Lattice Defects Diffuse Scattering from Thin Films of a Ge-Si System with Low-Energy Ar+ and Xe+ Bombardment During Molecular Beam Epitaxy (MBE) Growth

Homo?CGe, homo?CSi, and hetero?CSi_0.2Ge_0.8 alloy epitaxial layers, using molecular beam epitaxy (MBE), were grown on Ge and Si (001) substrates in order to study development of crystalline strains caused by ion bombardment during the growth of materials. Ion energies and ion/atom fluxes were used in the epitaxial growth, and significant lattice distortions along the growth direction developed. Using high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM), the form of distortion, caused by low-energy argon (Ar⁺) and xenon (Xe⁺) bombardment of the thin epitaxial films grown on the (001) substrates, were investigated. The isotropic point defects case (of spherical distortions) occurs in epitaxial thin films ??as-grown?? processes. The intensity distribution has two maxima, one from the distorted layer and the other from the original unaffected matrix. The significant changes in the 2?? location, peak broadening and integrated intensity from the secondary (004)* reflections were obtained as a function of aging temperatures in the grown layers. Defects-induced diffuse scattering close to and between Bragg reflections supplies information on the strain and symmetry of the distortions fields and yields the atomic structure of point defects (self-interstitial, vacancies, and small clusters). First, aging heat treatment affects the distribution of distortions obtained in local regions at the ??as-grown?? layer, which develops to a special topography of continued distortions at higher aging temperatures. At aging temperatures above 923 K (650 °C), this extra diffraction peak disappears. The TEM observations reveal the appearance of dislocation lines with dark and bright contrasts around them, interdislocation strain contrasts, and disordered point defects atoms in the silicon region with semicoherent interfaces. The ion bombardment-induced formations and injection of the different types of pointlike defects and defects clusters.     

Investigation of “shock-induced” and “shock-assisted” chemical reactions in Mo + 2Si powder mixtures

Chemical reactions occurring in Mo + 2Si powder mixtures under “shock-induced” (during the high-pressure shock state) and “shock-assisted” (due to bulk temperature increases subsequent to unloading from the shock state) conditions were investigated using shock recovery experiments performed under a range of loading conditions. Cylindrical implosion geometry experiments showed fully reacted mixed-phase eutectic microstructure (MoSi₂ and Mo₅Si₃) in axial regions and a partially reacted region (containing MoSi₂ spherules surrounding molybdenum particles in melted and resolidified silicon matrix) in outer peripheral areas of the compact cross sections. Planar-pressure geometry experiments showed a single-phase MoSi₂ microstructure in almost the entire compact. Calculations of the peak shock pressure and maximum mean bulk temperature in the different regions of the compacts and their correlation with the observed microstructures suggest that the formation of the mixed-phase and partially reacted products in the implosion geometry samples is due to “thermally initiated” liquid-liquid or solid-liquid reactions. In contrast, formation of the single-phase product in the planar-pressure geometry experiments is due to solid-state “pressure-initiated” reactions. The “thermally initiated” reactions are a result of large increases in shock-generated bulk temperatures, produced in time scales of thermal equilibration following unloading from the high-pressure state; hence, these are referred to as “shock-assisted” chemical reactions. However, the pressure-initiated reactions occur during the rise to the peak pressure and in time scales of pressure equilibration; hence, these are referred to as “shock-induced” chemical reactions.     

200, 020, and 002 X-ray peaks in tempered Fe-18 Ni-C martensites

Separate 200, 020, and 002 X-ray peaks were recorded for 0.0, 0.4, and 0.8 wt pct carbon (18 pct Ni) martensites after tempering between 25 and 500°C. The carbon bearing martensites studied here have been tempered initially enough to eliminate the “high tetragonality” 002 peak usually recorded for as-quenched martensite and the present results apply to tempered martensite only. The peak maximum is taken to determine the lattice parameter and the peak shape is recorded. At all carbon levels and after all tempering treatments, the “crd parameter is larger than or equal to the “a” or “b”. The relative enlargement is very small (0.08 pct) for the lowest carbon level and for any carbon level after severe tempering (500°C for 15 min). For the two higher carbon alloys tempered at temperatures below 400°C (for 15 min) the “c” parameter is significantly larger than the “a” and “b” and for the 0.4 wt pct C alloy the “b” is significantly smaller than the“a” whereas in the 0.8 pct C alloy the “b” is slightly larger than the “a”. Within experimental error the mean volume of the unit cell does not change during the tempering studied here and is nearly unaffected by the initial carbon content. This indicates that little (at most 0.1 wt pct) carbon is dissolved in tempered martensite. In the low carbon alloy the peaks are symmetric and sharpen symmetrically during tempering. In the higher carbon alloys the peaks are nearly symmetric and sharp after severe tempering. After less severe tempering the 002 peak is asymmetrically broadened toward lower9 values (higher lattice parameters) whereas the 200 and 020 peaks are asymmetrically broadened toward higher 0 values corresponding to lower lattice parameters. This collection of results is tentatively interpreted as being due to strains in martensite due to transformation induced substructure and precipitated carbides.     

