The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable phase was identified in the first stage of dissolution nucleating at the - boundaries. Interface dislocations at the - boundaries forming a perpendicular net in the 110 directions were identified as of screw character and of Burgers vector b=1/2a 110. During dissolution a change of shape of precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 110 directions.     

Microstructure stability of fine-grained silicon carbide ceramics during annealing

Fine-grained silicon carbide ceramics with an average grain size of 140 nm or smaller were prepared by low-temperature hot-pressing of very fine -SiC powders using Al₂O₃-Y₂O₃-CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850°C and by evaluating quantitatively the grain growth behavior using image analysis. The phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure.     

A transmission electron microscopic study of the Bethany iron meteorite

The Bethany iron meteorite which is a part of the Gibeon shower is a fine octahedrite with zoned plessite fields of various sizes. The optically irresolvable microstructural details inside the plessitic fields have been studied by transmission electron microscopy, and the crystallographic relationships between the primary kamacite () and the parent taenite (), and between the and particles in the coarse plessite, have been examined using electron diffraction. In the case of primary kamacite the orientation-relationship with was close to the Nishiyama-Wasserman relationship, whereas, for the plessitic , the orientation-relationship with was close to Kurdjumov-Sachs. It was also found that the (111) and (110) planes were not strictly parallel. Additionally, measurements of the composition profile through the zoned plessite have been made using STEM microanalysis technique, and related to microstructure.     

Microstructure and properties of supersaturated ZA27 alloy during natural ageing

The decomposition of the supersaturated solid solution of ZA27 alloy at room temperature has been investigated by X-ray diffraction, TEM and mechanical properties testing. Based on the results obtained, both continuous precipitation and cellular reaction occur during the decomposition process. The continuous precipitation follows the sequence: ₁ + spherical GP zones ₂ + elliptical GP zones ₃ + R + . The cellular reaction can be written: + + . The properties of the alloy depend on the microstructure. After 1 month of ageing, a series of changes of microstructure have taken place. The properties of the alloy are: _b=500 MPa, =13%,H _v=148.     

X-ray studies of iodine sorption in some silver zeolites

After saturation sorption, in a static system, of molecular iodine onto some silver zeolites at 130° C, powder X-ray diffraction analysis revealed sharp reflections due to crystalline -Agl. However, this -Agl contained only a fraction of the sorbed iodine and it seems likely that this -Agl involves unlocalized ions such that a continuous three-dimensional network of -Agl forms in the zeolite channels. Water in the zeolite channels is probably the underlying cause: heating to 200 to 300° C in air destroyed the sharp -Agl reflections and no such reflections were observed if the sorption was carried out with the zeolites dehydrated beforehand. -Agl formation was enhanced when the zeolite framework was broken down by heating in air at 800° C, heating in water or brine at 200° C, or -irradiation.     

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mössbauer spectra of epitaxial -Fe₁₆N₂ and -Fe₈N films have been studied and their differences are discussed in detail. The Mössbauer spectrum of -Fe₁₆N₂ can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mössbauer spectrum of -Fe₈N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for -Fe₁₆N₂ than for -Fe₈N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for -Fe₁₆N₂. The iron moments tend to locate in the film plane for -Fe₁₆N₂ and to arrange perpendicularly to the film plane for -Fe₈N.     

The redistribution of alloying elements by rapid-heating solution treatment of an (α +β) titanium alloy

Thin-foil X-ray energy-dispersive chemical analysis of an (+) titanium alloy has been used to establish the compositional profiles which develop between the and phases as a result of heating into the field at a rate of 100 K sec^–1. The results show that phase transforms to via a diffusional mechanism as it is progressively enriched in-stabilizing solutes. Complete diffusional transformation of to occurs 160 K above the transus but the phase remains chemically inhomogeneous. On subsequent quenching at a rate of 400 K sec^–1, phase containing less than approximately 10wt% Mo equivalent of -stabilizing solutes transforms to martensite. The proposed transformation mechanisms are consistent with the results of resistivity measurements made during the heating and cooling cycles.     

Transformation-induced plasticity in two-phase (α+γ) Fe-Cr-Ni alloys

The relationship between the fracture elongation and the value of martensite per unit tensile strain has been studied for the two-phase ( + ) Fe-Cr-Ni alloys containing 10%, 35% and 52% 7. The elongation has a peak in the elongation-test temperature curve. The peak elongation is dependent upon both the delay of necking and a suitable value of martensite per unit tensile strain.     

Microstructures of titanium-aluminides produced by laser surface alloying

The microstructures of Ti-Al layers (from 43–80 at %Al) produced by laser surface alloying of titanium substrate with a powder feed technique have been investigated. The laser processing parameters were; 1.8 kW laser power, 3 mm beam diameter, 7 mm s^–1 traverse speed, and values of powder flow rates of aluminium ranging from 0.07–0.11 g s^–1. The microstructures were dendrites of ₂ and interdendritic regions of ₂+ in the Ti-43 at %Al alloy; dendrites of either ₂ or ₂+ with interdendritic in the Ti-50 at %Al alloy; dendrites of ₂+ with interdendritic in the Ti-55 at %Al alloy; single phase in the Ti-60 at %Al alloy and TiAl₃ dendrites and Al solid solution in the interdendritic regions in the Ti-80 at %Al alloy. The microstructures were fine and comparable to those produced by other methods of rapid solidification processing. The microstructures of the Ti-50 and Ti-55 at %Al alloys were in agreement with the existence of the peritectic reactions:L + andL + , in the Ti-Al system.     

