Effect of heat treatment on the microstructure and mechanical property of vacuum die-casting (VDC) NZ30K Mg-alloy were systematically investigated by means of optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), hardness test and tensile test. The results show that the as-cast alloy is composed of a surface zone and a central region. Fine α-Mg matrix and Mg12Nd eutectic compounds were observed in the surface zone and the central region, besides, coarser externally solidified crystals (ESCs) existed in the central region. During solution treatment the grain growth of the central region was more significant than that of the surface zone, which can be explained by the grain growth model of unhomogenized structure, i.e.v=M0 exp (-Q/RT) A (1/D1-1/D2). The optimized heat treatment of the alloy was 540oC×6 h+200oC×8 h. Compared with the as-cast alloy, the ultimate tensile strength and yield strength of the peak-aged alloy enhanced from 186.0±1.5 MPa to 223.6±4.1 MPa and from 131±2.5 MPa to 172.8±2.9 MPa respectively, with a decreased elongation (from 6.6±0.4 % to 4.2±0.3%). The strength enhancement may be mainly attributed to the plate-shaped β" precipitates, which could block the dislocation motion effectively. The fractography of surface zone exhibited ductile fracture pattern at different states. However, the fractography of central region showed quasi-cleavage, cleavage and quasi-cleavage fracture patterns for the as-cast, as-solutioned and peak-aged alloys, respectively.
TaN films were deposited on monocrystalline silicon wafer via plasma enhanced atomic layer deposition with Ta[N(CH3)2]5 as precursor and NH3 plasma as coreactant. The as deposited films were characterized by means of atomic force microscopy, X-ray photoelectron spectroscopy, four-point probe and X-ray reflection. The results show that the as-deposited film consists mainly of TaN with small quantities of C and O. As the deposition temperature increases from 250oC to 325oC, the ratio of Ta/N increases from 46:41 to 55:35, and the C-content (atomic fraction) decreases from 6% to 2%. Meanwhile, the resistivity of the film gradually decreases from 0.18 Ω?cm to 0.044 Ω?cm, and the film density increases from 10.9 g/cm3 to 11.6 g/cm3. After annealing at 400oC for 30 min, the film density shows an increment of ~0.28 g/cm3 on average, and the film resistivity decreases to 0.12-0.029 Ω?cm. Further, the barrier performance test results indicate that the TaN film of 3 nm in thickness deposited at 250oC demonstrates a perfect barrier function after annealing at 500oC for 30 min.
Diamond coatings were deposited on hard alloy cutting tool of WC-Co8% via hot filament chemical vapor deposition (HFCVD) technology. Two type of coatings, namely, monolayered coating consisted of microcrystalline with grain size of 1.2 μm and multi-layered coating consisted of nanocrystalline with grain size of 200 nm, were prepared by adjusting the methane concentration in the reaction chamber. Then the cutting performance of WC-Co8% cutting tools coated with the two coatings was comparatively assessed via machining 7075 aviation Al-alloy under dry cutting conditions without lubrication. The results show that after cutting for 2 h, coating partially spalled off and the tool edge became blunted for the tool with monolayer diamond coating, in the contrary, the tool edge remains intact and the coating does not fall off for the tool with multilayed diamond coating. Furthermore, Rockwell indentation test of flat samples of hard alloy with coatings revealed that the area of delamination induced by indentation for the multilayered coating is 1/5 to 1/10 of that for the monolayered one. Accordingly, the cracking resistance of the multilayered diamond coating should be better. It follows that the multilayered structure can be adopted to enhance the adhesion of diamond coatings to the substrate, thereby effectively increase the service performance of diamond coating tools.
The microstructure and mechanical property were investigated for Ti-Nb-Zr Ti-alloys with varying contents of (24~30)Nb and (8~12 mass fraction%)Zr. The results show that the increase of Nb- and Zr-content is favorable for suppressing α" martensite- and ω phase- formation, while the alloys composed of single β phase can be obtained as their electron-to-atom ratio higher than about 4.19, which is much less than the ratio 4.24 for Ti-Nb binary alloys. The Ti-30Nb-(8~12)Zr alloys of single β phase exhibit low Young's modulus of about 62 GPa and high strength-to-modulus of about 0.9%, which imply that these alloys possess superior biomechanical compatibility rather than Ti-Nb binary alloys.
Composites with co-continuous structure of Si3N4/1Cr18Ni9Ti were prepared via a two-step process, namely gel casting and pressure casting. The phase composition, macro- and micro-structure of the composites were characterized. The erosion rate in flow slurry composed of water and quartz sand, as a function of impingement angle, flow velocity, sand content and erosion time was assessed in comparison with the plain 1Cr18Ni9Ti. Results show that Si3N4/1Cr18Ni9Ti composites exhibited a perfect co-continuous phase structure with a good combination between Si3N4 and 1Cr18Ni9Ti; the fluctuation of erosion rate as a function of impingement angle of this composites is smaller than that of 1Cr18Ni9Ti; the erosion rate of composites has an exponent relationship with flow velocity (E∝V 0.67), while there is a linear relationship between the erosion rate and flow velocity for 1Cr18Ni9Ti; the erosion rate of this composites decreases gradually with the increasing erosion time and then stabilizes, while that of 1Cr18Ni9Ti is hardly changed; There is a linear relationship between the erosion rate and sand content in the slurry for the two materials. The composite with co-continuous structure of Si3N4/1Cr18Ni9Ti exhibits superior erosion resistance, in contrast with the plain 1Cr18Ni9Ti steel.
