共析钢温变形过程的组织球化与超细化

利用热模拟压缩变形、SEM和热磁法实验,研究了共析钢在600~700℃变形过程的组织演变规律,探讨了变形及随后保温过程对珠光体球化、组织超细化和渗碳体溶解、再析出的影响.实验表明:共析钢温变形过程中发生珠光体片层溶断,渗碳体逐渐球化,以及伴随铁素体动态回复再结晶的同时细小弥散渗碳体颗粒在基体析出的过程.提高温度有利于上述复合过程的进行.形变组织经过保温后,亚微米级别的等轴铁素体晶粒和渗碳体颗粒弥散分布的复相组织的均匀性程度有所提高.温变形过程中渗碳体溶解和在铁素体内再析出的事实得到证实.     

炭泡沫中Si含量对碳化硅多孔陶瓷 组织和性能的影响

研究炭泡沫预制体中Si添加量的变化对碳化硅多孔陶瓷的组织及性能的影响. 以中间相沥青添加一定质量分数的Si粉为原料,经发泡工艺制备含Si的炭泡沫预制体并结合反应烧结工艺制得碳化硅多孔陶瓷. 对碳化硅多孔陶瓷的微观形貌、相组成、孔隙率、孔筋密度和抗弯强度进行分析与测试. 结果表明:随着炭泡沫预制体中Si量的增加,碳化硅多孔陶瓷的孔隙率下降,孔筋密度增加,抗弯强度提高. 当Si的质量分数为50%时,多孔陶瓷孔筋完全由SiC相组成,孔筋密度为3.14g/cm3,多孔陶瓷的抗弯强度达到23.9MPa,对应孔隙率为55%.     

Study on high efficient electric discharge milling of silicon carbide ceramic with high resistivity

A new method which employs a group pulse power supply for electric discharge milling of the silicon carbide ceramic with the resistivity of 500 Ω·cm is presented. Due to the good machining stability and high pulse utilization, the material removal rate （MRR） can reach 72.9 mm^3/min. The effects of high-frequency pulse duration, high-frequency pulse interval, peak voltage, peak current, polarity, rotate speed and group frequency on the process performance have been investigated. Also the EDMed surface microstructure is examined with a scanning electron microscope （SEM）, an X-ray diffraction （XRD）, an energy dispersive spectrometer （EDS） and a micro hardness tester. The results show that the conditions of smaller high-frequency pulse duration and pulse interval, higher peak voltage and peak current, and positive tool polarity are suitable for machining the SiC ceramic. The optimal rotate speed is 1090 r/min and the preferable group frequency is 730 Hz. In addition, there is a small quantity of iron on machined surface when machining with steel electrode. The average grain size of the EDMed surface is smaller than that of the unprocessed, and the micro hardness of machined surface is superior to that of the unprocessed.     

Negative differential resistance in single-walled SiC nanotubes

A two-probe system was established for a finite （7, 0） silicon carbide （SIC） nanotube coupled to Au （111） surfaces via Au-C bonds. Using the non-equilibrium Green function （NEGF） combined with density functional theory （DFT）, the above system was studied for its electronic transport properties. Negative differential resistance （NDR） was observed when the bias voltage was greater than 1.4 V. Because the transport properties of the system were sensitive to the applied bias voltage, NDR might be caused by the fluctuation of the transmission coefficient with the bias voltage.     

Electron theory analysis about the influence of Cr and Mn on the carbide spheroidization by heat treatment

It is found that the distribution of covalent electron pair number and covalent bond energy was nonuniform in carbides Fe 3C, (Fe,Cr) 3C and (Fe,Mn) 3C, in which the energy difference between the strongest and the weakest bonds is very great. A criterion of covalent bond breaking during the heat treatment of M 3C pattern carbide is presented as follows. If the heat energy \%E\-\%w from heat treatment is higher than the diffusion activation energy \%Q\% of carbon atom in γ_Fe, the less the covalent bond energy \%E\% α is, the earlier the covalent bond α will be broken. According to the criterion, why the breaking of (Fe,Mn) 3C network was easier than that of Fe 3C network and why the breaking of (Fe, Cr) 3C network was more difficult than that of Fe 3C network are explained. It is indicated that Mn promoted the carbide to be spheroidized and Cr hindered the carbide from spheroidization during the heat treatment of carbides spheroidization.     

反应烧结碳化硅陶瓷内筒的损毁机制

以X射线衍射仪、扫描隧道电子显微镜、能量散射光谱仪等手段对在悬浮预热器内筒上使用前后的反应烧结碳化硅陶瓷进行分析,研究该陶瓷应用于悬浮预热器上的损毁机制。碳化硅陶瓷中残存金属硅和表面的碳化硅在高温使用工况下首先氧化成SiO2,SiO2在K2 O( g)、Na2 O( g)、KCl( g)、NaCl( g)等蒸气以及氯化物作用下黏度降低,形成覆盖于陶瓷表面的氧化层,继而被高速的气固流体冲蚀和磨损掉,并导致新的界面出现。如此循环,使碳化硅陶瓷的外侧逐渐变薄和断裂,直至损毁。提高陶瓷的致密性和降低残余硅含量是改进反应烧结碳化硅陶瓷在悬浮预热器中使用性能的有效途径。     

Effect of graphite powder as a forming filler on the mechanical properties of SiCp/Al composites by pressure infiltration

(38vol% SiCp + 2vol% Al₂O_3f)/2024 Al composites were fabricated by pressure infiltration. Graphite powder was introduced as a forming filler in preform preparation, and the effects of the powder size on the microstructures and mechanical properties of the final composites were investigated. The results showed that the composite with 15 μm graphite powder as a forming filler had the maximum tensile strength of 506 MPa, maximum yield strength of 489 MPa, and maximum elongation of 1.2%, which decreased to 490 MPa, 430 MPa, and 0.4%, respectively, on increasing the graphite powder size from 15 to 60 μm. The composite with 60 μm graphite powder showed the highest elastic modulus, and the value decreased from 129 to 113 GPa on decreasing the graphite powder size from 60 to 15 μm. The differences between these properties are related to the different microstructures of the corresponding composites, which determine their failure modes.     

高铬铸铁型自保护药芯焊丝堆焊组织与性能分析

研制了一种自保护高铬铸铁型药芯焊丝,对其堆焊金属组织与性能进行了分析,结果表明:堆焊金属表面硬度达到HRC60以上,堆焊金属显微组织主要为马氏体+残余奥氏体+M1C3,型碳化物;初生碳化物主要沿堆焊层向母材方向生长,其表面硬度为HVl783,侧面为HVll27:共晶碳化物围绕在初生碳化物周围生长,其显微硬度为HV830:在相同磨损条件下磨损1h后,堆焊金属相对耐磨性为Q235钢的14倍左右,在药芯中加入适量的稀土氧化物能提高堆焊金属的耐磨性.     

新型TiC钢结硬质合金致密化技术

采用行星式球磨机对Fe-3.0Cr-3.0Mo-0.5Cu.0.5C-33TiC新型钢结硬质合金混合粉末进行高能球磨,对烧结后的合金进行热等静压和锻造处理,研究球磨时间、热等静压及锻造工艺对合金组织、性能及孔隙度的影响.研究结果表明:高能球磨使粉末细化和成分均匀化,促进烧结进程,随球磨时间的延长,合金组织细化且致密;热等静压和锻造工艺可进一步减小和消除合金中烧结后残留的孔洞等缺陷;锻造作为最后工艺,可使TiC颗粒之间的连接破碎,促使硬质相颗粒弥散分布,从而改善烧结组织,提高合金的致密性及力学性能.