Ti-2Al-2.5Zr单道次皮尔格轧制过程中显微组织和织构的演化

研究了单道次皮尔格轧制过程中Ti-2Al-2.5Zr材料的变形行为和织构的演变规律。结果表明,在轧制过程中,｛102｝孪晶和柱面滑移是最容易被激活的2种变形模式,｛102｝孪晶的产生使得晶粒在轴向上的位向从<100>转向<110>。并且,在不同瞬时Q值和等效应变量下,滑移和孪生导致｛0001｝极图中最大极密度点在切向上发生变化。     

正火温度对低活化马氏体钢组织及力学性能的影响

对低活化马氏体钢丝材进行1000~1100 ℃保温60 min的正火处理,随后在790 ℃保温90 min进行回火处理,研究正火温度对低活化马氏体钢丝的显微组织及力学性能的影响。结果表明,正火后,丝材的显微组织由粒状珠光体转变为板条状马氏体,碳化物粒子大部分回溶于基体中,正火温度的升高加速碳化物粒子的回溶,在1100 ℃实现完全回溶;原奥氏体晶粒尺寸随正火温度升高显著增大(由1000 ℃的7.4 μm增至1100 ℃的34.9 μm)。回火处理后,马氏体板条尺寸变宽,板条间的位错密度显著降低,析出相沿晶界、晶粒内部析出、球化及长大,其中M₂₃C₆(M以Cr为主)相为短棒状,分布在晶界,而MX(M以Ta为主)相为椭球状,分布在马氏体板条内部。经1000 ℃×60 min正火+790 ℃×90 min回火后能够获得最佳的综合力学性能,其抗拉强度为745.7 MPa,断后伸长率为18.9%。     

HT9钢的高温蠕变性能研究

??The tensile creep of HT9 steel was measured at 700 and 800?? with different stress levels. Stress exponent was fitted by power law relation. Rupture time vs. minimum creep rate of HT9 steel was fitted by M- G relationship and modified M- G relationship. The fracture morphology after creeping and the creep mechanism and damage mechanism were analyzed by scanning electron microscopy, transmission electron microscopy and X- ray diffraction. The results showed that the minimum creep rate and creep rupture time of HT9 steel obeyed a linear relationship with the stress in double logarithmic coordinates, which could be described by M- G and modified M- G relationship. The stress exponent increased with the temperature. The dislocations bypassed the second phase particles during the creep process according to the Orowan mechanism. The fracture had a distinct dimple structure, and some of the second phase particles coarsened. The oxidation of HT9 steel was obvious during the creep at 800??. The main precipitates were M23C6 during the creep, which showed different forms, with significant differences in the size of the precipitated phases. The damage mechanism of HT9 steel included external cross- sectional area loss, material microstructure degradation, environmental damage, etc. There may also be internal sectional area loss.     

Loading sequence effect on fatigue life of Type 316 stainless steel

The change in fatigue life due to variable cyclic loading was investigated experimentally in order to consider the loading sequence effect in fatigue damage assessment for a component design, and the reason for the change was discussed. Strain-controlled fatigue tests, that is, two-step, surface removal two-step, repeating two-step and periodical overload tests were conducted using Type 316 stainless steel specimen in a room temperature laboratory environment. The high-low loading amplitude sequence for the two-step test, and the repeating two-step and periodical overload tests showed a shorter fatigue life than that predicted by the linear damage accumulation rule. On the other hand, the low–high loading amplitude sequence for the two-step test exhibited a longer fatigue life. The reduction in the fatigue life was mainly attributed to the change in effective strain amplitude. The fatigue life reduction due to the loading sequence effect could be assessed conservatively by determining the allowable number of cycles for effective stress amplitude. Namely, by assuming the crack mouth was fully opened in the assessment, predicted fatigue life became shorter than the experimental results. It was concluded that the margin of 1.3–2.3 should be considered in the design fatigue curve in order to take account of the reduction in fatigue life due to the loading sequence effect.     

