Duplex microstructure models containing multi-variants in each β transformed (βT) grains are established, and then, the high temperature tensile deformation of Ti-6Al-4V alloys with different microstructure features was investigated via the rate-dependent crystal plasticity finite element simulation by taking all slip systems in the α and β phases into consideration. The spatial distributions and time evolution of the stress and strain in various grains and phases are analyzed in detail, and a new method is proposed to evaluate quantitatively the deformation consistency. Simulation results showed that αp underwent higher strain distribution rather than βT, and inter-crossing high strain bands formed in the duplex microstructure and distributed symmetrically with respect to the tensile direction. The surrounding structure formed between αp and βT grains can enhance the differences in the local strain distribution. Increasing the volume fraction of αp may reduce the strain allocation in αp, the consistency coefficient of strain first decrease rapidly and then stabilized. As the thickness of αs increase, the feature of high strain bands weakened and the consistency coefficient of strain increased. The consistency coefficient of strain for βT containing double αs variants is usually lower than that with single or three αs variants.
The microstructural evolution and softening mechanism of 2205 duplex stainless steel during hot deformation were investigated by high temperature compression tests at different temperatures ranging from 850°C to 1250°C with different strain rates of 0.1 s-1, 1 s-1 and 10 s -1. The microstructures after hot compression were characterized by means of optical microscope and scanning electron microscope equipped with an electron back-scattered diffraction (EBSD) system. The results show that the peak stress and the corresponding strain decrease with the increasing temperature. The stress-strain curves show typical characteristics of dynamic recrystallization or dynamic recovery under different hot compression conditions. The transition temperature at which the stress-strain curve changes from dynamic recrystallization type to dynamic recovery type increases from 1150°C to 1050°C as the strain rate increases from 0.1 s-1 to 1 s-1 and 10 s-1. The phase transformation of ferrite to austenite was dominant in the microstructure of 2205 duplex stainless steel when deformation temperature increased from 850℃ to 950℃. With the increasing temperature, the strength difference between ferrite and austenite decreased gradually, and the transformation from austenite to ferrite was predominant in the microstructure of 2205 duplex stainless steel when the deformation temperature was above 950℃. Ferrite and austenite in the microstructure of 2205 duplex stainless steel exhibited different softening mechanisms under the hot compression conditions adopted in this work. Ferrite was softened by dynamic recovery and dynamic recrystallization, while austenite with lower stacking fault energy was softened by a limit degree of dynamic recovery.
Composites of epoxy resin modified polylactic acid (ePLA)/low melting point nylon 6 (LMPA6) were prepared via method of “melting extrusion—hot stretching—quenching”. The crystallization behavior, thermal, rheological and mechanical properties of the composites were investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA), rheometer and electronical stretching machine. Results show that the addition of LMPA6 changed the crystal structure of PLA, and further significantly changed the cold crystallization temperature and melting temperature of cold crystallization of the composite; The addition of LMPA6 could enhance the thermal stability and the glass transition temperature (Tg) of the ePLA/LMPA6 composites, while the storage modulus (G') decrease nonlinearly when the strain (γ) exceeds the critical strain (γC), namely the “Payne” effect appeared; The composites exhibited the characteristics of non-Newtonian fluids—"shear thinning", and which was more pronounced with the increasing content of LMPA6; When the LMPA6 content was 7%, the microfibrous structure appeared in the composite, thereby which exhibited the best compatibility; The addition of LMPA6 could enhance the strength and toughness of the composites to a certain extent, especially, when the LMPA6 content was 7%, the tensile strength (72.8 MPa) and the impact strength (5.0 kJ/m2) reached the extreme value, which were 10.8% and 78.6% higher than that of modified PLA (65.7 MPa, 2.8 kJ/m2), respectively.
The nanosized particles of LiF were successfully prepared via liquid synthesis method with ammonium fluoride and lithium hydroxide as raw materials, anhydrous ethanol as solvent and crystallization-controlling agent. The structure and morphology of LiF particles were characterized by means of XRD, SEM, TEM and particle size analyzer. The precursor mainly composes of LiF, NH4F and LiOH·H2O. The thermal analysis of the precursor indicates that the decomposition temperature of ammonium fluoride is about 190°C. Nano-powders of LiF has a single cubic crystallographic structure after calcinations at 220~400℃. The nano-LiF particles with average size of about 80nm are full in crystallinity, uniform in size and good in shape.
Transparent conductive thin films of fluorine Transparent conductive thin films of fluorine doped tin oxide (FTO) were prepared via spray-pyrolysis (SPD) process using with methanol, ethanol, isopropanol, n-butanol and deionized water as solvents respectively. The films were tested and characterized by X-ray diffraction, scanning electron microscope, four-point probe resistance meter, Hall effect meter and UV-visible spectrophotometer. The effects of solvent on the structure, morphology and photoelectric properties of FTO films were investigated. The results show that when the solvents were with different solvents, the prepared FTO thin films showed the same crystallographic, i.e. a tetrahedral rutile structure, and however their surface morphology and particle size were significantly different. Among others, the FTO thin films prepared with methanol as the solvent showed a dense and full pyramid-like structure al shape, with uniform grain size, compact structure, and the best comprehensive optical and electrical properties, namely, the best resistivity of the film can reach 4.43×10-4 Ω·cm, the carrier concentration is 9.922×1020 cm-3, the figure of merit of the film is 1.63×10-2 Ω-1, and the visible light transmittance is greater than 75%.
