Deformation Localization and Shear Fracture of a Rapidly Solidified Al-Fe-V-Si Alloy at Elevated Temperature

The tensile and fatigue behavior of a dispersoid strengthened, powder metallurgy Al-Fe-V-Si alloy at ambient and elevated temperatures was investigated. The results show that the strength and ductility of the alloy decrease significantly with increasing temperature and decreasing strain rate. Micro-structural examinations reveal that this change in mechanical behavior with increasing temperature is related to the mode of deformation of the alloy. Further observations show that localized shear deformation is responsible for the losses in both strength and ductility of the alloy at elevated temperature.     

In-situ Dendrite/Metallic Glass Matrix Composites:A Review

<正>The advanced fabrication of in-situ dendrite/metallic glass matrix(MGM) composites is reviewed.Herein,the semisolid processing and Bridgman solidification are two methods,which can make the dendrites homogeneously dispersed within the metallic glass matrix.Upon quasi-static compressive loading at room temperature,almost all the in-situ composites exhibit improved plasticity,due to the effective block to the fast propagation of shear bands.Upon quasi-static tensile loading at room temperature,although the composites possess tensile ductility, the inhomogeneous deformation and associated softening dominates.High volume-fractioned dendrites and network structures make in-situ composites distinguishingly plastic upon dynamic compression.In-situ composite exhibits high tensile strength and softening(necking) in the supercooled liquid region,since the presence of high volume-fractioned dendrites lowers the rheology of the viscous glass matrix at high temperatures.At cryogenic temperatures,a distinguishingly-increased maximum strength is available;however,a ductile-to-brittle transition seems to be present by lowering the temperature.Besides,improved tension—tension fatigue limit of 473 MPa and four-point-bending fatigue limit of 567 MPa are gained for Zr_(58.5)Ti_(14.3)Nb_(5.2)Cu_(6.1)Ni_(4.9)Be_(11.0) MGM composites. High volume-fraction dendrites within the glass matrix induce increased effectiveness on the blunting and propagating resistance of the fatigue-crack tip.The fracture toughness of in-situ composites is comparable to those of the toughest steels and crystalline Ti alloys.During steady-state crack-growth,the confinement of damage by in-situ dendrites results in enhancement of the toughness.     

Effects of Microstructure on the Tensile, Fracture Toughness and Fatigue Behaviour of Gamma Titanium Aluminides

The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour is analyzed for deformation temperatures between RT and 950℃, Fracture resistance behaviour and toughening mechanisms at RT and 800℃ are analyzed. and the inverse relationship botween ductility and toughness is explained using the crack initiation toughness. The preliminary results of load-controlled fatigue behaviour at 800℃ are interpreted using the tensile behaviour because deformation structure and fracture modes are similar under these two loading conditions     

A Damage-tolerant Bulk Metallic Glass at Liquid-nitrogen Temperature

Tensile tests and notch toughness tests were conducted on Zr61Ti2Cu25Al12 glass （ZT） at room temperature and liquid-nitrogen temperature. The tensile strength of ZT was improved from 1.63 GPa at room temperature to 1.72 GPa at liquid-nitrogen temperature. Micro-notches with a root radius of 1-3μm were introduced to test the notch toughness of ZT at room temperature and liquid-nitrogen temperature. The test results revealed that the notch toughness of ZT at liquid-nitrogen temperature is comparable to that of ZT at room temperature. The combination of high yield strength and notch toughness of ZT at liquid-nitrogen temperature is comparable to that of the best cryogenic engineering materials.     

