Volume 101, Issue 12

This paper reports on the role of repeated stress-induced martensite transformations at different temperatures (referred to as “training”) on the pseudoelasticity and microstructural evolution of as-grown Co 49 Ni 21 Ga 30 [001]-oriented shape memory alloy single crystals under compression. The training resulted in a reduction in transformation stress levels and a stable microstructure with a multi-variant configuration reducing the observed transformation strains. Training also improved the recoverability giving way to a large pseudoelastic temperature window of 400°C. In-situ microscopy revealed the martensite stabilization to be due to pinning of moving interfaces especially at temperatures above 120°C. An insight into the evolution of microstructure and stressüstrain behavior in terms of stress hysteresis changes with test temperature is provided, and the possible operant mechanisms are discussed.

A phenomenological model was utilized to describe diffusivities in the γ (fcc) /γü (L1 2 ) and A2/B2 phases of the NiüAl system. An effective strategy, which takes the homogeneity range and defect concentration into account, was developed in the present work to optimize the atomic mobilities of γü phase. Such a strategy results in a dramatic decrease in the number of atomic mobility parameters to be evaluated for the L1 2 phase. The measured composition- and temperature- dependent diffusivities in the NiüAl system have been well replicated by the present mobility descriptions. For the L1 2 phase, comprehensive comparisons show that with fewer model parameters the presently obtained mobilities yield a better fit to experimental diffusivities, compared with previous assessments. The mobility descriptions are further validated by comparing calculated and measured concentration profiles for various diffusion couples. The time-dependent Al composition profile for the annealed vapor Al / γ couple is accurately described for the first time.

The 450°C isothermal section of the ZnüFeüV ternary phase diagram has been determined experimentally using X-ray diffraction and scanning electron microscopy coupled with energy/wave dispersive spectroscopy. A new ternary compound T, containing 1.4 to 4.5 at.% V, 5.6 to 15.5 at.% Fe and 82.3 to 92.2 at.% Zn, was positively identified for the first time in this study. The existence of the rphase, together with two ZnüV compounds, VZn 3 and V 4 Zn 5 , at 450°C has been confirmed. Fe has no detectable solubility in ZnüV compounds. But it was proved that about 3.0 at.% Zn can dissolve into α-V at 450°C. Experimental results indicated that the maximum solubility of V in d, γ 1 , and γ was 0.5 at.%, 1.3 at.% and 1.8 at.% respectively, whereas that in ζ was less than 0.2 at.%.

Synchrotron radiography has enabled time and space resolved investigation of directional solidification of ternary AlüCuüAg eutectic. Due to a high spatial resolution the lamellar spacing of the eutectic can be identified. The morphological instability of a planar eutectic solidüliquid interface towards eutectic cells observed in sheet-like samples is investigated for different solidification conditions. The results are summarized in a stability diagram and compared to findings in bulk samples and theoretical predictions based on the criterion of constitutional undercooling. During growth of eutectic cells an intriguing dual phase fragmentation in the intercolumnar/cellular regions with Al2Cu fragments settling and Al-rich fragments rising in the melt was observed, which may be unique for coupled growth of multicomponent eutectics.

The Mg–Ge system is reassessed by means of the CALPHAD approach, supplemented with first-principles calculation. All of the experimental phase diagram and thermodynamic data available in the literature are critically reviewed and assessed by using thermodynamic models for the Gibbs energies of individual phases. Two models, the substitutional solution model and associate solution model, are employed to describe the Gibbs energy of the liquid phase. The enthalpy of formation for Mg2Ge is calculated by first-principles calculation with a desire to clarify the discrepancies in the literature data reported for the enthalpies of formation. Two sets of self-consistent thermodynamic parameters for the Mg–Ge system are finally obtained, with a substitutional solution model or associate solution model for the liquid phase. Some improvements have been made, compared with the previous assessments.

This work proposes an original quantitative approach to understand the backward plastic strain resulting from the unloading of a polycrystalline metal during an interrupted tensile test. Based on a prior model by Salama and Roberts, the main assumption is that the relaxed plastic strain is due to the small relaxation of mobile dislocation loops, also responsible for the internal stresses. During unloading, the number of dislocation loops per active source remains constant, but they move backwards as the applied stress decreases. Thus, the main challenge consists in the quantitative estimation of each hardening component and their variations, upon loading and during the unloading sequences, without reverse straining. The model is applied in the case of an interstitial-free steel, with excellent agreement. This study provides a more consistent framework related to the evaluation of the Bauschinger effects in metals avoiding the ambiguity on the choice of the plastic strain offset.