The creep behavior of a high strength compacted graphite cast iron (CGI) containing Cu, Mo and Sn under tensile load of 40~150 MPa in air at 623~823 K was investigated, while the creep damage was observed and the relevant mechanism of deformation and fracture during creep test was further analyzed. When the ratio T/Tm＞0.5 (T represents test temperature, Tm melt point of CGI) and the load is greater than 150 MPa, the creep deformation is significant. The creep deformation consists of matrix deformation, initiation and development of creep cavities at grain boundaries and debondings of the interface graphite/matrix. With the increasing temperature and tensile load, the creep deformation is gradually changing from grain boundary sliding to intragranular deformation. Two kind of cracks were observed in the microstructure of CGI: (1) cracks propagated preferentially in ferrite phase and connected with adjacent debondings of the interface graphite/matrix, (2) microcracks caused by nucleation and growth of creep cavities along grain boundaries. It is worthy to mention that the 3D network of the vermicular graphite in CGI may facilitate the inward diffusion of oxygen atoms throughout the sample of CGI, therewith induces the oxidation of the above mentioned two type cracks. Due to the difference in properties between graphite with ferrite and pearlite respectively, the debonding occurance for the inerface of graphite/ferrite may be easier than that of graphite/pearlite. In addition, pearlite in the microstructure may decompose significantly at 773 K and 823 K for 100 h, as a result, the lamellar cementite should be converted to short rods and granules at grain boundaries.
Needle-cokes (NCs) were synthesized via a two-step process, namely mesocarbon microbeads (MCMB) were firstly blended with coal tar pitch (CTP) at 180oC for ca 30 min, and which was then calcined in an autoclave filled with 0.5 MPa nitrogen at 1500oC for 5 h. The microstructure of mesophase, semi-cokes and the final product NCs was characterized by means of polarizing microscope, XRD and SEM. The resistivity and coefficient of thermal expansion (CTE) were measured by resistivity meter and thermal mechanical analyzer, respectively. The results show that the addition of moderate amount of MCMB could affect the formation of NCs, so that promote the formation of graphite-like layered structure, which significantly reduced the resistivity and CTE value of NCs. The structure of needle-cokes could be effectively improved with the increase of MCMB content (≤50 mass fraction%). However, the quality of needle-cokes began to decline as the content of MCMB exceeded 50%. The resistivity and CTE value (at 0-100oC) of NCs decreased by 27.9% and 45.7%, respectively, when the content of MCMB is 50% in feedstock, meanwhile, the corresponding graphitization degree increased by 46.2%, as comparing with the parent needle-coke.
The concentration of graphene in lubricant oil was measured by means of spectrophotometric method, in terms of an apparent linear relationship between the absorbance and the graphene concentration (0.0125~0.100 mg/mL) of lubricant oils. By making use of such linear relationship, the effect of the initial concentration of graphene, ultrasonic treatment time and surfactant content on the suspensibility and dispersibility of the graphene-modified lubricant oil were assessed, and thereby, the preparation processing was optimized. The as-prepared graphene-modified lubricant oil showed typically outstanding suspension dispersibility for long-term, which should be attributed to the optimization of ultrasonic time and surfactant concentration. The graphene-modified lubricant oil also presented excellent performance in friction reduction and wear resistance, namely the friction coefficient was found to decrease by 74.78% for the lubricant oil intermingled with 0.025 mg/mL graphene in comparison with the blank oil, correspondingly the wear scar diameter reduced by 28.33%.
Molybdenum phosphide (MoP) is successfully prepared by a facile 2-step process. First as precursor, nano metal Mo-powders were prepared via DC arc plasma method, which then react with red phosphorus through solid-phase reaction to yield MoP nanoparticles. The prepared MoP nanoparticles were further characterized by means of XRD and TEM. Results show that the MoP nanoparticles are spherical with particle diameter of 20-50 nm. As the anode material for lithium-ion batteries, MoP nanoparticles deliver the initial discharge capacity of 746 mAh/g at the current density of 100 mA/g and the capacity maintains at 241.9 mAh/g after 50 charge-discharge cycles. As the current density increased to 2000 mA/g the discharge capacity decreases to 99.90 mAh/g. The constant capacity of 247.60 mAh/g can be restored when the current density is back to 100 mA/g.
The reheat cracking susceptibility of the coarse-grained heat affected zone (CGHAZ) of SA508-4N steel for nuclear pressure vessel was evaluated by stress-rupture tests at high temperature. The microstructure of the CGHAZ and base metal (BM), as well as the crack- and fracture- morphology were characterized by means of Laser confocal microscope, scanning electron microscopy and transmission electron microscopy. The results show that the microstructure of base metal is tempered martensite, while carbon and chromium content can affect the size and distribution of carbides. The formation of martensite in CGHAZ is not conducive to the suppression of reheat cracking. The precipitation of carbides causes the difference in the strength for grain and grain boundary. When the strength of grain is greater than that of grain boundary, intergranular brittle fracture could emerge. However, when the strength difference between the grain and grain boundary is small, both of the transgranular- and intergranular-fractures could occur. The CGHAZ of SA508-4N steel is not sensitive to reheat cracking, and the resistance to reheat cracking in the CGHAZ of steel A is better than that of Steel B. It follows that the optimum parameters in the actual welding for steel A are as follows: the welding t8/5 is 25 s, and the post weld heat treatment temperature is 580oC.
