Microstructure of alumina ceramic/Ag–Cu–Ti brazing alloy/Kovar alloy joint

Abstract

The microstructure of the alumina ceramic/Kovar alloy joint brazed with Ag–35·2Cu–1·8Ti (wt-%) was studied. The effects of brazing temperature on the microstructure were also discussed. It was found that the microstructure of the joint brazed at 1173 K for 5 min was TiO + TiNi₃ + TiFe/eutectic Ag–Cu/TiFe₂ + TiNi₃/TiFe₂ + Cu (s.s) +Ag (s.s). When the brazing temperature was >1193 K, there was no TiO formed on the alumina ceramic/brazing alloy interface.     

Study on the grain refinement of A356 alloy by Al–3 wt-% VN master alloy

ABSTRACT

A novel Al–3?wt-% VN master alloy, mainly consisting of α-Al and VN phases, was successfully prepared by stir casting. The grain refinement performance of the master alloy on A356 alloy was then investigated. The results showed that the α-Al grain size of A356 alloy refined by Al–3?wt-% VN master alloy was 350?±?95?µm while that of A356 alloy treated by traditional Al–5Ti–B master alloy was 570?±?105?µm. Moreover, for A356 alloy with Al–3?wt-% VN addition, the good grain refining efficiency did not fade significantly within 30?min. The effectiveness of grain refinement might be attributed to VN particles, which acted as the heterogeneous nuclei of α-Al grains. Owing to the refinement strengthening, the yield strength, ultimate tensile strength and elongation of A356 alloy were improved.     

Effect of temperature and reactivity of molten 55Al–Zn alloy on Co based alloy coatings

Abstract

The short service life of bearings in galvanising industry is a result of a complex set of deterioration mechanisms. This work addressed the effect of temperature and molten bath reactivity on the material of the bearings respectively. Three commercial alloys, the Co–Cr–W Stellite 6 and Co–Cr–Mo–Si Tribaloy (T400 and T800) alloy systems, were deposited by plasma transferred arc on AISI 316L plates. Coatings were evaluated for the effect of temperature exposure on hardness, microstructure and sliding abrasive wear. The reactivity with the molten 55Al–Zn alloy was assessed by immersion tests in an industrial bath. Results showed that exposure at 600°C for 168 h resulted in an increase in hardness, microstructure changes and loss of wear resistance for the Stellite 6 coatings. A superior performance to temperature was shown by Tribaloy T800 with a stable abrasive wear resistance. The three alloys exhibited a strong reactivity with the 55Al–Zn molten bath. An intermetallic layer formed on the coatings as the Al from the bath reacted with elements from the Co based alloys. This reactivity consumed the coatings, causing a reduction on thickness particularly on those processed with the T800 alloy.     

Influence of alloy element partitioning on strength of primary α phase in Ti-6Al-4V alloy

The partitioning effect of Al(α-phase stabilizer) and V elements(β-phase stabilizer) on strength of the primary α phases in the α/β Ti-6 Al-4 V alloy with the bimodal microstructure was investigated.It was found that partitioning of Al and V elements took place in the Ti-6 Al-4 V alloy during the recrystallization process,leading to the variation of the content of Al and V elements in the primary α phases with changing the volume fraction of the primary α phase.Nanoindentation tests reveal a general trend that the strength of the primary α phases increases with decreasing the volume fraction of the primary α phases,and such trend is independent on the loading direction relative to the c-axis of the α phase.The enhanced strength is attributed to the increase of the content of Al element in the primary α phase,but it is not dominated evidently by the change of the V content.The solid solution strengthening contributed from both the elastic strain introduced by the solute atoms and the variation of the density of states was estimated theoretically.     

Fatigue-crack growth in 2090 AlLi alloy

The fatigue-crack growth kinetics of the 12.7 mm thick 2090-T8E41 alloy plate vary significantly with plate thickness location. The apparent fatigue-crack growth rates in the midthickness location are substantially lower than those in the surface region at intermediate and high applied stress intensity ranges. The lower crack growth rates are related to the extraordinarily tortuous fatigue profile which induces high crack closure levels. Such a tortuous crack profile appears to be primarily a consequence of the alloy's intense crystallographic texture and its great propensity for planar slip.     

