X-ray microtomography study of cavity coalescence during superplastic deformation of an Al–Mg alloy

Abstract

Coalescence between cavities induced during superplastic deformation of an Al–Mg alloy was studied, in particular by X-ray microtomography, which can provide three-dimensional images of the population of cavities through the volume of the material. From such images, the variation with strain of the number of cavities per unit volume was measured for various strain rates. For a given strain, the number of cavities increases with strain rate, but for a given strain rate, it is shown that significant interlinkage between cavities takes place in a large strain interval. Moreover, although the cavity volume fraction increases with strain, it becomes difficult to describe the population of cavities by a mean volume since the largest cavity may concentrate more than 70% of the total volume of cavities. For such circumstances, an interlinkage parameter was introduced as the ratio between the volume of the largest cavity and the total volume of cavities for given experimental conditions (strain and strain rate). This work illustrates the uncertainty of some conclusions directly drawn from data obtained using only two-dimensional techniques, such as quantitative metallography performed on polished sections.     

Evidence of FCC titanium hydride formation in β titanium alloy: An X-ray diffraction study

This study has shown a complete phase transformation from the bcc β phase Ti-30 wt% Mo alloy to the fcc δ phase titanium hydride by thermal hydrogenation treatment. This fcc δ hydride has a lattice parameter of 0.4444 (± 0.0001) nm and acomposition of TiH_1.96 and it is very brittle. It appears that the types of hydride formed depend on the phase regime where thermal hydrogenation takes place, i.e., the fcc δ hydride forms from the β phase regime, the fct (c/a > 1) γ hydride forms from the α phase regime, and a mixture of both hydrides forms from the α + β phase regime.     

The HREM study of precipitates in an Al−Zn−Mg alloy

A study of precipitate phases in an Al–Zn–Mg alloy in the T74 condition has been conducted by means of high resolution electron microscopy. It has been observed that G.P. zones, and phases exist simultaneously in the matrix. The G.P. zones are plate-like forming on (111) matrix plane. The phase has a hexagonal structure witha=0.496 nm,c=1.403 nm and the orientation relationship with the matrix is . A new orientation relationship between the phase and the matrix is found to be .     

Mössbauer study of martensitic transformations in an Fe-29.6% Ni alloy

The application of an external stress may form band shaped strain-induced martensites in the austenite structure of Fe alloys. Mössbauer spectroscopy and transmission electron microscopy techniques were used to clarify certain properties of strain-induced martensite in an Fe-29.6% Ni alloy. The reverse transformation mechanism between thermal plate martensite and the matrix austenite was also studied. Mössbauer spectroscopy made it possible to examine the same area of the austenitic thin foils during the thermal cycles, and the volume fraction changes were determined. The habit plane and orientation relationship of strain-induced martensite were measured from the electron diffraction patterns and the latter parameter was found to be K-S type as with thermal plate martensites of the Fe-Ni alloys. The isomery shifts caused by the deformation and cycling procedures were also calculated for both austenite and martensite structures and the hyperfine magnetic field parameter of Fe-29.6% Ni strain-induced martensite was found to be equal to that of Fe-Ni-C alloys reported earlier.     

Phase separation in immiscible silver–copper alloy thin films

Far from equilibrium, immiscible nanocrystalline Ag–Cu alloy thin films of nominal composition Ag–40 at.% Cu have been deposited by co-sputter deposition. Both X-ray and electron diffraction studies indicate that the as-deposited films largely consist of nanocrystalline grains of a single alloyed face-centered cubic (fcc) phase. However, detailed three-dimensional atom probe tomography studies on the same films give direct evidence of a nanoscale phase separation within the columnar grains of the as-deposited Ag–Cu films. Subsequent annealing of these films at 200 °C leads to two effects; a more pronounced nanoscale separation of the Ag and Cu phases, as well as the early stages of recrystallization leading to the breakdown of the columnar grain morphology. Finally, annealing at a higher temperature of 390 °C for a long period of time leads to complete recrystallization, grain coarsening, and a complete phase separation into fcc Cu and fcc Ag phases.     

Problem solving in sintering of tungsten heavy alloy product — a case study

The authors in DMRL are engaged for last three decades in development and production of a repertoire of tungsten heavy alloy products mainly for defence applications where stringent quality control is imperative. An exhaustive knowledge-base has been built over the years with inputs from literature and in-house use R&D. This knowledge-base generally ensures continuous and smooth production of quality products. But, there was a case where this knowledge-base of sintering had been proved to be incapable of ascertaining and solving certain production problem. This particular problem was analysed and solved by Ishikawa technique. Based on the experience, the authors advocate that the knowledge of different statistical and problem solving tools is essential in the field of powder metallurgy.     

Phase stability in a powder-processed Al–Mn–Ce alloy

There has been considerable interest in Al-rich Al–Mn–Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al–5Mn–2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al₂₀Mn₂Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al₂₀Mn₂Ce, and Al₆Mn, with a small amount of Al₁₂Mn and Al₁₁Ce₃. Upon heat-treatment at up to 450 °C, the Al₂₀Mn₂Ce and Al₆Mn phases decompose to give a hard stable phase mixture with 72–73 % Al₁₂Mn plus 13–14 % each of Al₁₁Ce₃ and FCC Al. Heat treatments at 500 °C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al₃(Mn,Ce) phase, 9 % Al₁₂Mn, 8 % Al₆Mn, and 1 % Al₁₁Ce₃. The formation of large volume fractions of Al₁₂Mn for heat-treatments at up to 450 °C suggests that the presence of Ce may stabilize this phase, and that more dilute Al–Mn–Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.     

