The evolution of the secondary phase in G18CrMo2-6 heat-resistant steel induced by tempering at 680℃ for a series of durations was investigated by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray detector (EDX). It demonstrates that martensite/austenite (M/A) particles and M3C precipitate on bainite matrix after normalizing. During the tempering, decomposition of M/A particles into the M3C carbides in bainite matrix can be found. The increasing of tempering time results in the precipitation of MC, the spheroidization and refinement of M3C carbide as well as the precipitation and coarsening of M23C6 carbide at the grain boundaries.
In order to understand the bonding mechanism of hydroxyapatite (HA) particles for the 3D printedbone scaffolds with binders, the performance of three commercial binders i.e.PVP, PAM andPVAwas studied by means of molecular dynamics simulationin terms of cohesive energy density, binding energy and pair correlation function g(r), as well as mechanical properties. The results revealed that the relationship of the binding energies between the HA surface with the three binders is consistent with their cohesive energy densities, i.e. PAM > PVA > PVP. The analysis of g(r) indicated that the interfacial interactionof HA and binders could mainly be attributed to the ionic bonds and hydrogen bonds which formed between the polar atoms, functional groups in binder polymer and the Ca, -OH in HA, and the strength of ionic bonds is larger. TheYoung's modulus for the three interaction types of binders/HA can be ranked as the following sequence: PVA/HA > PAM/HA >PVP/HA, which are all inferior to that of the single HA. This conclusion is not completely consistent with the ranking of the relevant binding energies, which means that there is no specific intrinsic relation between the mechanical properties of the three binding types and the related viscidity of binders.
The W fiber/Zr-based metallic glass composite was prepared by infiltration and rapid solidification. The effect of the ratios of length to diameter of fibers on the compressive properties of the composite was investigated in detail. The results show that the yield strength firstly decreases with the increase of the length to diameter ratio then reaches a stable value when the ratio is greater than 1. The plastic strain has no obvious change when the ratio is greater than or equal to 1.25, while the plastic strain is bigger than 50% when the ratio is smaller than 1.25. The reason for these phenomena is the comprehensive effect of the friction force between pressure head and the end of the compressive sample, the change of the length to diameter ratio of the metallic glass fibers between W fibers and the mismatching between metallic glass matrix and the W fiber during deformation.
Composites of Multi-walled carbon nanotubes (MWNTs), carbon microspheres (CMSs) and Polyethylene terephthalate (PET) were prepared by melt blending method.The structures, flame retardancy and pyrolysis behavior of the composites were characterized by scanning electron microscope (SEM)and infrared spectroscopy (IR), as well as limiting oxygen index (LOI)method, vertical burning method (UL-94), cone calorimeter and thermal gravimetric analysis (TG).The results showed that the addition of 1% (mass fraction) of (1MWNTs + 0.5CMSs) into PET resulted in a good comprehensive flame retardancyof the composite. Moreover, in comparison with the pure PET and CMSs/PET, the composites MWNTs/CMSs/PET could reduce effectively the fire risk. The pyrolysis process of PET could be suppressed by the synergistic effect of MWNTs and CMSs due to the following that during burning, on one hand, MWNTs might give rise to form a three-dimensional network of compact carbon layeron the PET surface to decrease the melt drips; on the other hand, CMSs might produce a turbulent carbon layer on the surface of PET, preventing the oxygen and heat from entering the interior of PET and thus releasing the non-flammable CO2 to reduce the concentration of combustible gas in the surrounding environment.
The oxidation behavior of a superalloy Ni-4.66Cr-5.87Co-7.54Mo-2.90W-4.97-Ta-6.32Al at 900℃ and 1000℃ in air has been investigated by means of TGA, XRD and SEM/EDAX. Results show that the oxidation rate in the initial stage is rapid, then with the oxidation time the oxidation weight gain tends to be smooth, but the oxidation kinetics curve for the superalloy later presents a wavy-like change and much obviously for the higher oxidation temperature. The oxide scales consist of two layers both at 900℃ and 1000℃. Of which , the outer layer is mainly composed of NiO、Ni2Cr2O4、Ni2CoO4 and CoTa2O6, while the inner layer is a thin scale of Al2O3. The continuous oxide layer of Al2O3 is formed on the alloy surface to restrain the growth of oxide scale and decrease the oxidation rate. The precipitates of internal oxide (Al2O3) and internal nitride (AlN) formed in the superalloy after exposure for 300h at 900℃ and 1000℃; the internal oxidation zone forms on the surface of the superalloy just beneath the outer layer of the oxide scale, while the internal nitridation zone forms below the internal oxidation zone; with the increasing temperature the internal oxidation zone and internal nitridation zone became thicker, simultaneity the size of internal nitride and internal oxide increases.
The high-temperature single-pass compression test was carried out by utilizing Gleeble-1500D thermal simulation compression testing machine for Si-Mn-Cr-B alloy steel, while its thermal deformation behavior was studied by different strain rate (0.01-5 s-1) in temperature range 950-1200℃ for the same deformation degree i.e. 50%. The influence of strain rate and temperature on the stress - strain curve and microstructure evolution was explored, and the thermal deformation constitutive equation of Si-Mn-Cr-B alloy steel was established on the basis of Arrhenius hyperbolic sine function. The results show that: the peak stress of the high temperature deformation of the Si-Mn-Cr-B alloy steel increases with the increase of strain rate and the decrease of deformation temperature, but high temperature deformation is controlled by thermal activation energy and its thermal activation energy is Q=372.6 kJ/mol; dynamic recrystallization occurs during deformation, and the average size of the dynamically recrystallized grains all shows a trend of increase with the increase of temperature and the decrease of strain rate.