TAHA processing of a 6201 aluminium alloy†

In order to gain better understanding of the effect of pre-aging on the tensile properties of 6201 Al alloy, different TAHA processes have been performed in this study. A 6201 AI alloy was first solution treated, followed by pre-aging at 135°C for 30?minutes (some specimens were without pre-aging). The specimens were then cold rolled to 60, 70 or 80% reduction in thickness followed by final aging at 140, 150 or 160°C for 1 to 20?hours. For specimens of 60% cold reduction, the ultimate tensile strength (UTS) and yield strength (YS) were significantly improved by pre-aging. The improvement in strengths by pre-aging was most (least) significant if the final aging temperature was 140°C (160°C). For specimens of 70% reduction, the improvement in UTS and YS by pre-aging was not as significant. For specimens of 80% cold reduction, lower strengths were found for pre-aged specimens as compared to specimens without pre-aging. Scanning electron microscopy revealed that there was close relationship between the fracture morphology and the resulting tensile properties. Proper combination of pre-aging and cold work led to finer, denser and more uniform distribution of precipitates which resulted in better tensile properties     

Room temperature age-hardening in a β titanium alloy

A metastable titanium alloy containing 10 wt % Zr and 12 wt % V has been found to undergo a substantial age-hardening reaction at temperatures as low as 20° C. The reaction involves continued growth of athermal-phase particles produced during water quenching from the-phase field. The morphology of the as-quenched is retained, implying the absence of long-range diffusion during ageing: this is consistent with the low value of the activation energy measured (93 kJ kg mol^–1). It is suggested that the growth is caused by unpinning of/ interfaces as a result of the short-range motion of interstitials present in the alloy. The age-hardening produces a severe loss in tensile ductility and inhibition of stress-induced martensite formation.     

Effect of Zr addition on properties of Al–Mg–Si aluminum alloy used for all aluminum alloy conductor

The effects of Zr addition on mechanical property in the aged Al–Mg–Si alloy exposed to thermal-resistant treatment (180–250 °C) have been studied by using both Brinell Hardness tests and tensile tests. The softening process at 180 °C and 230 °C has been investigated by transmission electron microscope (TEM). The Arrhenius Model is introduced to simulate the strength evolution in the thermal-resistant treatment. The results show that tensile strength and thermal-resistant property are improved by addition of Zr, and both the Brinell Hardness and Tensile Strength could maintain no less than 90% of their initial values when the alloy is exposed to heat treatment at 180 °C for 400 h and 230 °C for 2 h. The presence of rod-shaped phases and coarsening particles results in decreasing the hardness of the sample. The relationship between thermal-resistant life and temperature is derived by the Arrhenius Model. When the Al–Mg–Si–Zr alloy is heated at 130 °C, the duration described in the Arrhenius plot could reach to 40 years.     

Pt/Cr and Pt/Ni catalysts for oxygen reduction reaction: to alloy or not to alloy?

Bimetallic systems such as Pt-based alloys or non-alloys have exhibited interesting catalytic properties but pose a major challenge of not knowing a priori how the electronic and chemical properties will be modified relative to the parent metals. In this work, we present the origin of the changes in the reactivity of Pt/Cr and Pt/Ni catalysts, which have been of wide interest in fuel cell research. Using spin-polarized density functional theory calculations, we have shown that the modification of Pt surface reactivity in Pt/Ni is purely of geometric origin (strain). We have also found that the Pt-Ni bonding is very weak, which explains the observed instability of Pt-Ni catalysts under electrochemical measurements. On the other hand, Pt/Cr systems are governed by strong ligand effect (metal-metal interaction), which explains the experimentally observed reactivity dependence on the relative composition of the alloying components. The general characteristics of the potential energy curves for O2 dissociative adsorption on the bimetallic systems and the pure Pt clarify why the d-band center still works for Pt/Cr despite the strong Pt-Cr bonding and high spin polarization of Pt d-states. On the basis of the above clarifications, viable Pt-Cr and Pt-Ni structures, which involve nano-sized alloys and non-alloy bulk catalyst, which may strike higher than the currently observed oxidation reduction reaction activity are proposed.     

Lüders elongation in AI-Mg alloy AA5182

Abstract

Aluminium-magnesium alloys such as AA5182 exhibit a yield point elongation, or Lüders strain, when deformed in the annealed O temper, and the extent of the Lüders strain is a function of processing history and the deformation conditions. The yield point effect is usually attributed to Mg solute pinning, but is has been appreciated for some time that factors such as grain size, and cooling rate from the annealing temperature, affect the Lüders strain, and it has not been clear how this behaviour is consistent with solute pinning. In this paper it is shown that the presence, or absence of Lüders effect (Lüdering) is related to the grain boundary structure, and processing which results in a high concentration of grain boundary dislocations removes the yield point effect, whereas fabrication routes which produce defect free bondaries result in a Lüders strain. The extent of the Lüders strain, and its dependence on grain size and deformation conditions, is reasonably consistent with some recent models of propagative plastic instabilities.     