Anomalous high activation energy for creep in nanostructured 3YTZP/Ni cermets

The plastic behavior of cermets based on a 3 mol% yttria-stabilized tetragonal zirconia matrix that incorporates nanometric nickel inclusions (3YTZP/n-Ni), with 2.5, 5 and 10 vol.% of nickel content, has been studied by constant load tests in compression carried out in argon atmosphere. The microstructure of these composites consists of nanometric nickel inclusions homogeneously dispersed into a fine-grained zirconia matrix (about 200 nm). The microstructural and mechanical results obtained show that the creep behavior is controlled by the zirconia matrix as in 3YTZP-based cermets with micrometric Ni inclusions (3YTZP/μ-Ni); whereas the stress exponent values are similar to those of high-purity monolithic 3YTZPs, anomalous high values of the activation energy have been measured. The ceramic/metal interface plays a crucial role for creep properties; the strong TZP/n-Ni interface matching can be at the origin of these high values of the activation energies for creep.     

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical C_f/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.     

Fabrication of ultrafine grained FeCrAl-0.6 wt.% ZrC alloys with enhanced mechanical properties by spark plasma sintering

Low mechanical strength, especially at high temperatures, is the key problem that limit the application of FeCrAl alloys as the accident tolerance fuel (ATF) cladding materials. Dispersion strengthening by carbide nanoparticles is an effective way to improve mechanical properties at high temperatures. In this work, an ultrafine grained FeCrAl-0.6 wt.% ZrC alloys with excellent mechanical properties were fabricated successfully by mechanical milling and spark plasma sintering. The effect of milling speed on powder characteristics, microstructure and mechanical properties of FeCrAl alloys were investigated. The particle size of the powders increase significantly after milling at 400 rpm, while it has a lower oxygen content. Increasing the milling speed decreased the resultant grain size and improved relative density. Transmission electron microscope (TEM) demonstrated the nano ZrC particles uniformly distributed in the matrix at higher milling speed, which effectively promotes grain refinement and dispersion strengthening. The results of mechanical properties show that the tensile strength, percentage elongation and hardness of FeCrAl-0.6 wt.% ZrC alloys at room temperature (RT) reached up to 1.05 GPa, 349.86 HV and 12.1%, respectively, after milling at 400 rpm. It is worth noting that the FeCrAl-0.6 wt.% ZrC alloy also exhibited a good high-temperature strength more than 110 MPa at 800 ℃, which is about 55.4% and 24.7% higher than previously reported FeCrAl-0.5 wt.% ZrC and FeCrAl-1.0 wt.% ZrC alloys, but the plasticity is reduced. The results demonstrated that the excellent mechanical properties were not only attributed to the dispersion strengthen by nanosized ZrC, a good interface bonding between Fe matrix and nanosized ZrC, but also the ultra-fine grained structure induced by the milling process.     

温轧FeCrAl钢退火组织演变对力学性能的影响

摘要：研究采用电子背散射衍射技术（EBSD）对温轧及退火态Fe-13Cr-4.5Al-2.2Mo-1.1Nb（质量分数,%）钢的织构、晶界类型和Laves相进行了表征,并讨论了对力学性能的影响。结果表明,300℃温轧变形,70% 的样品出现显著的变形不均匀组织,利用Taylor因子解释了不均匀变形特征,晶粒取向以变形织构α、γ和<100>//ND为主,比例分别为43.3%、39.0%和17.1%,屈服强度和抗拉强度为1298.1MPa和1371.6MPa,伸长率4%。750～800℃退火30～60min后,再结晶晶粒尺寸小于10μm,γ织构比例减少至11.9%～15.5%,此时屈服强度为790～860MPa,抗拉强度为840～890MPa,伸长率为20%左右。1000℃退火5min后再结晶晶粒明显长大,γ织构增加至39.1%,此时屈服强度、抗拉强度和伸长率分别为567.7MPa、800.7MPa和25.6%。1000℃时随退火时间增加,γ织构增加至50%以上,Laves相的钉扎是γ织构增加的原因。600℃温轧的微观组织和300℃温轧的类似,但屈服强度和抗拉强度略有下降,伸长率增加。