The surface of carbon fibers was firstly modified with a new water borne sizing agent O3PPA, and then composites of the modified carbon fibers/ polycaprolactam resin were prepared with polycaprolactam resin as matrix. The above prepared products were characterized by means of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), fiber strength extensometer and universal material testing machine. The results show that the optimum mass fraction and adsorption capacity of O3PPA were 1% and 5mg·g-1 respectively. The fracture strength of the O3PPA modified carbon fiber monofilament increased by 12% and the dispersibility of short carbon fibers in polycaprolactam resin was significantly improved. Furthermore, the flexural strength and interlaminar shear strength of the modified carbon fibers/polycaprolactam resin composite increased by 35% and 46% respectively compared with those of plain carbon fibers/polycaprolactam resin composite.
The effect of cooling rate, undercooling degree and recalescence temperature on the morphology of the primary Si-phase and the mechanical property of Al-20%Si alloy were investigated by means of high-precision thermometer, optical microscope (OM) and scanning electron microscopy (SEM). The results showed that the average size (D) of the primary Si in Al-20%Si alloy is a power function of the cooling rate (v) as D=260.6v-3/4, and linearly related to the recalescence temperature (Tm) as D=0.25Tm-143.12; Reducing the recalescence temperature of the primary Si growth was the key to control the grain growth, the copper mold with high thermal storage coefficient may be favourable to the sustainable reduction of the nucleation temperature and recalescence temperature of the primary Si, so that the primary Si size was small; The critical supercooling degree of 70 K was needed for the transformation of the primary Si growth from facet-like to non-facet-like ones, which is consistent with the theoretical calculation (74 K). With the increase of cooling rate and the undercooling degree, while the decrease of recalescence temperature, the solidified microstructure of Al-20%Si alloy was refined remarkably, correspondingly, the tensile strength of the Al-20%Si alloy increased from 167 MPa to 210 MPa and the elongation increased from 2.14% to 3.89 % respectively.
The polished and as received 6082-T6 Al sheets of 6 mm in thickness were subjected to friction stir welding (FSW) aiming to revealthe effect of surface oxide film on the microstructure and fatigue property of the FSW joint. The results show that high quality FSW joints with a high joint strength coefficient of 81% could be produced by a high welding speed of 1000 mm/min, for the unpolished and polished butt surfaces. Similar fatigue properties were obtained for the two type of FSW joints with the above two surface states, and the fatigue strength was 100 MPa. Most weld joints failed at the heat affected zones, and only a few weld joints failed at the nugget zone (NZ) during fatigue tests. The fatigue strength increased to 110 MPa for the NZ for plates with the above two surface states, and it is revealed during fatigue test that the fatigue cracks did not initiate on and propagate along the so called“S-line”.
Micro-/nano-metered TiC particulates dispersion strengthened 304 stainless steel (TiC-304SS strengthened steel) were prepared by in-situ reaction technology with 2% and 5% TiC (in volume fraction) respectively. The high temperature creep properties of the plain 304SS and two TiC-304SS strengthened steels were investigated. The results show that the in situ formed TiC particulates, most of which exhibited polygonal shape, were distributed uniformly in the matrix of 304 SS and are well bonded with the matrix. Moreover, TiC particulates present a significant effect on the grain refinement of the steel matrix. It reveals that being subjected to creep test by100 MPa at 700oC for 200 h, the grains of the plain 304SS grew up evidently with elongated shape along the loading direction, in the contrary, the grain growth tendency of the TiC-304SS strengthened steels seems to be inhibited, thereby, the creep deformation was effectively reduced. The above results imply that dislocation motion in the three steels accords with dislocation climb mechanism. Besides, the values of apparent creep stress exponent and activate energy of the two TiC-304 strengthened steels are higher than that of the plain 304SS. It is proposed that the enhancement of creep performance of TiC-304SS strengthened steel may be ascribed to the enhanced threshold stress and load transfer barrier, as well as the microstructural strengthening effect.
Tribological behavior of experimental medium chromium wear-resistant cast steels with different niobium addition was investigated by using the MM-W1 pin-on-disc friction and wear tester at room temperature. The microstructure of the worn surface and subsurface were characterized by means of scanning electron microscopy and energy dispersive spectrometer, aiming to reveal the effect of Nb content on the wear behavior and mechanism for the steels. The results show that the addition of 0.2% (mass fraction) Nb can effectively increase the wear resistance of the steel due to the formation of discontinuous rod-like Nb carbides and the effect of grain refinement. An excess addition of Nb can reduce the wear resistance due to the occurrence of coarse mesh carbides and microstructural distortions, which then causes spallation of the worn surface. The 0.2%Nb test steel presented an evolvement of wear mechanism from slight abrasive wear to adhesive wear and oxidation wear.