The effect of Bi-content on microstructure evolution of Al-Bi monotectic alloy and solidification progress ofmiscibility gap under the action of gravity was studied by air cooling to desired temperatures and thenquenching, as well as bynumerical simulation. The average diameter and volume fraction of Bi-rich droplets in Al-10%Bi monotectic alloy are 1.1 and 12.8 times of those in Al-5%Bi monotectic alloyrespectively. A huge amount of large-size drops rich inBi gather in the lower part of Al-10% Bialloywith low solidification uniformity.The average size of drops rich in Bi in the alloy of Al-5%Bi with a uniform solidification; There's no large-size drops rich in Bi in the solidifiedmonotecticalloy with Al-3.4%Bi, and its solidification is uniform.The analysis demonstrates that the increase of the amount of Bi-rich phase may enhance the gravity action on the drops, thereby aggravate the solidification process with separation and decomposition of liquid phasesin the immiscible area, and enlargethe size and number of drops rich in Bi,whilethe synergistic action of Ostwald ripening and collision coagulation of drops yielda huge amount of large-size drops rich in Bi gathering in the lower part of the solidifying alloy, thus aggravates the macro segregation of the solidified monotectic alloy.
The effect of Fe content on the phase constituent, microstructure and mechanical properties of CrMoVNbFex high-entropy alloys were investigated by using of XRD, SEM, EDS and microhardness tester. The results reveal that CrMoVNbFex alloys exhibited as a solid solution of single bccphase without Fe, whereas bcc solid solution+ intermetallic σ phase was observed with the increase of Fe content. The as-cast alloys show a microstructure with typical casting dendrites. It was found that Mo mainly exist in the dendrite, Fe and Cr concentrated mainly in the interdendriticspace and the Nb content in the interdendriticspace was slightly higher than that in the dendrite. With the increasing Fe content, the microstructure of alloys is refined and the hardness is enhanced significantly and the maximum hardness value of the CrMoVNbFex high-entropy alloys reaches HV950.
The mechanical property of TiAlN coatings of large thickness deposited on stainless steel substrate by arc ion plating (AIP) was systematically investigated. The results indicated that the thickness of the coatings deposited by AIP with the increasing flow of N2 by way of cycle or stepwise could reach 68.79 μm and 64.48 μm respectively, and those coatings show fairly well mechanical performance. The depth profile of residual stress of the coatings presented a general trend that the stress increased gradually from the coating/substrate interface to the top surface. The average compressive stress of the coatings is lower than 1 GPa, and its surface hardness almost reaches 2000 HV. The former coating has lower friction coefficient and wear rate, whereas the later one shows better coating/substrate adhesion.
Dynamic phase transformation behavior and CCT curves of 22 mm thick X80 pipeline steel were investigated using Gleeble simulator. Comparative analysis of microstructure, mechanical properties and precipitation behavior was carried out for the steel cooled by different ways. Results reveal that even under cooling rate of 40℃/s, the microstructure still consists of mainly the refined lath bainite (LB), while the end phase of transformation temperature reaches about 400℃. The strength of coil increases without obvious change in toughness due to ultra fast cooling. The proportion of Quasi-polygnoal ferrite (QF) reduces as well as M/A island, while that of lath bainite (LB) rises instead. Precipitation of Nb(C, N) has been inhibited by ultra fast cooling after rolling, which increases the proportion of precipitate in α-Fe with more refined particle size and promotes the precipitation strengthening effect.
Ingeneral,high-temperature salt-corrosion may usually induce mechanical stresses and phase transformation within ZrO2,as the main componentof thermal barrier coatings (TBCs), therewith further cause the failure of TBCsfor hot section components of gas turbine. Yb/Y co-doped zirconia (YbYSZ) powder was synthesized by a coprecipitation-calcination method, thenYbYSZ ceramic pallets were obtained by cold pressing and subsequent sintering at high temperature. The corrosion behavior of thepallets coated with a film of powder mixture CaO-MgO-Al2O3-SiO2 (CMAS) was examined in air at 1250℃ for different time intervals. The elemental diffusion and phase transformationwith YbYSZ after high-temperature corrosion wereinvestigated by XRD, SEM and EDS. The results showed that among othersthe elementYbin YbYSZ reacted preferenyially with CMAS and dissolved into the molten saltCMAS. The loss of Yb could suppressed the segragation of Y from the rest YbYSZ.Consequently, it stabilized the metastable tetragonal phase (t'-zirconia). The optimal dose of Yb is 5 mass% for the minimal yield of monoclinic ZrO2 in the YbYSZ after corrosion test.
As magnetron sputtering copper films would flake from silicon substrates in the ultrasonic cleaner, this paper analyzed the states of motion and stress of samples in the ultrasonic medium. By calculation, it was found that the tension-tension cycle stress caused by forced vibration was the main reason of interface damage. Then the ultrasonic mechanical model was established and the film-substrate adhesive strength was calculated. The results showed that the adhesive strength values gotten by the ultrasonic test method were in the same order of magnitude compared with that of scratch test results. In addition, this ultrasonic test method was used to test adhesion of copper films on diamond substrates. The influence of ultrasonic parameters, substrate morphology as well as composition on adhesive strength was also discussed.