Structure characterization of a laser-processed Al–Mo alloy

The surface structure of a laser-processed Al–Mo alloy has been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractory (XRD). The alloy was prepared by first laser alloying a mixture of Al and Mo powders into an Al substrate and then laser remelting the alloyed surface. Following the first laser alloying process, the needle-like equilibrium phases (Al5Mo(h) and Al5Mo(r)) are formed with a broad size ranges and distribute inhomogeneously in the -Al solid solution matrix. This coarse structure is replaced by a finer, uniform dispersion of dendrites after the subsequent laser remelting. Four basic types of solid states precipitates are observed: (1) irregularly shaped particles constructing the dendrites and having a nearly Al5Mo stoichiometry; (2) needle-like particles which is the Al5Mo (r) phase; (3) Faceted particles having a cubic structure with a stoichiometry close to Al7Mo; (4) tiny, equi-axed particles, with a rather narrow particle size distribution and a cubic structure. © 1998 Chapman & Hall     

Thermochemical processing with hydrogen of super-α2 alloy

Thermochemical processing (TCP) with hydrogen, including hydrogenation, intermediate heat treatment and dehydrogenation, of super-₂, a type of Ti₃Al-based titanium aluminide alloy, was carried out in order to examine the microstructural variations in the alloy. It is shown that a number of microstructural changes, such as the amount, morphology and distribution of different phases in the alloy, can be brought about by TCP. Acicular or Widmanstatten ₂ was found to form in the retained B2 matrix under TCP with an intermediate heat treatment of furnace cooling from high temperature, instead of mainly equiaxed ₂ under the normal treatment. The precipitation in the microstructures for alloys undergoing TCP with the intermediate -solution heat treatment followed by ageing while dehydrogenating was found to be more regular in morphology and homogeneous in distribution. Both the amount of primary ₂ and the size of the primary grains in the microstructure could be controlled by suitable TCP. Some of the reasons leading to the microstructural variations caused by TCP are discussed.     

Phase relation in titanium-aluminide alloy — an X-ray study

Results of X-ray diffraction studies on titanium aluminides stabilized by niobium, vanadium and molybdenum are reported to establish a phase relation in the Ti-25Al-10Nb-3V-1Mo at% (Ti-25-10-3-1) alloy. It is shown that the composition of the phases probably deviated slightly from ideal stoichiometry Ti₃Al for ₂ and Ti₂AlNb for -type; its partial ordering in of the -phase type and the phase relation is 64% -type and 36% ₂ phase.     

Mechanism of high temperature oxidation of laser surface engineered TiC/Al alloy ‘composite’ coating on 6061 aluminium alloy

Abstract

Structural, morphological, and kinetic investigations into the oxidation behaviour of a laser surface engineered TiC/Al alloy ‘composite’ coating on 6061 aluminium alloy are discussed in the present work. Specimens of 6061 alloy coated with TiC were oxidised at 473, 673, and 873 K in air for 50 h to study the effects of long duration exposure on the coating. Severe oxidation of the coating was observed above 673 K and delamination/separation of the coating occurred in the specimen exposed to 873 K. The oxidation kinetics were studied by thermogravimetric analysis (TGA) of pure TiC (coating material), 6061 alloy (substrate material), and the TiC/6061 aluminium alloy ‘composite’ coating in the temperature range 473–873 K. Oxidation and oxidation rate within the lower temperature range (473–573 K) were comparable for both TiC and 6061 alloy, whereas both oxidation and oxidation rate were several times greater for TiC compared with 6061 alloy above 573 K, which illustrates accordingly two different oxidation behaviours below and above 573 K for the ‘composite’ coating TiC/6061. These effects were further demonstrated for different TiC contents in the TiC/6061 ‘composite’ coating by preparing specimens with various precursor thicknesses.     

Oxide bifilms in aluminium alloy castings – a review

Abstract

Aluminium alloy castings are most widely used in automobile industry because of their light weight, better castability and improved properties. The liquid aluminium surface easily oxidises during melting, transferring and pouring operation which may entrain oxide films into the casting. Research work has shown that the entrainment of this surface film and formation of bifilms in castings appear to be the source of most of the casting defects leading to a significant reduction in the mechanical properties of aluminium alloy castings. In this paper, the phenomenon of formation of oxide bifilms in aluminium alloy castings, effect of these bifilms on casting properties and their assessment techniques are discussed. For enhancing the quality of casting, research should focus towards development of process techniques for healing of bifilms in liquid metal during solidification.     

Dynamic fracture of binary Al–Li alloy in torsion

Abstract

To investigate the mechanical properties of a binary Al–2·8Li alloy at high strain rates in as received (solution treated) and aged conditions, quasistatic and dynamic torsion tests were carried out. With increasing strain rate, the strength and ductility of the as received alloy increased and a change of the fracture mode from intergranular to transgranular was also observed. However, in the aged alloy, the intergranular fracture mode was predominant at both quasistatic and dynamic rates. At the grain boundary offsets on the fracture surface of the aged alloy, a large number of very fine dimples were observed. This evidence suggests that a ductile precipitate free zone may be present there.

MST/1043