Materials Science Forum Vols. 675-677

Abstract: Honeycomb sandwich structures are generally designed to carry flexural loads. However, mechanical properties can be influenced by accidental impacts, or service conditions. Thus a nondestructive detection testing is desired for them. In contrast to many conventional nondestructive evaluation (NDE) techniques, acoustic emission (AE) technique permits continuous damage inspection, classification and identification of failure modes in real time. In this work the fracture process of steel honeycomb sandwich beams has been investigated by in-situ AE technique. Pre-cracks were made both for L-direction and W-direction specimens subjected to three-points bending loads. Damage initiation sites were observed in the vicinity of the crack tip. A series of curves among the AE hits, AE amplitude, AE energy and loading time were obtained. Damage characteristics were discussed based on the above parameters. The results indicate AE characteristic parameters can reflect the damage and failure process of specimens. A good agreement was found between the experimental and analytical results.

Abstract: The effect of dissimilar welding of 7050/2024 aluminum alloy on mechanical properties and microstructure were analyzed and investigated. The post-welding heat treatments of weldments were carried out and recorded for microstructure, tensile test, hardness and fracture surface in different conditions. The experimental results indicate that the post-welding heat treatment, filler metal chose can increase the mechanical properties. The microstructure and fracture surface of dissimilar welding can evaluate the distribution of precipitation strengthening phase in fusion zone, heat affected zone and its fracture type.

Abstract: The paper reports an analysis of the effect of high current pulsed electron beam(HCPEB) on microstructure transformations and wear resistance of hypereutectic Al-Si alloys. HCPEB treatment with 2.5 J /cm2 energy density leads to the formation of “halo” centered on primary Si, composition homogeneity, the formation of supersaturated solid solution of Al and grain refinement of top melted surface layer. The wear resistance of 15 pulse-treated Al-17.5Si and Al-20Si alloys is drastically improved by a factor of 6.5 and 2, respectively. The increase of hardness in modified surface layer has a positive effect on wear of hypereutectic Al-Si alloys.

Abstract: SAPs (Super Absorbent Polymers) are a group of polymeric materials that have the ability to absorb a significant amount of liquid many times their own weight from the surroundings and to retain the liquid within their structure without dissolving and to desorb it in the dry environment. The shrinkage and cracking performance of SAP-modified concrete was studied by experimental investigation, which express as total cracking area from early-age plastic cracking tests and shrinkage rate (restrained and unrestrained) from shrinkage tests respectively. The effect of SAP on shrinkage and cracking of concrete was compared with that of using pre-soaked ceramsite or perlite. The results show that SAP can effectively improve the cracking and shrinkage of concrete with achieving a 30-50% decrease. SAP particles improve the shrinkage and cracking performance of concrete in three ways as absorption-desorption, some absorbed water being bonded with hydrogen bond of macromolecule and filmforming on SAP particles. The coupling effect of SAP retards the water losing and change the moisture distribution in concrete, and thus reducing the cracking and shrinkage of concrete.

Abstract: Phosphoaluminate cement (PAC) is a kind of new cementitious material which has many special properties compared to Portland cement (PC). PAC sets quickly and develops early-high strength. In order to investigate the hydration mechanism, the hydration products and microstructure of PAC were studied with x-ray diffraction (XRD), electron scanning microscope (SEM) and electrochemical impedance spectroscopy (EIS). Heat evolution of PAC was also measured. The results show that the hydration mechanism of PAC is different from Portland cement, which is caused by the special minerals including CxP, CA(P), phase L, and so on. The main hydration products of PAC are calcium phosphorus aluminates hydrate (C-A-P-H), calcium phosphate hydrate (C-P-H), aluminates hydrate (C-A-H), the corresponding hydration microcrystal as well as gels. Also, there is no calcium hydroxide produced during hydration. The hydration procedure of PAC is divided into four stages which are dissolution and induction, acceleration, deceleration, stabilization.