Aging embrittlement and grain boundary

“Clean” 3.5NiCrMoV steels with limited contents in trace elements (P, Sn, As, Sb) are commonly provided for manufacturing big rotor shafts. The possible increase in temperature in future steam turbines has promoted the development of “superclean” steels characterized by an extra drastic decrease of manganese and silicon contents. Their higher cost in comparison to “clean” steels leads to concern above which temperature they must be considered as mandatory for resisting aging embrittlement in operation. 3.5NiCrMoV “clean” steel samples (Mn = 0.30 pct; Si = 0.10 pct) were aged at 300 °C, 350 °C, and 400 °C for 10,000 hours up to 30,000 hours. No embrittlement results from aging at 300 °C and 350 °C, but holding at 400 °C is highly detrimental. Auger spectroscopy confirms that, when aging at 400 °C, phosphorus is the main embrittling trace element. It is suggested that grain boundary embrittlement is associated with the building of a layer that contains, on the one hand, Ni and P and, on the other hand, Mo and Cr. Head of the Testing and Head of the Testing and Head of the Testing and     

Computer study of tweed as a precursor to a martensitic transformation of a Bcc lattice

Using molecular dynamics computer simulations we have studied a bcc crystal with an interatomic potential having a second derivative which passes through zero near second neighbor distances. This causes an inherent tetragonal instability which manifests itself as random dynamic tetragonal distortions at high temperature (“dynamic tweed”), quasi-static and somewhat spatially correlated tetragonal distortions at intermediate temperatures (“static tweed”), and an apparent martensitic transformation to a close-packed structure at low temperatures. The effect of simple defects (vacancies and interstitials) on the development and spatial localization of the pretransformation structure (tweed) and the subsequent martensitic transformation will also be discussed. This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformation in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.     

Low-Temperature Carburization of the Ni-base Superalloy IN718: Improvements in Surface Hardness and Crevice Corrosion Resistance

“Case-hardening” of the Ni-base superalloy IN718 has been achieved by low-temperature gas-phase carburization. After carburization under optimum conditions, the hardened surface layer (the “case”) has about twice the hardness of the core (HV of ≈800) and contains ≈12 at pct carbon in interstitial solid solution. This causes a lattice parameter expansion of ≈1 pct perpendicular to the surface and, because of the mechanical constraint provided by the noncarburized core below, develops a large biaxial surface compressive residual stress (≈1.9 GPa) parallel to the surface. Microstructural studies and X-ray diffractometry reveal no carbide precipitates in the case. In agreement with this observation, low-temperature carburization does not compromise the ductility and actually improves the crevice corrosion resistance of the alloy.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensitic R transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain = 0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensiticR transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain=0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.     

The influence of Mo and Co on the embrittlement of an Fe-Ni-Mn alloy

In the “as rolled” condition an Fe-6 Ni-5 Mn maraging type alloy was found to be brittle exhibiting intergranular fractures. The addition of 2.5 pct Mo and 5.0 pct Mo increased the impact toughness of the “as rolled” material and changed the mode of brittle fracture to transgranular cleavage. The addition of 9 pct Co embrittled the alloy. On aging Mo and Co raised the peak hardness of the base Fe-6 Ni-5 Mn alloy, however, aging led to rapid embrittlement. The base alloy and an alloy containing 2.5 pct Mo showed brittle intergranular fractures on aging. The addition of 5 pct Mo gave rise to brittle transgranular cleavage fractures on aging at 450°C, but at temperatures less than 450°C there was always up to 20 pct intergranular fracture present in brittle fractures. At temperatures greater than 475°C brittle intergranular failure occurred in the 5 pct Mo alloy due to a grain boundary film of M₆C and Fe₂Mo. This paper is based upon a thesis submitted by D. R. Squires in partial fulfilment for a higher degree of CNAA at Sheffield Polytechnic.     

The kinetics of carbide precipitation in silicon-aluminum steels

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100} _α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed. This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.     