The availability of large high-quality silicon carbide (SiC) single crystals is a key issue in the development of the full potential of SiC-based device technology. This paper mainly discusses SiC crystal growth in the graphite crucible with several geometrical structures by using the physical vapor transport method to investigate the influence of graphite crucible structures on SiC crystal diameter enlargement. Metallographic microscopy and X-ray diffraction are applied to the obtained cross-sectional slice of SiC ingot. In a growth run with a crucible structure of cylinder platform seed-holder and additional cone-shaped inner furnace close to the seed crystal, the SiC crystal diameter increases only 4–5 mm and the growth front exhibits a smooth segment of spherical shape; but the diameter of SiC single crystal decreases when the inside diameter of the crucible is close to that of the seed crystal and the angle α between the cone-shaped platform and cylinder platform is larger than 20°. When using the crucible with a cone-shaped platform seed-holder and cone-shaped inner wall, the SiC crystal growth will proceed along the axial direction as well as the radial direction, and the diameter can be enlarged by about 12 mm in a single growth run.

The thermal and chemical stability of Cr 2 AlC, Cr 2 AlC–NiAl and Cr 2 AlC–α-Al 2 O 3 have been studied. No enthalpic effects indicating phase transformations or chemical reactions were observed for the Cr 2 AlC–α-Al 2 O 3 powder mixtures at T < 1476°C, while a melting onset temperature of 1310°C was measured for the Cr 2 AlC–NiAl powder mixture by differential thermal analysis. However, the formation of chromium carbide is also predicted to occur below this temperature based on our thermodynamic calculations, and was observed by X-ray diffraction of powders annealed at 1000 and 1300°C. The carbide formation is caused by the reaction of Cr 2 AlC with NiAl and can be understood by considering the large solid solution range of Al in NiAl. These results indicate that the application potential of Cr 2 AlC–NiAl composites is limited by the Al solubility in the NiAl.

Microstructures and compressive properties of Zn25Al3Cu alloy and Zn25Al3Cu/Al 2 O 3 particulate composites with large reinforcing particles (250 μm) were examined. The composites were obtained by the compocasting technique through infiltration of 3, 8 and 16 wt.% Al 2 O 3 particles into the semi-solid metal matrix. The influence of temperature in the range from 70 to 170°C on compressive yield strength of the matrix alloy (as-cast and thixocast Zn25Al3Cu alloy) and the composites was investigated. Above 70°C compressive yield strength of all materials decreases, but the rate of decrease is different for the matrix alloy (as-cast and thixocast) and composites. It was found that the abrupt decrease in compressive yield strength of the matrix alloy occurred at temperatures higher than 70°C, whereas composites retained relatively high values of compressive yield strength till the end of the testing temperature range regardless of the amount of reinforcing particles.

In this paper, Incoloy800 and 304 stainless steel (SS304) were clad by explosive welding. Accordingly, hardness, tensile strength, shearing strength, bending properties and microstructure of the clad plates were evaluated. The bonding interface showed a wavy morphology. Line scanning results indicated that there was no diffusion area through the bonding interface. Whereas, a colorful zone was observed at the wave ridge of the base metal (white area in optical microscopy and black area in scanning electronic microscopy). Energy dispersive spectroscopy analysis indicated that compositions and contents of the colorful zone were the same as that in the base metal. Hardness testing showed that the highest value of hardness was obtained in the colorful zone. Thus, the colorful zone was an intensive deformation area in SS304, which was produced by shock pressure from collision. Mechanical tests showed that the clad plate was acceptable according to ASTM specification.

The damping capacity of three different FeüCrüMgüAl/Si alloys was measured using the conventional cantilever beam method. The samples were heat-treated in vacuum at 1100°C for 1 h followed by slow cooling (120°C h ü1 ). The vibration damping capacity of the alloys was definitely influenced by the amount of grain boundary precipitates as well as by the chromium concentration in the precipitates. The three different alloys investigated were FeüCrüMgüAl, FeüCrüMgüSi, and FeüCrüMgüAlüSi. It was found that the FeüCrüMgüAlüSi alloy exhibits the highest damping capacity with the smallest amount of grain boundary precipitates and least chromium concentration. Among the remaining alloys, although the chromium concentrations in the grain boundary precipitates were almost the same, the amount of grain boundary precipitates in FeüCrüMgüAl was lower than in FeüCrüMgüSi, and hence the damping capacity of FeüCrüMgüAl was higher than the other.

