Amount Of Phase Transformation Research Articles

Ultra-high tensile strength via precipitates and enhanced martensite transformation in a FeNiAlC alloy

Abstract Aging and quenching at critical temperatures were applied to a hot-rolled (HR) Fe-24.86Ni-5.8Al-0.38C (mass%) dual-phase alloy to obtain B2 precipitates with various volume fractions and sizes. Higher yield strength and stronger strain hardening were achieved in the aged samples compared to these for the HR sample, and the corresponding deformation mechanisms were carefully revealed. The aged samples show stronger hetero-deformation induced hardening compared to that for the HR sample. The amount of phase transformation during tensile tests is much higher for the aged sample compared to that for the HR sample due to the reduced stability of the austenite phase, which can be attributed to the high local stress level induced by undeformable and hard B2 precipitates around the austenite grains and reduction of nickel and aluminum in the austenite phase by diffusion from the austenite phase to the B2 precipitates during aging. Deformation-induced lath martensite with high dislocation density can be observed after tensile deformation, and these transformation-induced dislocations should have great impact on the strain hardening. Moreover, the density of twins becomes much higher for the martensite grains after tensile deformation, and these deformation-induced nanotwins should contribute significantly to the strain hardening, as the dynamic Hall-Petch effect.

Molecular dynamics simulation and experimental investigation of structural transformation and graphitization in diamond during friction

Molecular dynamics simulation of friction is conducted by sliding SiC hemisphere on diamond bulk. The mechanism of structural transformation and graphitization occurred on diamond surface is investigated, and its dependence on load pressure, temperature and sliding velocity are analyzed. The analyzation of radial distribution function g(r) proves graphitization occurrence of diamond. Contrast results show temperature and stress both promote formation of sp2-hybridized phase while sliding velocity changing brings no difference in amount of phase transformation. Furthermore, existence and change of stress are the key factors to structural transformation and graphitization. Finally, actual friction test with different loading force is carried out. The comparative Raman results give evidence that more graphite content is detected in the friction test with higher loading force of 7 N than that with loading force of 2 N.

Study on fracture behaviors of nanocomposite ceramics under a high cycle stress

In order to clarify the fracture behaviors of nanocomposite ceramics under ultrasonic vibration–aided grinding, the stress under ultrasonic was analyzed by non-local elastic theory. A similarity model of bending fracture experiment was built to allow online observation of fracture process, according to the actual conditions of fracture damage under ultrasonic vibration–aided grinding and the similarity law. The experiments validated the theoretical analysis: as the ultrasonic frequency increases, fatigue life is distinctively shorter, and the fatigue crack extends faster and is more stable; the deflection angle decreases as the ultrasonic frequency increases, and it is biggest (47.53°) under 19.9 kHz; the phase transition occurs in the fracture process and the amount of phase transformation increases with the increase in ultrasonic frequency and zirconia content, and it can reach the maximum value when the zirconia content is about 25% and then declines; the greater the ultrasonic frequency, the more obvious the transgranular fracture, the more microcracks on the crystal layers, and the more smooth the fracture surface.

Nanocrystalline β-sialon by reactive sintering of a SiO 2–AlN mixture subjected to high-energy ball milling

A mixture of powders of silica and aluminum nitride is subjected to high-energy ball milling for different milling times. This material is subsequently compacted by uniaxial pressing and sintered at 1450 °C. The resulting pellets are crushed and analysed by X-ray diffraction. For short milling times, the amount of phase transformation is minimal and the resulting material mostly consists of cristobalite and aluminum nitride. For long milling times, nanocrystalline β-SiAl 2O 2N 2 is mainly produced.

Quantitative characterization of various tetragonal zirconia polycrystals (TZPs)

The microstructures , Y 2 O 3 content and phases of 3 mol% yttria-doped tetragonal zirconia polycrystals (TZPs) were quantitatively investigated. Five yttria-doped zirconia samples were observed by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) or electron probe microanalyzer (EPMA) equipped with wavelength dispersive spectroscopy (WDS). The amount of phase transformation was determined by X-ray diffractometry (XRD). Operating conditions of EDS or WDS were controlled based on the needs of statistical analysis. The Student's t distribution in a confident interval of 99% was adopted and applied to reveal the level of homogeneity of Y 2 O 3 content in five TZP samples. Based on the results and analytical methods, the quality, including sintered density, the fraction of large grains, the transformation capability, and the homogeneity of Y 2 O 3 content of TZP can be distinctly verified.

Phase Transformation in Y2O3‐Partially‐Stabilized ZrO2 Polycrystals of Various Grain Sizes during Low‐Temperature Aging in Water

ZrO2‐2 mol% Y2O3 crystals with average grain sizes from 0.51 to 0.96 µm were prepared by sintering in air at 1400°C for 2 to 100 h. The tetragonal‐to‐monoclinic phase transformation associated with the low‐temperature degradation was investigated to clarify how the presence of water directly affects the influence of grain size on transformation. The specimens were exposed to water at 80–120°C, a temperature range in which transformation by thermal activation is difficult in the absence of water. Contrary to expectations, this type of low‐temperature transformation did not accelerate monotonously with increasing grain size. Instead, the amount of phase transformation first decreased, reaching a constant value, and then increased with increasing grain size. Such interesting results can be explained satisfactorily by the combined influences of grain size on the nucleation process, because of preferential dissolution of yttrium at the grain boundaries, and the intrinsic transformability of Y2O3‐doped tetragonal ZrO2 grains.

Ion-induced phase transformation in type 304 austenitic stainless steel by rare-gas ion irradiation

Foil specimens of austenitic stainless steel (type 304 stainless steel) were irradiated with 50– 400 keV Ar, Kr, and Xe ions in the fluence range of 1×1020–1×1021 ions/m2 at room temperature. It has been shown by glancing angle x-ray diffraction (GXRD) measurement that the amount of ion-induced phase transformation of γ →α′ in the foil specimens depends strongly on irradiation conditions such as ion species, energy, and fluence; the amount of the phase transformation at saturation increases linearly with increasing ion range. An ion-induced new peak was found in the GXRD pattern, which is shown to be due to solidification of implanted rare-gas atoms in the stainless steel. The solid phase inclusions of the rare-gas atoms have a close correlation with the phase transformation. It is shown that the phase transformation is induced by the formation of highly pressurized rare-gas inclusions, while the formation of the inclusions depends on a depth distribution of implanted rare-gas atoms.