Energy band theory and the lattice dynamics of rare gas crystals

The results of a simple local-density energy band model (X) are used to carry through a calculation of the phonon dispersion relations in fcc Ne and Ar. Adiabatic phonons, calculated from the computed X total energy surface, are perturbed by the Fröhlich Hamiltonian with electron-phonon matrix elements calculated from the X energy bands. The use of low-order perturbation theory gives results in fair comparison with observed values, but shows the general scheme to be feasible. Except for questions of the exact physical content of the X model itself, the calculation is entirely ab initio, and is believed to be the first such achieved in insulators.Work supported by Research Corporation and National Science Foundation, and based in part on the Ph.D. thesis of J. P. Worth.     

Effect of crystalline phase, orientation and temperature on the dielectric properties of poly (vinylidene fluoride) (PVDF)

The effect of crystalline phase, uniaxial drawing and temperature on the real () and imaginary () parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 10² and 10⁶ Hz. Samples containing predominantly and phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. phase samples were also obtained from melt crystallization and from commercial films supplied by Bemberg Folien. Different molecular orientations were obtained by uniaxial drawing of and phase samples. The results showed that the crystalline phase exerts strong influence on the values of and , indicating that the _a relaxation process, associated with the glass transition of PVDF, is not exclsively related to the amorphous region of the polymer. An interphase region, which maintains the conformational characteristics of the crystalline regions, should influence the process decisively. The molecular orientation increased the values of for both PVDF phases and modified its dependence with temperature over the whole frequency range studied. The influence of the crystallization and molecular orientation conditions on the dc electric conductivity (_dc) were also verified. The value of _dc was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio.     

Grain-boundary sliding and intergranular cavitation during superplastic deformation of α/β brass

Intergranular and interphase cavitation in binary alpha/beta brass has been investigated in tension at 600° C under conditions of superplastic deformation. The sites for nucleation of cavities has been studied by quantitative metallography and the cavities are observed to nucleate preferentially at- interfaces. The process of cavitation is associated with grain boundary sliding and cavity nucleation occurs at points of stress concentrations in the sliding interfaces. Measurements of grain and phase boundary sliding at various interfaces demonstrate that sliding occurred on- boundaries more readily than on- and-gb interfaces. The predominance of- interface cavitation is believed to be as a result of greater sliding at the- boundary and of an unbalanced accommodation of sliding adjacent to this type of boundary.     

Morphology of nitrated layers in titanium alloys of different structural classes

This paper generalizes the results of investigations of the effect of nitrogen on titanium alloys at temperatures of isothermal exposure of 800 – 1100°C and gas pressures of 10⁵ – 10^–1 Pa. As a result of nitration, a gas-saturated area appears on the surfaces of the alloys (in addition to the nitride area). Its qualitative and quantitative characteristics depend on the phase composition of the alloys. The gas-saturated area consists of two layers. For all the investigated alloys, the first layer (adjacent to the nitride area) can be described as the -titanium structure stabilized with nitrogen. If the saturation temperature is lower than the temperature of the polymorphic transformation, then the structure of the second layer is identical to the alloy matrix. As a result of nitration in the (+)--region, this structure undergoes the transformation and, for - and pseudo--alloys, turns into an -grain of smaller size but with higher etchability compared to the -structure of the first layer. For (+)-alloys, this is mainly the -phase (-plates) in the -transformed structure. For -alloys, the morphology of the gas-saturated area does not depend on the saturation temperature and is identical to other structural types nitrated in the -region.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 35–41, September–October, 1993.     

Specific analysis of EBSD data to study the texture inheritance due to the β → α phase transformation

A specific processing of EBSD data is proposed to study the texture inheritance of titanium or zirconium alloys. A non standard misorientation map is calculated to localise the colonies inherited from the same parent grain. The calculation of the parent orientation from its inherited variants detailed in previous works has been adapted to the data obtained from an automated EBSD analysis. Finally, a method to derive the orientation map of the parent phase from that of the inherited phase is proposed. The resulting and COMs are used to study some aspects of the variant selection occurring in the transformation of a T40 sample.     

Eutectic solidification characteristics of Bridgman grown AI-3Fe-0.1V alloy

Eutectic solidification characteristics of Al-2.85 wt%Fe-0.12 wt% Valloy have been investigated by steady-state growth over the range of solidification front velocity from 51 to 1030m/s and temperature gradient 8 to 15 K/mm. Increasing growth velocity displaced the Al-Al3Fe eutectic by Al-AlxFe eutectic rather than by the Al-Al6Fe eutectic obtained for the binary Al-3 wt%Fe alloy. A fully Al-AlxFe eutectic structure has been obtained for the first time in the vanadium-containing alloy over the growth velocity range from 71 to1030m/s except at 100 and 510m/s where some Al dendrites were present in the eutectic matrix. TheAl-AlxFe eutectic was observed to undergo a morphological transition from lamellar to rod-like with increasing growth velocity concurrently with formation of a cellular eutectic structure. It was found that the relationship = Av–1/2, between eutectic spacing and growth velocity v, was applicable with A = 22.4 ± 1.8 and 13.8 ± 2.1 m3/2s–1/2 for lamellar and rod-like Al-AlxFe eutectics, respectively.     

