Papers by Keyword: Microstructure

Abstract: The effects of variations in the rate of post-austenitization cooling of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.2%Mo-1.5%Ni-0.05%Nb (mass%) transformation-induced plasticity (TRIP)-aided steel with a lath martensite structure matrix on the Charpy impact toughness were investigated, with the aim of improving the material properties for automotive body applications. When cooled at 1.2°C/s after austenitization, the TRIP-aided steel showed a higher upper-shelf Charpy impact absorbed value (90 J/cm²) and a lower ductile-brittle fracture appearance transition temperature (−126°C), compared with the values determined (82 J/cm², −98°C) for steel cooled at 53.5°C/s. The lower cooling rate yielded a higher volume fraction and carbon concentration of metastable retained austenite, finer martensite-austenite constituents, and a lower carbide fraction in the wide lath martensite structure in the TRIP-aided steel. These improved microstructural characteristics resulted in superior impact toughness.

Abstract: The direct quenching of low-carbon steel has been shown to be an effective way of producing ultra-high-strength, tough structural steels in the as-quenched state without tempering. However, in the present study, the influence of tempering at 500 °C has been studied in order to evaluate the possibilities of widening the range of strengths that can be produced from a single base composition. The chosen composition was 0.1C-0.2Si-1.1Mn-0.15Mo-0.03Ti-0.002B. In order to compare direct quenching with conventional quenching, two pre-quench austenite states were studied: a thermomechanically rolled, non-recrystallized, pancaked austenite grain structure and a recrystallized, equiaxed grain structure. Quenched and quenched-and-tempered microstructures were studied using FESEM and FESEM-EBSD. The as-quenched microstructures of the reheated and quenched and direct quenched specimens were fully martensitic and martensitic-bainitic, respectively. In both cases, tempering made the needle-shaped auto-tempered carbides of the as-quenched materials more spherical. In the case of the direct quenched (DQ) material, tempering led to a notable increase in the size of the grain boundary carbides. Prior austenite grain size and effective grain size after quenching were larger in the case of reheated and quenched material (RQ). Tempering had no effect on effective grain size. The crystallographic texture of the DQ material showed strong {112}<131> and {554}<225> components. The RQ material also contained the same components, but it also contained an intense {110}<110> and {011}<100> components. The effects of these microstructural changes on tensile, impact toughness and fracture toughness are described in part II.

Abstract: A recently introduced processing route consisting of cold rolling and recovery annealing allows the production of TWIP steels with high yield strength along with appreciable uniform elongation due to the thermal stability of mechanically induced nanoscale twins. A wide range of strength-ductility combinations was obtained using recovery and recrystallization annealing of 30%, 40%, and 50% cold-rolled Fe-23Mn-1.5Al-0.3C TWIP steel. Texture measurement during cold rolling and annealing was proven to be a suitable tool to determine the optimal deformation degree and annealing time for this processing method. As a consequence, texture analysis can be used to predict the final materials properties.

Abstract: Effect of cold rolling prior to ageing on a dispersion of secondary phases and mechanical properties at room temperature for an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr (wt. %) alloy, which was solution treated and water quenched initially, was examined. It was shown that cold working leads to significant increase in density of lattice dislocations that induces the formation discrete agglomerates of the θ′-phase on the {100} planes. Strain of 7% provided increased aspect ratio (length to thickness) of plates that leads to moderate increase of strength. Imposing of higher strains leads to increased lattice dislocation density and the formation of deformation-induced boundaries. Precipitation of the coarse particles of secondary phases on these boundaries takes place. The high yield stress (YS) of 535 MPa and ultimate tensile strength (UTS) of 570 MPa, were attained after cold rolling with a reduction of 80% followed by ageing at 190°C for 2 h. The effect of plastic deformation prior to ageing on the precipitation behavior and strengthening of Al-Cu-Mg-Ag alloy is discussed.

Abstract: A high carbon steel (HCS) and low carbon steel (LCS) bimetal was fabricated by centrifugal composite casting. Two different thermomechanical treatments (TMT1 and TMT2) were employed to improve the mechanical properties of the bimetal. TMT1 process includes 60% of overall reduction by hot compression with temperatures of 1100 and 800 ^oC, respectively. While TMT2 process involves 60% of overall reduction using the two-step deformation method, which is a combination of non-isothermal compression cooling from 1100 to 800 ^oC and isothermal compression at 800 ^oC. The flow stress behavior, microstructural evolution and microhardness variation were analysed. Experimental results show that both TMT processes contributed to the improvement in mechanical properties resulting from a refinement of the grain size and an increase of density of pearlitic lamella in HCS layer. However, TMT2 process gave a better efficiency and a more significance in improvement of properties with the evidence of the same overall reduction leading to a higher microhardness.