X-ray diffraction study of nonstoichiometry in CuInSe2 + §

CuInSe₂ alloys with the stoichiometric Cu : In ratio and varying Se content were studied by x-ray diffraction. The lattice parameters of the alloys, atomic positional parameterx, and the bond lengths lcu-se and l_in-Se were determined. The lattice parameters were found to increase with decreasing Se content on the Se-poor side of CuInSe₂ and to pass through a flat minimum around 50.5 at. % Se. The results are interpreted as indicating that the dominant intrinsic defects in Se-deficient CuInSe₂ are Cu interstitials. In the Se-enriched alloys containing more than ≃50.5 at. % Se, the dominant defects are cation vacancies, Se interstitials, and/or Se_Cu and Se_In antisite defects.     

Phase and microstructural development in alumina sol–gel coatings on CoCr alloy

Phase transformation of -Al₂O₃ to -Al₂O₃ in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above 1200 °C. Addition of -Al₂O₃ seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 °C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the -Al₂O₃ phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of -Al₂O₃ grains at the expense of the precursor fine-grained -Al₂O₃ matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.     

Internal melting and coarsening of liquid droplets in an Al–Cu alloy: a 4-D experimental study

In conventional melting experiments of pure monocrystalline metals, the phase transformation starts at the sample surface and progresses inwards according to thermal gradients. In solutionized alloys, traces of internal melting are usually observed after reheating and quenching from the semi-solid state. The formation and development of these liquid pockets are not fully understood despite their significance in semi-solid processing, where the formability is greatly influenced by the distribution of liquid within the feedstock material. In situ X-ray microtomography was performed in this study to shed light on this phenomenon. We report in detail the melting and isothermal holding of a model binary alloy where a remarkable number of liquid droplets were observed to develop and coalesce. Various computational tools have been used to study their statistical evolution as well as the local ripening mechanisms involved. We analysed an interesting case of particle coarsening which differs from classical case studies by the fact that the fast-diffusing liquid phase is entrapped within the slow-diffusing solid medium.     

Phase characterization in as-cast F-75 Co–Cr–Mo–C alloy

Microstructural characterization of a cast acetabulum of ASTM F-75 alloy has been carried out in order to clarify conflicting reports from the literature. The present investigation revealed that although sigma (σ) and M₂₃C₆ carbide were the only secondary phases formed in the face centered cubic cobalt-base alpha matrix (Co-α), as identified by X-ray diffraction, the observed microstructure was quite complex. Scanning and transmission electron microscopy indicated the presence of coarse and fine lamellar cellular colonies, grain boundary film carbide, and different types of coarse blocky particles, including single-phase σ, dual-phase σ/M₂₃C₆, a binary eutectic comprised of σ and Co-α phases, and a three-phase feature comprising the binary eutectic and solid state formed M₂₃C₆. The carbide has probably formed during cooling from casting due to σ metastability. While it is proposed that the some lamellar cellular colonies were formed by discontinuous precipitation, it is not clear whether all lamellar structures present in the as-cast alloy occurred due to the same mechanism. The results obtained for the tensile properties are discussed in view of the observed microstructure.     

Enhanced bioactivity of ZnO nanoparticles—an antimicrobial study

Abstract

In this study, we investigate the antibacterial activity of ZnO nanoparticles with various particle sizes. ZnO was prepared by the base hydrolysis of zinc acetate in a 2-propanol medium and also by a precipitation method using Zn(NO₃)₂ and NaOH. The products were characterized by x-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Bacteriological tests such as minimum inhibitory concentration (MIC) and disk diffusion were performed in Luria-Bertani and nutrient agar media on solid agar plates and in liquid broth systems using different concentrations of ZnO by a standard microbial method for the first time. Our bacteriological study showed the enhanced biocidal activity of ZnO nanoparticles compared with bulk ZnO in repeated experiments. This demonstrated that the bactericidal efficacy of ZnO nanoparticles increases with decreasing particle size. It is proposed that both the abrasiveness and the surface oxygen species of ZnO nanoparticles promote the biocidal properties of ZnO nanoparticles.     

Enhanced bioactivity of ZnO nanoparticles—an antimicrobial study

In this study, we investigate the antibacterial activity of ZnO nanoparticles with various particle sizes. ZnO was prepared by the base hydrolysis of zinc acetate in a 2-propanol medium and also by a precipitation method using Zn(NO₃)₂ and NaOH. The products were characterized by x-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Bacteriological tests such as minimum inhibitory concentration (MIC) and disk diffusion were performed in Luria-Bertani and nutrient agar media on solid agar plates and in liquid broth systems using different concentrations of ZnO by a standard microbial method for the first time. Our bacteriological study showed the enhanced biocidal activity of ZnO nanoparticles compared with bulk ZnO in repeated experiments. This demonstrated that the bactericidal efficacy of ZnO nanoparticles increases with decreasing particle size. It is proposed that both the abrasiveness and the surface oxygen species of ZnO nanoparticles promote the biocidal properties of ZnO nanoparticles.