Abstract: Steel Fiber Reinforced Self-stressing Concrete (SFRSSC for short) is a new type of high performance cementitious composite with self-expansive performance and high tensile resistance. It can be used as a joint material in the new bridge construction or the old bridge rehabilitation. Because when SFRSSC is restrained by steel bars and other terminal conditions, it can create chemical pre-stressing force to enhance the cracking moments of the continuous beams. For purpose of utilizing the properties of SFRSSC, the primary goal of this research is to apply SFRSSC as a joint material to build continuous bridges. Firstly, the model experiments of 8 continuous Tbeams with SFRSSC layers are carried out. Secondly, based on the model experiments, flexural performance of the beams reinforced by SFRSSC layers is investigated. Owing to enhancement of steel fibers and self-stress induced by steel bars, the layers greatly improve the first-crack strength and stiffness of the continuous T-beams. The test results obviously indicate that the composite SFRSSC-RC continuous T-beams enhance the crack moment 51.4%～121% more than conventional concrete continuous beams. Furthermore, SFRSSC can help cancel out the relative deformation and stress due to new concrete shrinkage between new and existing concrete during the process transforming simply supported beams into continuous beams. It is concluded that flexural performance of continuous T-beams strengthened by SFRSSC is more greatly improved than that strengthened by conventional concrete.

Abstract: Three types of TiAl-Nb ternary alloys are obtained by arc-melting and heat treatment, which are γ-TiAl single phase, γ-TiAl + α2-Ti3Al duplex phase and γ-TiAl +α2-Ti3Al +Nb2Al multiple phase alloy. The phase stability is studied using X-ray diffractometer and electron probe microanalyzer. Mechanical properties are investigated through compress and tensile tests at RT to 1373K. It is found that, their mechanical properties are related to their microstructures. The deformability of alloy B is better than that of alloy A and C at RT due to small grain size reduced by α2 phase appearance, lamellar microstructure and interfaces. The deformability of alloy B increases significantly with increasing temperature, the elongation of alloy B achieves 40.4% at 1173K. The fracture mode of alloy B changed from brittle transgranular failure at room temperature to ductile intergranular failure at 1173K. While, both of alloy A and C have not increased its deformability at 1173K and showed brittle transgranular failure at both temperatures. Alloy C was a very brittle material at room and high temperature due to Nb2Al phase appearance, which reducing the continuity of (γ+α2) lamellar structure.

Abstract: This paper presented how to build finite element model based on UG NX, and simulated the cold expanding process by DEFORM software. The deformation, distribution and development trend of velocity, equivalent stress, equivalent strain and equivalent strain rate were predicted. The punch’s load-time variation curves in X, Y and Z direction were also obtained.

Abstract: The microstructure evolution during annealing in large strain deformed Fe-32%Ni alloy was investigated by transmission electron microscope (TEM). Firstly, the Fe-32%Ni alloy was subjected to multi-axial forging at temperature of 773K and a strain rate of 10-3 s-1 to cumulative strain of 9.0, and then the large strain deformed specimens were annealed at temperature of 973K with different time. The results show that the grains of Fe-32%Ni alloy were obviously refined by severe plastic deformation, and the ultra-fine grains with mean size of about 300nm were obtained when the cumulative strain amounted to 9.0. The large strain deformed microstructure evolves homogeneously and gradually to equiaxed structures with straight and smooth grain boundaries when annealed at temperature of 973K, and there is no observable nucleation stage found during annealing. The annealing process involves two sequential processes i.e. recovery followed by normal grain growth, and the microstructure evolution mechanism is considered as continuous recrystallization.

Abstract: Selective laser melting (SLM) is an advanced manufacturing technology, which is flexible in building three-dimensional (3D) metallic parts. In this work, SLM experiment of a multicomponent Ni-based composite powder, which consisted of Ni, Cr, Fe, and Al powders, was conducted with favorable forming ability. The SEM, EDX, and XRD analysis were used to characterize the surface morphology, microstructure, and phase structure of as-formed Ni-based alloy. The XRD analysis showed that the as-received phase structure was Ni based solid solution. The SEM analysis of surface morphology revealed that metal agglomerates or balls were very easily formed in SLM surface, between which some pore channels existed. The surface condition and porosity could be improved by increasing laser energy input, because of a higher molten temperature and accordingly better flowing and flatting characteristics. The SEM analysis of microstructure showed that the crystalline grains were in cellular and columnar shape. Moreover, the grains were very fine with average dimensions about 5μm, due to the rapid cooling rate with rapid laser beam moving. The EDX analysis illustrated that the element contents of starting powder were uniformly distributed in as-prepared sample. A case investigation into SLM of this composite powder to form an impeller was also performed.