Abstract: Thermal barrier coatings are commonly used in gas turbines for protection against high tem-perature and oxidation. Life prediction of oxidation protective coatingsmay be done bymicrostructure-based techniques such as -depletion based life criteria. In this study, a thermal barrier coating sys-tem, with an overlay NiCoCrAlY coating as bond coat, was oxidised up to 10000 h at 900 C. Themicrostructure was studied and related to Al depletion. It was found that a -depletion based lifecriterion could not be used for the studied coating composition and temperature as it would be tooconservative. A 0-depletion based model was instead suggested and supported by interdiffusion sim-ulation.

Abstract: An aluminum alloy with a chemical composition of Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.) and an initial grain size of ∼22 μm was subjected to equal-channel angular pressing (ECAP) at 593 K up to a total strain of ~12. Extensive grain refinement provided the formation of fully recrystallized structure with an average grain size of ∼0.6 μm. The mechanical properties of the alloy in two different structural conditions were examined at temperatures ranging from 77 to 293 K. It was shown that ECAP highly enhanced the strength, ductility and fracture toughness of the material over the wide temperature interval. Positive effect of grain refinement tends to increase with decreasing temperature due to suppression of brittle intergranular fracture. At ambient temperature, the extensive grain refinement provides +65% increase in yield stress (YS) and ductility, concurrently. At 77 K, YS increase is + 77%, and the ductility increase is +113% owing to grain refinement. Effect of the grain size on fracture toughness at cryogenic temperatures is discussed.

Abstract: In turbine engines, Ni or Co based alloys are used at high temperature, either as base materials, superalloys, or deposited on the surface of superalloys, as coatings. In the present study, two different MCrAlY overlay coatings, Ni and Co based, on a Ni based superalloy IN792 were aged for different times in air at three temperatures, 900°C, 1000°C and 1100°C. The aging processes were simulated by using DICTRA software by focusing on the interdiffusion behavior in the superalloy-coating systems. The results of simulation captured the main microstructural features observed and were used to analyze the diffusion behavior of alloying elements and the corresponding microstructure development. It was found that coating composition and temperature affected significantly the microstructure near the superalloy-coating interface, and their relations were mapped as a summary.

Abstract: New titanium alloys for biomedical applications have been developed primarily with the addition of Nb, Ta, Mo, and Zr, because those elements stabilize the β phase and they don’t cause cytotoxicity in the organism. The objective of this paper is to analyze the effect of molybdenum on the structure, microstructure, and selected mechanical properties of Ti-15Zr-xMo (x = 5, 10, 15, and 20 wt%) alloys. The samples were produced in an arc-melting furnace with inert argon atmosphere, and they were hot-rolled and homogenized. The samples were characterized using chemical, structural, and microstructural analysis. The mechanical analysis was made using Vickers microhardness and Young’s modulus measurements. The compositions of the alloys were sensitive to the molybdenum concentration, indicating the presence of α’+α”+β phases in the Ti-15Zr-5Mo alloy, α”+β in the Ti-15Zr-10Mo alloy, and β phase in the Ti-15Zr-15Mo and Ti-15Zr-20Mo alloys. The mechanical properties showed favorable values for biomedical application in the alloys presenting high hardness and low Young’s modulus compared with CP-Ti.

Abstract: Light-weight design is one of the main drivers for material development in the automotive industry. For optimum weight reduction new materials and their fatigue behavior under real cyclic service loads have to be taken into account (Gassner test). Currently the casted components made from Austempered Ductile Iron (ADI) show better service fatigue life for variable load cases than some traditional forging steels because of it’s inherent retained austenite. The traditional forging steels are the precipitation hardening ferritic-pearlitic steels (PHFP steel) and the martensitic quenched and tempered (Q&T) steels. The next steel generation for forged components in the drive train might be bainitic steels with an optimized microstructure with respect to cyclic behavior. Depending on the chemical composition and the heat treatment it includes a ferritic primary phase and a secondary phase, which consists of either carbides, martensite, retained austenite or M/A constituents. By alloying of more than 1% Si the formation of cementite will be suppressed and a carbide free bainite (CFB) will be formed. The secondary phase of this CFB contains retained austenite, which has the possibility to close crack tips by local compression stresses due to the transformation to martensite. As a result of this CFB exhibits better cyclic properties than the commonly used forging steels. The materials and process design as well as results of the fatigue behavior will be presented.