Microstructural development of a gas-atomized and hot-pressed super-α2 alloy

A variety of heat treatments have been employed to explore the microstructure in Ti-25Al-10Nb-3V-lMo alloy prepared by gas atomization and hot pressing. These treatments include quenching by oil cooling and water cooling and aging at temperatures between 530 °C and 950 °C. Quenching transformations from the β-phase field include the formation ofO phase in oil quenching and β (disordered) +O phase in water quenching. The metastable β phase decomposes intoO + “Ω”,O, or α₂ + β_o/B2 phase when the as-quenched alloy is aged at various temperatures. By comparing the selection area diffraction patterns, it has been found that the ordered w phase in the alloy studied in this article is distinct in structure to the “Ω type” (P3m1) and B8₂ phase which are formed in the parent matrix of the ordered β(B2,D0₃) phases. It has also been shown by X-ray diffraction (XRD) analyses that the lattice parameters of the as-agedO phase do not remain constant in the alloy at various temperatures.     

Investigation of behavior of elemental sulfur in H<Subscript>2</Subscript>O-OH<Superscript>−</Superscript>-CN<Superscript>−</Superscript> solutions

The processes of dissolution of elemental sulfur in S-H₂O-OH⁻-CN⁻ systems are investigated. These processes simulate the technological processes of cyanidation and the sorption leaching of Au from the biocakes of gold recovery plants in the range of concentrations of the CN⁻ ions of 0–0.02 mol/dm³ and the range of concentrations of the OH⁻ ions of 0–0.1 mol/dm³. Using the simple-lattice planning method of the experiment, mathematical models and “solvent composition-amount of dissolved sulfur” diagrams are constructed for temperatures of 20, 30, and 40°C. It is established that, at t = 20°C, the strongest effect on the solubility of S in the mentioned solvents exerts the concentration of cyanides, while, at t = 30 and 40°C, the larger contribution to an increase in transfer of S into the solution gives the OH⁻ content. The role of hydrolysis that the CN⁻ ion plays in increasing the solubility of sulfur in the studied systems is shown.     

Lattice transformations related to unique mechanical effects

Athermal and stress-induced martensitic transformations are examined in various alloys of the large family which exhibit the unique “memory” and/or “superelastic” shape memory effects (SME). Such mechanical effects are found to be intimately related to details of the martensitic and premartensitic reaction paths in each system. A common feature of various “uncommon” systems is that the usual phenomenological crystallographic analysis cannot completely describe the martensitic transformation in these systems. Addiional features represented by lattice “shuffles” or low-wavelength lattice waves, and the mechanistic role of transformation dislocations are examined. A common thread in various systems such as TiNi, CuZn, AuCd, In-Tl, and so forth, is viewed in terms of evidence related to alloying (electronic entropy) effects on lattice instability of the parent phase. Instability reflected by premonitory phenomena can be given considerable generality when related to observations in systems which exhibit similar dynamic lattice transitions, such as the second-order “athermal omega” lattice transition in Group IV-base systems. The importance of reversibility in martensitic transformation of SME alloys is emphasized. Comparisons with more common non-SME martensitic alloys are made. This paper is based on a presentation made at a symposium on “Phase Transformations in Less Common Metals: A Dialogue,” held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals.     

Poisson Effects on X-Ray Diffraction Patterns in Low-Temperature-Carburized Austenitic Stainless Steel

Austenitic stainless steel was carburized at low temperature to generate a hard surface layer. X-ray diffractometry (XRD) revealed that this “case” contained an expanded fcc lattice and significant residual stresses due to the interstitial carbon. The XRD patterns also exhibit consistent variations with crystallographic orientation. Using published elastic constants for austenitic stainless steel and appropriate approximations for the XRD elastic constants, the XRD peak position variations can be accounted for by orientation-dependent Poisson effects due to biaxial residual stresses. The XRD patterns of specimens containing either compressive or tensile residual stresses were consistent with this hypothesis. This article is based on a presentation given at the “International Conference on Surface Hardening of Stainless Steels,” which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS.

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Plutonium—an Element Never at Equilibrium

The relentless deposition of energy from the α-particle decay of plutonium damages its crystal lattice and transmutes plutonium into other elements over time (principally, helium, americium, uranium, and neptunium). At cryogenic temperatures (4 K), lattice damage causes significant volume expansion of pure plutonium and contraction of face-centered-cubic stabilized alloys, and both appear to lose crystallinity at long irradiation times. At room temperature, much of the lattice damage is annealed out because defects produced by self-irradiation are sufficiently mobile. Nevertheless, plutonium’s delicate balance of stability with changes in temperature, pressure, or chemistry may be affected by self-irradiation. For example, at room temperature the lattice of fcc plutonium alloys expands and exhibits nanoscale bubbles at irradiation levels <0.1 displacements per atom (dpa). In addition to self-irradiation damage, it is now generally agreed that most fcc alloys previously believed to be thermodynamically stable at room temperature are in fact metastable. They undergo eutectoidal decomposition to α-plutonium, plus the nearest intermetallic compound. However, for most practical purposes, the kinetics of phase decomposition are too slow to be of concern. So, although plutonium may not be “far” from equilibrium, it is never at equilibrium because of the very nature of its radioactive decay. Surface reactions in plutonium can be increased catastrophically by the presence of moist air or hydrogen. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.     

