Carbon nanotubes (CNTs), dispersed in absolute ethanol, were evenly mixed into Ti/MgH2 powders by wet milling. Then, we applied the vacuum hot-pressed sinteringmethod to the CNTs/TiMg composite materials. An optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and a field emission scanning electron microscope (FESEM) were used for the microstructure observation and phase analysis of samples. The mechanical properties were measured via the micro-vickers hardness. The results show that the main phases in the composites were Ti, Mg and C. Meanwhile, a small amount of Ti-Mg solid solution phase was also found. The cross-section morphology of the composites shows that the melted magnesium fills the grain interface during extrusion and that the composites have a better compactness.The microstructures of the composites have been greatly refined as the CNT contents increased. The structure of the composites was further refined when 0.5 wt.% CNTs were added. The fracture surface is obviously a ductile fracture. The microhardness increases obviously with the CNT content increasing. When the content of the CNTs is 1.0 wt.%, the microhardness of the composites reaches 232 HV, which is 24% higher than that of the matrix.
The formation and growth kinetics of the reverted austenite during tempering in 13Cr supermartensitic stainless steel were investigated by a combination X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). The reverted austenite precipitated at the martensite blocks, sub-blocks, laths and grain boundaries. The growth kinetics was established by Johnson-Mehl-Avrami (JAM) kinetics equation according to the volume fraction of the equilibrium reverted austenite at room temperature. The Avrami exponent value is 0.5, and the activation energy was estimated to be 369 kJ/mol, the kinetic model indicates that the mechanism of reverted austenite is diffusion-controlled and the growth of reverted austenite closely relies on the diffusion of the nickel (Ni) element. The experimental measured orientations of the reverted austenite are in good agreement with the theoretical ones, implying that the reverted austenite has the same orientation with the surrounding martensite, which meets the Kurdjumov–Sachs (K-S) orientation relationship. The orientation relationships minimize the strain energy of the phase transformation by reducing the crystallographic mismatch between phases.
The microstructure and microhardness of X70 pipeline steel were investigated after conducting different processing routes. The microstructure was characterized using optical and electron microscopy. Scanning electron microscopy equipped with electron backscattered diffraction (EBSD) and transmission electron microscopy techniques were applied for investigation of different thermal processing treatment conditions. Mechanical properties were characterized by a microhardness tester. The results show that the microstructure mainly consists of granular bainite, acicular ferrite and a small amount of M/A constituents under hot rolling states. There are many dislocations inside the acicular ferrite. The thermal simulation experiments show that the microstructure becomes homogeneous with the increase in cooling rate. The acicular ferrite morphology becomes fine and uniform, and the content of M/A constituents increases at the same compression amount. The compression gives rise to the accumulated strain and stored energy, which accelerate the transformation of acicular ferrite and refine the microstructure of the pipeline steel. The microhardness rises with the increase in deformation ratio and cooling rate. The microstructure of the pipeline steel subjected to the isothermal quenching process is ultrafine ferrite and M/A islands. When the isothermal quenching temperature reaches 550 °C, a small amount of upper bainite appears in the microstructure. With the increase in isothermal quenching temperature, the microhardness decreases. Acicular ferrite is a better candidate microstructure than ultrafine ferrite for the pipeline steels.
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