Effect of Post-weld Heat Treatment on Properties of Friction Welded Joint Between TC4 Titanium Alloy and 40Cr Steel Rods

Dissimilar metal joining of Ti–6Al–4V(TC4) titanium alloy to as-rolled 40 Cr steel rods was conducted with friction welding, and the effect of post-weld heat treatment(PWHT) on the microstructure and mechanical properties of the resultant joints was investigated. The average tensile strength of the as-welded joints reached 766 MPa and failure occurred in 40 Cr steel base metal. However, after PWHT at 600 °C for 0.5, 1, 2 and 3 h, the tensile strength of the joints decreased and fracture happened through the interface with quasi-cleavage features. The bending angle of specimens was improved from 9.6° in as-welded state to 32.5° after PWHT for 2 h. The tensile strength of the joint was enhanced by martensitic transformation near the interface in as-welded state. Sorbite formed near the interface in PWHT state and improved the bending ductility of the joint. Ti C brittle phase formed at the interface after PWHT for 0.5 h and deteriorated the tensile strength and bending ductility of the joint. After PWHT for 2 h, no Ti C phase was detected at the interface. The microhardness on the interface in as-welded state was higher than that after PWHT, indicating that the decrease of microhardness around the interface could be accompanied by degradation of tensile strength but improvement of bending ductility of the joints.     

Comparison of Impact Properties for Carbon and Low Alloy Steels

The impact properties of hot rolled carbon steel(used for the manufacture of reinforcement steel bars) and the quenched & tempered(Q&T) low alloy steel(used in the pressure vessel industry) were determined.The microstructure of the hot rolled carbon steel contained ferrite/pearlite phases,while that of the quenched and tempered low alloy steel contained bainite structure.Impact properties were determined for both steels by instrumented impact testing at temperatures between-150 and 200 C.The impact properties comprised total impact energy,ductile to brittle transition temperature,crack initiation and propagation energy,brittleness transition temperature and cleavage fracture stress.The Q&T low alloy steel displayed much higher resistance to ductile fracture at high test temperatures,while its resistance to brittle fracture at low test temperatures was a little higher than that of the hot rolled carbon steel.The results were discussed in relation to the difference in the chemical composition and microstructure for the two steels.     

Influences of Ag and Au Additions on Structure and Tensile Strength of Sn-5Sb Lead Free Solder Alloy

It is important, for electronic application, to decrease the melting point of Sn-5Sb solder alloy because it is relatively high as compared with the most popular eutectic Pb-Sn solder alloy. Adding Au or Ag can decrease the onset melting temperature （233℃） of this alloy to 203,5℃ and 216℃, respectively. The results indicate that the Sn-5Sb-i.5Au alloy has very good ultimate tensile strength （UTS）, ductility, and fusion heat, which are better than both those of the Sn-5Sb-3.SAg and Sn-5Sb alloys. The formation of intermetallic compounds （IMCs） AuSn4 and Ag3Sn enhanced the SbSn precipitates in the solidification microstructure microstructure stability, while retained the formation of thus significantly improved the strength and ductility For all alloys, both UTS and yield stress （σy） increase with increasing strain rate and decrease with increasing temperature in tensile tests, but changes of ductility are generally small with inconsistent trends.     

Correct Interpretation of Creep Rates: A Case Study of Cu

Traditionally the deformation resistance in creep is characterized by the minimum creep rate εminand its sensitivity to stress(stress exponent n) and temperature(activation energy Q). Various values of constant n have been reported in the literature and interpreted in terms of specific mechanisms. The present case study of coarse-grained Cu at 573 K yields a stress exponent n = 9 for εminin tension and a relatively low activation energy. The evolution of the deformation resistance with strain at constant tensile creep load and comparison with creep in compression without fracture indicates that the tensile εminresult from transition from uniform deformation to strain localization during fracture. This is confirmed by the results of creep in compression where fracture is suppressed. Both the tensile εminand the compressive creep rate at strains around 0.3 can be described using existing equations for quasi-stationary deformation containing the subgrain boundary misorientation θ as structure parameter. While in the latter case constant θ leads to monotonic increase of n with stress, the tensile nine-power-law results from variable θ, and has no simple meaning. The result of this case study means that uncritical interpretation of minimum tensile creep rates as stationary ones bears a high risk of systematic errors in the determination of creep parameters and identification of creep mechanisms.     

