mechanical properties determined by uniaxial tensile tests. It was found that AHSS of third generation can be obtained by thermal cycles simulating CAG lines through modifications in chemistry of a commercial AISI-1015 steel, having an ultimate tensile strength of UTS = 1020–1080 MPa and an elongation to fracture of Ef = 21.5–25.3%, and microstructures consisting of a mixture of ferrite phase, bainite microconstituent and retained austenite/martensite islands. The determination coefficient obtained by multiple linear regression for UTS and Ef was R2 = 0.94 and R2 = 0.84, respectively. In addition, the percentage error for UTS and Ef was 2.45–7.87% and 1.18–16.27%, respectively. Therefore, the proposed model can be used with a good approximation for the prediction of mechanical properties of low-alloyed AHSS.
First-order phase transitions (FOPT) and second-order phase transitions (SOPT) are commonly observed in Cu alloys containing lanthanide elements, due to their electronic configuration, and have an important effect on the optimization of their magnetocaloric effect (MCE). Alloys containing rare earths have the best magnetocaloric response; however, these elements are very expensive, due to their obtaining and processing methods. The present work reports the effect of using 3d transition elements and thermal treatments on the microstructure and MCE of Cu-11Al-9Zn alloys with 6.5 wt % Ni and 2.5 wt % Fe. It was found that thermal treatments of quenching and normalizing, as well as the use of Ni and Fe, have an important influence on both the resulting phases and MCE of the investigated alloy. MCE was calculated indirectly from the change in the magnetic entropy (-∆S m ) under isothermal conditions, using Maxwell´s relation; it was found that samples subjected to normalizing presented a higher magnetocaloric effect than samples with quenching, which was related to the greater disorder in the alloy, due to the coexistence of β 1 + β phases. Magnetochemistry 2019, 5, 48 2 of 12The entropy of such a system can be considered as the sum of two contributions: the entropy related to magnetic ordering and the entropy related to the temperature of the system. Application of a magnetic field will order the magnetic moments comprising the system, which are disordered by the thermal agitation energy, and consequently, the entropy depending on the magnetic ordering (the magnetic entropy, S m ) will be lowered. If a magnetic field is applied under adiabatic conditions, when any heat exchange with the surroundings is absent, then the entropy related to the temperature should increase in order to maintain constant the total entropy of the system constant. Increasing this entropy implies that the system heats up and increases its temperature [5].Unlike conventional refrigeration, magnetic refrigeration (MR) does not use gases that can contribute to the global heating and the deterioration of the ozone layer. However, so far only a few prototype refrigeration machines have been presented worldwide, and there are still many scientific and technological challenges to overcome [6,7]. While in conventional refrigeration a fluid undergoes compression/evaporation processes, causing a change in the temperature of the system, MR is based on a magnetization/demagnetization process. Therefore, the efficiency of the equipment used in MR depends strongly on the magnetic behavior of the solid refrigerant [8,9]. MCE can be obtained by promoting first-order transitions (for example, the martensitic transformation) due to a high crystal lattice disorder, which results in an increase of the total system entropy that leads to better cooling capacity [9]. When the material exhibits a ferromagnetic-paramagnetic transition (second-order transition) during service, the refrigeration capacity can be also improved, since the disorder of the system increases ...
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