Advanced Materials Research Vol. 893

Abstract: In this work, two approaches for determining critical stress and strain for initiation of dynamic recrystallization (DRX) of the AISI 4340 steel were presented.The first one applied a polynomial function to represent relationship between work hardening rate and flow stress. Secondly, Cingara constitutive model were employed. To investigate hot deformation behavior of the steel, compression tests were performed at different temperatures between 850 °C and 1150 °C and strain rates between 0.01s^-1and 10 s^-1. Obviously, both methods provided different values of critical stress and critical strain.Accuracy of the first method depended on fluctuations of the fitted strain hardening curve. On the other hand, results of the Cingara model was primarily related to the described flow curves up to their peak points. It could be noticed that the DRX occured during hot deformation of the examined steel started when the normalized critical stress and strain reached the values of 0.735 and 0.324, respectively.

Abstract: AZ31 magnesium alloy samples were compressed to different strains at room temperature and examined through the optical microscope, X-ray diffraction (XRD) and scanning electronic microscope. The results show that the produced twins were mainly the {102} type, and then the {101} type and {102}-{101} type. The size and amount of the twins increased with the strains growth, and after the saturation of twins in the grains the samples fractured. The compressed texture with the basal planes perpendicular to the compression direction also become stronger with strain increase. The {102} twinning deformation played an important role in changing the microstructure and properties of the magnesium alloy at room temperature.

Abstract: The electrochemical noise (EN) of N80 steel in NaCl solution has been studied by the wavelet and the equivalent circuit theory. Spectrum analysis showed that the corrosion data is no transient peaks, the variations of potential and current showed typical characteristics of uniform corrosion. Wavelet analysis results show that the different time energy fluctuations of different size ranges, low-level occur fluctuations repeatedly and high-level appear concave curve. Analysis shows that it is mainly due to uniform corrosion process, and the control step is diffusion process of corrosive ion. The applications of equivalent circuit analyze electrochemical noise data in theory, and describe the corrosion process by the components of potential noise and current noise power spectral density

Abstract: Austenitic stainless steels especially 316L has been used extensively in many sectors including construction, medical and household appliances due to their highly resistance to corrosion attack, reasonable cost and excel in mechanical properties. However, in corrosive media, 316L are susceptible to localised corrosion attack especially in seawater and high temperature. The corrosiveness of media increased as the anions contents increased. This paper presents the corrosion mechanism of 316L exposed to high concentration of sulphate in the salinity of seawater. The solution (media) was prepared according to the same composition as seawater including pH, salinity and dissolved oxygen. The corrosion mechanism were characterized to breakdown potential (E_b) of 316L which are the potential once reaches a sufficiently positive value and also known as pitting potential. This is the most point where localized corrosion susceptibility to evaluate and considered a potential, which could be an appropriate point according to any given combination of material/ambient/testing methods. The E_b value were identified at 4°C, 20°C, 50°C and 80°C and compared with E_b value of 316L in seawater. The E_b value of 316L in high sulphate are higher compared to seawater in every temperature which elucidate that some anions accelerate corrosion attack whereas some anions such as sulphate behaves as inhibiting effect to 316L.

Abstract: Mechanical properties of Al-12Zn-2.4Mg-1.2Cu alloy extruded sheet were investigated by tensile tests. Microstructures were investigated by optical microscopy (OM) and scanning electron microscope (SEM). The result shows that no matter in the L direction or in the T direction, the tensile strength and yield strength decrease with the increasing of the extrusion temperature in different states. The tensile strength and yield strength in the L direction are higher than in the T direction at different extrusion temperatures and different treatment states. When temperature is 340°C, the highest tensile strength is 780 MPa and the highest yield strength is 753 MPa in the two-stage solution and two-stage aging state. The reason for the higher mechanical properties are in the L direction in different states is mainly depend on the distribution direction of the grains.

Abstract: The effect of tempering temperature on the microstructures and mechanical properties of a microalloy low carbon bainitic steel was investigated by microscopic analysis and testing of mechanical properties. The results show that the microstructures of the tested steel primarily consists of lath bainite, granular bainite, quasipolygonal ferrite and little acicular ferrite at different tempering temperatures. With the tempering temperature increasing, the proportion of lath bainitie decreases, while the volume of granular bainite and quasipolygonal ferrite increases. At the tempering temperatures of 550-650°C and tempering time of 1 hour, the steel was mostly composed of granular bainite, quasipolygonal ferrite and a little lath bainite, which a good combination of strength and toughness can be obtained.

Abstract: This paper presents the influence of H₂SO₄ and NaCl concentrations on the corrosion mode of AISI 304 stainless steel in H₂SO₄-NaCl aqueous solutions. Immersion test was conducted at room temperature using U-bend specimens. The specimens in annealed and sensitized conditions were immersed in aqueous solutions containing H₂SO₄ concentration in the range of 0 to 3.0 kmol/m³ with 1.5 kmol/m³ interval, and NaCl concentration in the range of 0.5 to 2.0 kmol/m³ with 0.5 kmol/m³ interval. Results showed that, specimens in sensitized condition have a higher mass loss and higher corrosion rate. In addition, the experimental result also indicates that the two solutes have synergistic effect on corrosion mode of the material that is stress corrosion cracking (SCC) and general corrosion, which occur within a specific range of concentration. SCC region became smaller as the H₂SO₄ and NaCl concentration increased. However, the occurrence of SCC was high when sensitization treatment was subjected to the specimens. The appearance of surface damage and crack morphology were revealed by using scanning electron microscopy (SEM).

Abstract: Martensitic high-carbon high-strength SAE 52100 bearing steel is one of the main alloys used as the principal alloys for rolling contact applications. We developed a novel repeated quenching method by induction heating in order to enhance the materials strength. The steel bar passed through the heated coil several times and finally quenched by cooling water. The rotating bending fatigue tests was done in order to investigate the fatigue strength of the steel bars. The heat affected zone (HAZ) was not expanded by the heat treatment however it was found that the thrice-induction-heating and once-quenching improved the fatigue strength. These two features indicated that the size of heat affected zone does not dominate the strength of heated-samples.

Abstract: he softening and static recrystallization behavior of typical Fe-Mn-Si-Al series TWIP steel between high temperature deformation passes was investigated by two-pass hot compress deformation experiments on Gleeble-3500 thermal simulated test machine. The dynamic model of static recrystallization was built according to the experimental results. The investigation of the effects of deformation temperature, deformation rate and pre-deformation shows that deformation rate is the most effective parameter, and the increase of deformation and pre-deformation can also promote the static recrystallization. The calculated static recrystallization activation energy of TWIP steel is about 147kJ/mol through the dynamic equation built. The results of model predict conform to the experimental results.

Abstract: This study investigated the plastic deformation behavior and microstructure evolution of the modified AA7075 alloy (using Eco-Mg in lieu of Mg element). Before the compression tests, the microstructure was observed and phase analysis was performed. As the conditions for the compression tests, temperature range was from 523K to 723K and strain rates were controlled from 10^-2 s^-1 to 10 s^-1 using Gleeble equipment. The flow stress of the modified AA7075 alloy with their small grain size and second phases were slightly higher than that of conventional AA7075 alloy. The activation energy for the plastic deformation of this alloy was about 135.9kJ/mol, a relatively lower value compared with the conventional AA7075 alloy (more than 140kJ/mol). The processing map of the modified AA7075-T6 alloy was plotted and compared with that of conventional alloys. This alloy showed similar formability with the conventional AA7075 alloy, i.e., similar area of stable deformation region. We also attempted to discuss plastic deformation behavior related to its microstructure.