Magnetic chitosan nanocomposites have been synthesized on the basis of metal (Fe, Co, Ni)-encapsulated carbon nanomaterials. The organic layer was identified using Fourier transform infrared spectroscopy and elemental analysis. The functionalized nanomaterials were employed to remove metal ions: cadmium, copper, nickel, lead and zinc from aqueous solutions. Their adsorption capacities were studied by the batch procedure. The concentration of cations extracted was determined by inductively coupled plasma mass spectrometry. The influence of different parameters (pH, amount of the support studied, contact time) on the cation extraction was investigated. The studies demonstrated excellent ability of copper and lead adsorption, and satisfactory adsorption of other ions applied by the supports studied. The nanocomposites with complexed cations can be easily removed from water with the help of an external magnet. They can then undergo regeneration in an acidic environment and can be reused in the analogous process repeatedly.

Fibrous mat was produced from eri silk fibroin by electro spinning. The fibrous mat was tested for tensile properties and in-vitro bio-degradation by enzymatic treatment. The tensile strength of the mat was 5.3 MPa and its elastic modulus was 49 MPa before bio-degradation. The weight loss obtained after 30 days of degradation was 34%. The in-vitro enzymatic degradation of fibrous mat was confirmed through Scanning electron microscopy and Fourier transform infrared spectroscopy studies.

The photodegradation of thin films of poly(para-tert-butylstyrene) with 265 nm radiation in the presence of oxygen, and as a function of irradiation time, has been studied using fluorescence, Fourier transform-infrared spectroscopy, and ultraviolet-visible spectroscopy. The influence of phthalate and terephthalate plasticizers on photo-oxidative degradation was also investigated. Blending with phthalate plasticizers was found to cause a higher efficiency of degradation than that obtained for doping with terephthalate plasticizers. The intensity of absorption was also found to increase with time of irradiation and in the intensity of a new absorption band at longer wavelength, thus indicating a possibility of photodegradation of polymer films. The analysis of the Fourier transform-infrared spectra of the irradiated and non-irradiated samples showed an increase in the intensities of the carbonyl and hydroxyl regions of the Fourier transform-infrared spectra providing evidence for the photodegradation as well as the photo-oxidation of polymeric chains. The increase in the analyzed ranges is attributed to the formation of alcohols, aliphatic ketones and to the increase in the number of polyene structures that resulted from hydrogen abstraction during photodegradation reactions.

The present work is focused on the synthesis of beryllium doped titania (Be +2 –TiO 2 ) at different percentages (0.25, 0.5, 0.75 and 1.0 wt.%) by the sol-gel method and its characterization using X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform-Infra red and Ultra violet-visible absorption spectroscopic methods. Diffraction peaks of anatase crystalline phase were present in both synthesized TiO 2 and Be +2 –TiO 2 . The presence of Be +2 ion in the TiO 2 structure caused a significant absorption shift towards the visible region and its presence was confirmed by X-ray photoelectron spectroscopy and Fourier Transform-Infra Red data. The photocatalytic efficiency of the synthesized Be +2 –TiO 2 and pure TiO 2 was evaluated by the degradation of aqueous methyl orange dye under visible light irradiation, where the degradation rate of methyl orange by Be +2 –TiO 2 was found to be higher than by pure TiO 2 . This can be attributed to more efficient electron–hole creation in Be +2 –TiO 2 in visible light and the electrons produced due to photosensitization of the dye can be scavenged by photoexcited doped TiO 2 in visible light.

The ASTEC/Sophaeros computer code has been developed to model transport of reactive gases and aerosols in pipe flows with extensive coverage of chemical and physical phenomena. As part of the ASTEC severe-accident code co-developed by IRSN in France and GRS (Gesellschaft für Anlagen- und Reaktorsicherheit) in Germany, it is principally used to evaluate so-called radioactive source terms released to the environment in the event of potential accidents affecting water-cooled nuclear reactors. A major uncertainty in the past concerned the impact of chemistry on radioactivity transfer where the treatment of chemical phenomena was recognized as inadequate. Work described here, involving mainly systematic identification of relevant species and thermodynamic- data verification, has vastly improved this situation leading in particular to creation of a comprehensive database with fully-referenced sources of information. Further prospective improvements in this area are described.