Optimum notch configurations for the chevron-notched four-point bend specimens

The failure sequence following crack formation in a chevron-notched four-point bend specimen is examined in a parametric study using the Bluhm slice synthesis model. Premature failure resulting from crack formation forces which exceed those required to propagate a crack beyond is examined together with the critical crack length and critical crack front length. An energy based approach is used to establish factors which forecast the tendency of such premature failure due to crack formation for any selected chevron-notched geometry. A comparative study reveals that, for constant values of ₁ and ₀, the dimensionless beam compliance and stress intensity factor are essentially independent of specimen width and thickness. The chevron tip position ₀ has its primary effect on the force required to initiate a sharp crack. Small values for ₀ maximize the stable region length, however, the premature failure tendency is also high for smaller ₀ values. Improvements in premature failure resistance can be realized for larger values of ₀ with only a minor reduction in the stable region length. The stable region length is also maximized for larger chevron base positions, ₁, but the chance for premature failure is also raised. Smaller base positions improve the premature failure resistance with only minor decreases in the stable region length. Chevron geometries having a good balance of premature failure resistance, stable region length, and crack front length are 0.20 ₀0.03 and 0.70₁0.80NASA Resident Research Associate at Lewis Research Center.     

Phase decomposition in extruded Zn-Al based alloy

Ageing characteristics of an extruded eutectoid Zn-Al based alloy were investigated using X-ray diffraction and scanning electron microscopy techniques. The extruded alloy consisted of Al rich phase and Zn rich _E and phases. The original cast eutectoid Zn-Al alloy was extruded at 250 °C. Both supersaturated _s and _s phase decomposed during extrusion and appeared as fine and coarse lamellar structures. The _E and phases particles formed in the original interdendritic region. It was found that two Zn rich phases _E and decomposed sequentially during ageing at 170, 140 °C. The decomposition of the _E phase occurred as a discontinuous precipitation in the early stage of ageing and the decomposition of the phase took place in a four phase transformation: + T + in the prolonged ageing. Two typical morphologies of the decomposition of the Zn rich phases _E and were distinctive in back-scattered scanning electron microscopy.     

Boundary layer similarity flow driven by power-law shear

Summary Similarity solution of the Prandtl boundary layer equations describing wallbounded flows and symmetric free-shear flows driven by rotational velocitiesU(y)=y are determined for a range of exponents and amplitudes . Asymptotic analysis of the equations shows that for <–1 no similarity solutions with proper algebraic decay exist. For wall-bounded flow, exact solutions found at =–1/2 and =1 correspond to an Airy function wall jet and uniform planar Couette flow. Numerical integration of the governing similarity equation reveals singular behaviour for wall-bounded flows as ₀ = –2/3, and no solutions are found in the range –1<–2/3. For >–2/3 the shear stressf(0) parameter is determined as a function of and . Symmetric free-shear flow solutions become singular as ₀ = –1/2 and no solutions are found in the range –1<–1/2. For >–1/2 the centerline velocityf(0) is determined as a function of and . An asymptotic analysis of the singular behavior of these two problems as ₀, given in a separate Appendix, shows excellent comparison with the numerical results. Similarity solutions at the critical values ₀ have exponential decay in the far field and correspond to the Glauert wall jet for wall-bounded flow and to the Schlichting/Bickley planar jet for symmetric free-shear flow.     

Phase transformation behavior in a multipurpose dental casting gold alloy during continuous heating

Phase transformation in a multipurpose dental casting gold alloy during continuous heating was studied by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. The behavior can be explained by the following reaction sequences in the nodule: ₁(FCC) + ₂ (L1₂) ₁(fcc) + ₂(L1₂) + (L1₀), where fcc is face centred cubic. A discontinuous precipitation with very fine nodules contributed to the hardening and the growth produced the softening. This multipurpose gold alloy is characterized by the introduction of a PtZn ordered phase with L1₀ structure instead of a CuAu I phase.     

Devitrification of sodium disilicate glass: a NMR study

Several crystallographically non-equivalent structures of sodium disilicate are known to exist. Successive heat treatments of the sodium disilicate glasses show -, -, - and -Na2Si2O5 as the devitrification products, depending upon the nature of heat treatment. At higher temperatures and for longer treatment times, only -Na2Si2O5 is observed but for intermediate temperature and time, and forms of the polymorphs are produced. For shorter times, all the polymorphs are formed. The results are verified using magic angle spinning nuclear magnetic resonance as well as X-ray diffraction. A way of estimating the unknown Si–O–Si mean bond angle of the polymorphs, with the help of the known values of the angles, is also presented.