Polybutylacrylate/poly (methyl methacrylate) Core-Shell Elastic Particles as Epoxy Resin Toughener: Part Ⅱ Toughness on DGEBA/DDM system

Mechanlcal properties of epoxy resin were investigated by adding core-shell elastic particles (CSEP). The results indicated that optimized core-shell ratio was 60/40 and the loading volume of CSEP was 10 phr (per hundred parts of epoxy resin by weight). The impact strength of modified systems increased apparently with the decrease of core sizes. However, the shearing strength changed gently with the particle sizes. CSEP with lightly crosslinked rubbery core showed more effectiveness on toughness than others. With solution blending, CSEP could be dispersed in epoxy matrix well, and the morphologies of dispersed rubber domains were controlled perfectly by CSEP whose structure was predesigned. A cavitation-shearing band toughness mechanism was observed from the SEM micrographs of fracture surface. It also was found that the deforming temperature (DT) of modified epoxy did not decline apparently.     

Flexural and Impact Resistance of FRC／Bamboo Laminate

The flexural and impact resistance of a newly developed FRC/bamboo laminate have been investigated.The laminate considered in this study was combined with reformed bamboo plate and extruded fiber reinforced cementitious (FRC) sheet.Innovated from the raw bamboo,reformed bamboo showed high tensile strength and high strength to weight ratio.It can not only remarkably strengthen the FRC sheet but also reduce the total weight of the laminate.Flexural and impact load,broken energy,deflection and duration were measured.Test results showed that the flexural strength value for the laminate can be improved to greater than 90MPa,while the impact resistance is increased more than 10 times for the laminate when compared with the FRC sheet only.     

聚乙二醇对环氧胶衣温度冲击开裂性能的影响

为开发一种满足航空环境要求的新型环氧胶衣,首先,研究了不同分子量、不同含量的聚乙二醇(PEG)改性环氧胶衣在室温下的冲击韧性及冲击断面形貌;然后,采用一种简单温度冲击试样模型,着重研究了PEG改性环氧胶衣在温度冲击(-50℃/30 min+90℃/30 min)下的开裂性能,并通过分析热膨胀及储能模量等的变化趋势对其进行了验证。结果表明:5wt% PEG-1000改性环氧胶衣的冲击强度比未改性环氧胶衣的提高了31.8%,达到最高值4.97 kJ·m-2;PEG-1000和PEG-2000改性环氧胶衣的冲击断面呈现出更明显的塑性变形形貌,胶衣的冲击韧性得到提高;15wt% PEG-400改性环氧胶衣在温度冲击下的宏观初始开裂性能达到最佳,平均初始开裂需要进行10.4次循环;改性环氧胶衣的耐开裂扩散性能基本上随着PEG分子量和含量的增加而提高,但裂纹宽度也会随PEG分子量的增加而扩大。      

Synergistic effects of oxidized CNTs and reactive oligomer on the fracture toughness and mechanical properties of epoxy

The soft modifiers added to improve the fracture toughness of epoxies generally deteriorate their mechanical properties. Hence, oxidized multi-walled carbon nanotubes (O-CNTs) were added to the epoxy modified with reactive oligomer. The NCO terminated reactive oligomer acts as a cross-linker between the O-CNTs and the OH groups of the epoxies. The impact strengths of the 15 wt.% oligomer modified epoxy containing 0.5 wt.% of O-CNTs at room temperature (RT) and cryogenic temperature (CT) are enhanced by 23.6% and 69.5% compared to that of the unmodified epoxy. In addition to increasing fracture toughness, the tensile strength (TS) of the modified epoxy/O-CNTs at CT is found to be 91.7 MPa, which is comparable to that of the unmodified epoxy (92.1 MPa). Hence, the attachment of O-CNTs to the reactive oligomer modified epoxy can be an efficient approach to toughen epoxy resins without compromising their tensile properties.     

