Synthesis of γ-TiAl based alloy by mechanical alloying and reactive hot isostatic pressing

Elemental Ti-48Al-2Cr-2Nb powders were mechanically alloyed (MA) for 8 h and 45 h. The MA powders were then consolidated by reactive hot isostatic pressing (HIP). The microstructure of the HIPed materials consisted of equiaxed γ-TiAl and α₂-Ti₃Al phases. During the high-temperature annealing of the HIPed materials, the α₂-Ti₃-Al phase transformed into a lamellar structure consisting of alternating laths of α₂-Ti₃Al and γ-TiAl. It is suggested that a high content of interstitial elements together with the microalloying elements of niobium and/or chromium in MA powders raises α/(α + γ) transus to a higher temperature.     

Estimation of impact-induced damage in CFRR laminates through ultrasonic imaging

Ultrasonic C-scans are used frequently to detect defects in composite components caused during fabrication and damage resulting from service conditions. Evaluation and characterization of defects and damage require experience and good understanding of the composites. Under low-velocity impact loading delamination is observed to be a major failure mode. In this paper, the results of extensive experimental work on damage of carbon fiber reinforced plastic (CFRP) laminates due to low-velocity impact are presented. The study was carried out on 10 different layups over four different thickness. Both drop-weight and projectile impacts simulating practical conditions like tool-drop and runway debris, respectively, were carried out at energy levels varying from 3 to 30 J. The ensuing delamination damage was determined by ultrasonic: C-scans using the pulse-echo immersion method for both projected and layer-wise distribution. While projected delamination was obtained by placing a gate over the backwall echo, layerwise distribution was obtained by successive time delay from the frontwall to the backwall echo covering each interface. Delamination areas were quantified accurately by processing the raw image data using a digital image processing technique. Based on the data obtained an empirical relation was established between the delamination area and the impact energy.     

Effect of microstructure on environmental embrittlement in a TiAl base alloy

The effect of different microstructure on environmental embrittlement in a chromium-containing TiAl alloy has been analyzed. The alloys having fully lamellar, nearly lamellar and duplex structure exhibited no apparent difference in the fracture strength and the tensile elongation at two test environments, vacuum and air. The near gamma alloy shows more strength higher and ductility in the vacuum than in the air. Tensile ductility increased to 3.5% in the vacuum, compared with 0.3% in the air. It can be argued that the gamma phase itself has relatively high intrinsic ductility, although a relationship with the ductility of alpha-2 is not yet clarified. An important factor to determine the environmental effect is the morphology and the volume fraction of alpha-2 phase in the alloy.     

MICROSTRUCTURE AND MECHANICAL BEHAVIOUR OF Al-Li-Zr AND Al-Li-Cu-Mg-Zr ALLOYS

The aging response,tensile and impact properties of two kinds of Al-Li based alloys havebeen studied.The microstructure,deformation as well as fracture behaviour in the alloys wereobserved with SEM and TEM.It was found that the mechanisms of deformation and fracturefor different heat-treated alloys with the same chemical composition are quite different.Thecauses leading to the drop of ductility,toughness as well as anisotropy in peak-aged alloyshave been analysed.Finally,possible methods to improve the ductility and toughness of the al-loys have been discussed.     

MECHANISM OF NON-EQUILIBRIUM SEGREGATION OF SOLUTE TO GRAIN BOUNDARIES

本文以空位-溶质原子拖曳机制为基础,并考虑了空位、溶质原子和复合体三者的反应平衡及晶界平衡偏聚的作用,得到了溶质原子的非平衡晶界偏聚动力学方程。对硼在奥氏体中等温及连续冷却时所产生的非平衡晶界偏聚现象进行了理论计算,得到了与实验较为一致的结果。     

Computer modeling metal flow in forging

By simulating the flow of metal in forging, engineers can eliminate defects and undesirable flow patterns. Localized deformation heating of the forging can be predicted and avoided, with a resulting improvement of quality.     

Characterizing the interface of nickel-zincate-aluminum thin films

The manufacture of plated computer memory disks requires a complex sequence of processing steps. One of the most important factors affecting disk surface topography and adhesion between nickel and aluminum was found to be the formation and diffusion of a zincate thin film at the nickel-aluminum interface. To improve the adhesion, the strength of the weak link (the zinc layer) must be increased. This can be accomplished by alloying the zinc layer with the aluminum and nickel layers by co-diffusion, which is accelerated at high temperatures, or by keeping the zinc layer very thin. Surface topography can be modified by changing the metallurgical properties of the substrate, promoting the diffusion of elements between layers at elevated temperatures, or adjusting the zincating bath parameters.

     

A new approach to the network modelling of capacitive irises and steps in waveguide

Classical network representations of waveguide discontinuities assume monomode operation and involve elements which depend not only upon geometry, but also on frequency. Later approaches to the problem consist of numerical techniques. These are unsuitable for synthesis since they do not yield equivalent networks and involve repeating the field analysis at each frequency, which is cumbersome for applications to wideband design. A recent approach permits to derive wideband equivalent networks with elements which depend only upon geometry. In this contribution capacitive irises, steps and combinations are treated. These discontinuities occur in filters, transformers, etc. where higher-order mode interaction cannot be neglected for closely spaced discontinuities. Whatever this spacing may be, higher order mode interaction always occurs in oversize guide. Higher order mode interaction fits naturally into the network modelling presented here. The resulting network representations are either in canonical Foster form of the first type or easily reducible to it. This approach is based on a Rayleigh–Ritz variational solution of the field problem and involves manipulations with small matrices only. The accuracy of the method is demonstrated for a number of basic configurations, while its flexibility is illustrated by providing simpler equivalent networks, applicable when extreme accuracy is not required. The elements involved are given in graphical or in simple analytical form.     

Antiseizure properties of aqueous solutions of compounds forming liquid crystalline structures

Amphiphilic compounds composed of a hydrophilic part and a hydrophobic alkyl chain were investigated. The compounds—sodium lauryl sulfate (SLS) and ethoxylated sodium lauryl sulfate (ESLS)—exhibit strong affinity for solid surfaces and form liquid crystalline structures in water. It is expected that they may become effective additives that could significantly modify antiseizure properties of water. They may extend the possibilities for applying water as an ecological base for lubricating substances. Rheological and structural X-ray studies as well as measurements of conductance and wetting angle were carried out. Their aim was to verify and validate the structures being formed in aqueous solutions of the compounds studied. Tribological studies were conducted to measure friction torque as a function of linearly increasing load by means of a four-ball apparatus. The tests were followed by measurement of the wear scar diameter on the balls. Based on the results obtained, antiwear properties were characterized by seizure and scuffing loads as well as by limiting pressure of seizure. The test results are highly surprising. The systems in which simple one-component aqueous solutions were the lubricating substance did not undergo seizure up to the maximum load of 8 kN designed for the tribological tester used. A number of typical antiwear additives in suitable oil bases do not exhibit such good characteristics. Also, the quantities characterizing seizure show relatively high values. Local maxima for the two compounds tested are formed in the area of low concentrations in the dependences of antiseizure properties as a function of concentration. It can be assumed that this is due to ordered structures being formed in the surface phase. The hexagonal phase of high viscosity of the order of several thousand Pa·s which forms in the bulk phase for ESLS and SLS solutions has no visible effect on antiseizure properties, whereas the existence of the lamellar phase for a 70% solution of ESLS affects an increase in the values being measured.