Thermal Transients During Processing of 3003 Al-H18 Multilayer Build by Very High-Power Ultrasonic Additive Manufacturing

Previous investigations suggested a gradient in bond microstructure along the height of a “build” made by very high power ultrasonic additive manufacturing—a rapid prototyping process that is based on ultrasonic seam welding. The bonding of foils is associated with the occurrence of dynamic recrystallization at the interfaces between them. To understand heating patterns across the build that may be responsible for such microstructure evolution, temperatures from different interface regions were recorded simultaneously during the fabrication of a 3003 Al-H18 multilayer build under a given processing condition. Thermal transients were observed over multiple interfaces of the build during welding of each layer. The temperatures were the highest for the layer processed and were found to diminish beneath with each subsequent layer. Such maximum temperatures also depended on the height at which the new layer was bonded. The occurrence of transients across the build is rationalized based on heat being conducted away from the processed layer.     

Secondary Silicon Carbide Formed in the Interaction of Nanosized Silicon Carbide with Iron Oxide

The characteristics of superfine powder composites formed in the interaction of nanosized silicon carbide with iron oxide in vacuum and argon at 1200 and 1400°C, respectively, are analyzed. Silicon carbide (β-SiC), iron silicide and carbide, silicon oxide, and silicon oxynitride are main components of the powder composites. The lattice parameter of SiC in the powder composites synthesized in the SiC–Fe2O3 system is determined. In the interaction in the SiC–Fe2O3 powder mixture in vacuum, secondary SiC is synthesized with a lattice parameter that corresponds to the standard parameter for cubic β-SiC. The interaction in an argon atmosphere is accompanied by the synthesis of secondary SiC with a decreased lattice parameter. The minimum lattice parameter (0.4336 nm) is 0.6% smaller than the standard parameter for cubic β-SiC. The morphology of the powder composite synthesized in the SiC–Fe2O3 system is studied. The average particle size of the powder composite decreases with increasing weight content of secondary SiC.     

The crystallography of hydride formation in zirconium: II. the δ → ε transformation

The phenomenological crystallographic theory of martensitic transformations has been applied to the transformation from δ (fcc) to ε (fct) zirconium hydride, using published lattice parameters. The habit plane, orientation relationship, lattice invariant shear, and interface characteristics were determined by transmission electron microscopy and diffraction. The shape strain was observed by interference microscopy. Good agreement between the predictions of the theory and the measured crystallography was obtained. The predicted and observed lattice invariant shear was twinning on 101. These twins which are found within alternating bands of hydride variants produce a herringbone morphology, and the bands produce a roof gable type of surface relief. For a given plate, the measured habit plane, twin plane, unique Bain contraction axis, and orientation relationship were mutually consistent with the respective predictions for a single variant. The magnitude of the lattice invariant shear was in excellent agreement with the predicted value. The interfaces separating the e hydride bands were found to be of two types, which alternated, often filling an entire grain. One of these, termed a spear interface, was found to be a twin plane, across which the twinned regions of the two bands “matched-up”. The other, termed an impingement interface, was found to have twin regions which did not “match-up”. This morphology can be explained as a pair of ε-hydride plates which share a spear interface. When two growing spears impinge, the resulting impingement interface is of the second type. Formerly with the University of Illinois.     

A high resolution transmission electron microscope study of early stage precipitation on dislocation lines in Al-Zn-Mg

The weak beam transmission electron microscopy technique was employed to study the early stages of precipitation on dislocation lines in Al-3.87 wt pct Zn-1.79 wt pct Mg. The heterogeneous precipitation sequence was found to follow the homogeneous sequence in this alloy. The interaction between the initial coherent precipitate particles and the strain fields of the catalyzing dislocations produced “gaps” of background intensity at precipitate locations along the otherwise continuous weak beam images of the dislocation lines. A simple model was developed to relate a distribution of measured weak beam gap lengths to a particle size distribution at a given aging treatment. In this manner the growth kinetics of the initial precipitate phase was observed; it was found that the precipitation followed the Cottrell-Bilbyt ^2/3law, suggesting that matrix dislocations may assist the growth of heterogeneous precipitates in a manner analogous to grain boundary “collector plates.” Weak beam microscopy was found to be superior to standard bright field microscopy for the current study. Particles too small to be visible in bright field were revealed in weak beam. Weak beam observations also indicated that the coherent precipitate particles were positioned asymmetrically about the dislocation cores.