混合型固化剂对环氧树脂室温和低温力学性能的影响

研究了一种刚性和柔性胺混合型固化剂（芳香胺DETD和聚醚胺D-400）固化环氧树脂浇铸体的力学性能、材料断裂表面的微观形貌和玻璃化转变温度等性能。结果表明:当D-400加入量占固化剂总量的40%时,其室温拉伸强度呈现最大值,为82.52 MPa,弹性模量为2.30 GPa,与未加D-400的体系相比分别提高了6.3%和14.4%,其低温冲击强度提高了14%。对冲击断面形貌进行扫描电子显微分析表明:D-400的加入致使断口形貌变得粗糙,抗开裂能力得到提高。热分析实验结果显示,体系的玻璃化转变温度随着D-400含量的增加而降低。此外,还探讨了环氧树脂体系低温增韧机制。      

The reinforcing effect of graphene nanosheets on the cryogenic mechanical properties of epoxy resins

The reinforcing effect of graphene in enhancing the cryogenic tensile and impact properties of epoxy composites is examined at a weight fraction of 0.05–0.50%. The micro-structure and cryogenic mechanical properties of the graphene/epoxy composites are investigated using scanning electron microscopy, transmission electron microscopy, small-angle X-ray scattering and mechanical testing techniques. The results show that the graphene dispersion in the epoxy matrix is good at low contents while its aggregation takes place and becomes severer as its content increases. And the cryogenic tensile and impact strength at liquid nitrogen temperature (77 K) of the composites are effectively improved by the graphene addition at proper contents. The cryogenic Young’s modulus increases almost linearly with increasing the graphene content. Moreover, the results for the mechanical properties at room temperature (298 K) of the graphene/epoxy composites are also presented for the purpose of comparison.     

Micro-crack behavior of carbon fiber reinforced thermoplastic modified epoxy composites for cryogenic applications

Three different types of thermoplastics, poly(ether imide) (PEI), polycarbonate (PC), and poly(butylene terephthalate) (PBT) were used to modify epoxy for cryogenic applications. Carbon fiber reinforced thermoplastic modified epoxy composites were also prepared through vacuum-assisted resin transfer molding (RTM). Dynamic mechanical analysis (DMA) shows that the storage moduli of PEI, PC, and PBT modified epoxies are 30%, 21%, and 17% higher than that of the neat epoxy respectively. The impact strength of the modified epoxies at cryogenic temperature increases with increasing thermoplastic content up to 1.5 wt.% and then decreases for further loading (2.0 wt.%). The coefficient of thermal expansion (CTE) values of the PBT, PEI, and PC modified epoxies also decreased by 17.76%, 25.42%, and 30.15%, respectively, as compared with that of the neat epoxy. Optical microscopy image analysis suggests that the presence of PEI and PC in the carbon fiber reinforced epoxy composites can prevent the formation of micro-cracks. Therefore, both the PEI and PC were very effective in preventing micro-crack formation in the composites during thermal cycles at cryogenic condition due to their low CTE values and high impact strength.     

石墨烯-多壁碳纳米管协同增强环氧树脂复合材料的低温力学性能

为了提高环氧树脂的低温力学性能,采用石墨烯与多壁碳纳米管(MWCNTs)协同改性环氧树脂,系统研究了石墨烯-MWCNTs/环氧树脂复合材料的室温(RT)和低温(77K)力学性能。结果表明:当石墨烯的质量分数为0.1wt%,MWCNTs的质量分数为0.5wt%时,纳米填料的加入可同时改善环氧树脂的低温拉伸强度、弹性模量和冲击强度;在此最佳含量下,石墨烯-MWCNTs/环氧树脂复合材料在RT和77K时的拉伸强度皆达到最大值,比纯环氧树脂的拉伸强度分别提高了11.04%和43.78%。石墨烯和MWCNTs能协同提高环氧树脂的低温力学性能。     

可溶性聚醚醚酮对复合环氧/DDM的改性

热熔法制备了可溶性聚醚醚酮（s-PEEK）/E-51/多官能度环氧树脂复合体系,测试了体系的冲击强度、高温拉伸剪切强度和玻璃化温度,用扫描电镜（SEM）观察体系的微观结构,并与聚醚醚酮（PEEK）改性进行对比。结果表明,复合环氧体系加入两种聚醚醚酮后冲击强度下降,但含量为5g时出现较大值;体系的玻璃化温度随着s-PEE...     

Microstructure and mechanical properties of epoxy hybrid nanocomposites modified with acrylic tri-block-copolymer and layered-silicate nanoclay

In this study, processing, morphology and mechanical properties of acrylic tri-block-copolymer and organophilic layered-silicate nanoclay modified epoxy hybrid nanocomposites were investigated. The acrylic tri-block-copolymer preferentially self-assembled into spherical micelles in the epoxy matrix, and predominantly intercalated and few exfoliated platelets were observed with nanoclay. Three-phase ternary nanocomposites showed coexistence of both intercalated nanoclay and nanostructured block-copolymer in epoxy. Experimental results revealed that the block-copolymer significantly enhanced fracture toughness. Increased toughness of epoxy coincided with a reduction of tensile stiffness and strength. The nanoclay filled nanocomposites exhibited superior stiffness and slight improvement in tensile strength while compromising ductility. Optimum property enhancement was observed in the case of epoxy hybrid nanocomposites. Mechanical properties of the hybrid nanocomposites depend on microstructure, dispersion state and the ratio between organic and inorganic nanofiller contents.     

Cryogenic mechanical behaviors of carbon nanotube reinforced composites based on modified epoxy by poly(ethersulfone)

Cryogenic mechanical properties are important parameters for epoxy resins used in cryogenic engineering areas. In this study, multi-walled carbon nanotubes (MWCNTs) were employed to reinforce diglycidyl ether of bisphenol F (DGBEF)/diethyl toluene diamine (DETD) epoxy system modified by poly(ethersulfone) (PES) for enhancing the cryogenic mechanical properties. The epoxy system was properly modified by PES in our previous work and the optimized formulation of the epoxy system was reinforced by MWCNTs in the present work. The results show that the tensile strength and Young’s modulus at 77 K were enhanced by 57.9% and 10.1%, respectively. The reported decrease in the previous work of the Young’s modulus of the modified epoxy system due to the introduction of flexible PES is offset by the increase of the modulus due to the introduction of MWCNTs. Meanwhile, the fracture toughness (K_IC) at 77 K was improved by about 13.5% compared to that of the PES modified epoxy matrix when the 0.5 wt.% MWCNT content was introduced. These interesting results imply that the simultaneous usage of PES and MWCNTs in a brittle epoxy resin is a promising approach for efficiently modifying and reinforcing epoxy resins for cryogenic engineering applications.     

Simultaneously enhanced cryogenic tensile strength and fracture toughness of epoxy resins by carboxylic nitrile-butadiene nano-rubber

Epoxy resins are important matrices for composites. Carboxylic nitrile-butadiene nano-rubber (NR) particles are employed to improve the tensile strength and fracture toughness at 77 K of diglycidyl ether of bisphenol-F epoxy using diethyl toluene diamine as curing agent. It is shown that the cryogenic tensile strength and fracture toughness are simultaneously enhanced by the addition of NR. Also, the fracture toughness at room temperature (RT) is enhanced by the addition of NR. On the other hand, the tensile strength at RT first increases and then decreases with further increasing the NR content up to 5 phr. 5 phr NR is the proper content, which corresponds to the simultaneous enhancements in the tensile strength and fracture toughness at RT. Moreover, the comparison of mechanical properties between 77 K and room temperature indicates that the tensile strength, Young’s modulus and fracture toughness at 77 K are higher than those at RT but the failure strain shows the opposite results. The results are properly explained by the SEM observation.