The activation and the spreading of deformation in a fully lamellar Ti–47 at.% Al–1 at.% Cr–0.2 at.% Si Alloy

The spreading of deformation in a lamellar Ti–47?at.% Al–1?at.% Cr–0.2?at.% Si alloy deformed under compression is studied at 25°C and 600°C. This microstructure is largely dominated by twin-related variants which are separated by either twin interfaces or thin α ₂ slabs. The alloy deforms at both temperatures by ordinary dislocations and twins. Deformation in a particular γ variant and its adjacent twin-related variant involves the same kind of glide system, either ordinary dislocations or twins. This property is found to be true for all twin-related lamellae. The occurrence of this correlated glide is explained by the introduction of the notion of pilot and driven orientations. The lamellar orientation in which the operating glide system is activated on the basis of Schmid factor considerations is termed the pilot orientation. It imposes its deformation system on to the twin-related lamella, called the driven orientation, whose deformation may not involve the slip system most favoured by the applied stress.     

Defect-dependent nitride surface layer development upon nitriding of Fe–1 at.% Mo alloy

Upon nitriding of binary Fe–1 at.% Mo alloy in a NH₃/H₂ gas mixture under conditions (thermodynamically) allowing γ′-Fe₄N_{1– x} compound layer growth (nitriding potential: 0.7?atm^?1/2 at 753?K (480?°C) – 823?K (550?°C)), a strong dependency of the morphology of the formed compound layer on the defect density of the specimen was observed. Nitriding of cold-rolled Fe–1 at.% Mo specimens leads to the formation of a closed compound layer of approximately constant thickness, comparable to nitriding of pure iron. Within the compound layer, that is, in the near-surface region, Mo nitrides are present. The growth of the compound layer could be described by a modified parabolic growth law leading to an activation energy comparable to literature data for the activation energy of growth of a γ′-Fe₄N_{1? x} layer on pure iron. Upon low temperature nitriding (i.e. ?793?K (520?°C)) of recrystallized Fe–1 at.% Mo specimens, an irregular, ‘needle-like’ morphology of γ′-Fe₄N_{1? x} nucleated at the surface occurs. This γ′ iron nitride has an orientation relationship (OR) with the matrix close to the Nishiyama–Wassermann OR. The different morphologies of the formed compound layer can be interpreted as consequences of the ease or difficulty of precipitation of Mo as nitride as function of the defect density.     

Crystallographic characteristic of the structural defects of martensite in Fe–32 at % Ni alloy

This crystallographic analysis of the structure has been based on the physical theory of large plastic deformation developed by V. V. Rybin, V. I. Vladimirov, and A. E. Romanov. The same terminology and its physical meaning as applied to plastic relaxation of elastic stress, the occurrence of which accompanies the γ→ α transformation, has been used in this study.     

Initiation of discontinuous precipitation at interphase boundaries in a two-phase Zn–6.3 at.% Ag alloy

Discontinuous precipitation (DP) was found to initiate at both η′–ε interphase and η′–η′ grain boundaries in a two-phase Zn–6.3?at.%?Ag alloy consisting of ε and supersaturated η′ phases. The η′–ε interphase boundaries at which DP has initiated illustrated a sinusoidal interface during ageing, which implies that the morphological instability is a prerequisite for the DP initiation at an η′–ε interphase boundary. The application of the morphological instability model for solid–solid interfaces has indicated that the interface protuberances grow into the supersaturated η′ and the interphase boundary becomes unstable since the observed wavelength of serrated η′–ε interphase boundaries was larger than the critical value predicted by the model. A solute-depleted region is therefore established in front of the η′–ε interphase boundary, which provides an appropriate site leading to a DP reaction. Based on this, a nucleation mechanism of DP at the interphase boundaries is proposed accordingly: the allotriomorphs of DP can be directly developed from ε protuberances of a serrated interphase boundary.     

Continuous-cooling α-to-γ transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy

The continuous-cooling transformation behaviour of Ti–45.5?at.%?Al–0.05?at.%?B alloy was quantitatively measured using a real-time resistivity–temperature–time measurement apparatus operating under a high vacuum. The addition of a small amount of B does not significantly alter the α–γ-phase equilibria but significantly raises the α–γ lamellar start temperature of Ti–45.5?at.%?Al alloy at most cooling rates. Furthermore, it markedly increases the critical cooling rate for the ordering reaction. The effect of B addition, which greatly stabilizes the lamellar structure up to a fast cooling rate, is to accelerate the lamellar formation kinetics; the lamellar spacing was nevertheless distinctively larger in a B-doped alloy. This is because lamellae in B-doped alloy nucleate heterogeneously on titanium borides at the grain boundary; the borides are effective nucleation sites particularly since local Ti depletion can occur near the interface of the growing titanium borides during cooling. In the absence of B addition, the lamellar structure starts to form only at temperatures below T ₀, suggesting that a large undercooling is required for the nucleation of lamellae even at the grain boundaries. On the other hand, the B addition greatly retards the kinetics of the α-to-α₂ ordering reaction by markedly increasing its critical cooling rate without a large change in the ordering temperature. This is believed to be due to its tendency to segregate strongly to the antiphase boundaries.     

Mesoscopic defects in the two-phase α + γ state of Fe–32 at % Ni alloy

An extreme discrepancy in the observed martensite structure when studying by means of optical and scanning microscopes in the same sample has been found. The results have been compared with data from the literature. An assumption on the effect of sample heating on the process of transformation of elastic spreads of fragments in the peripheral region of martensitic laminae into plastic spreads has been put forward.     

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe₄N_{1? x} ) was investigated upon nitriding in NH₃/H₂ gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe₂N_{1? z} ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.     

Effect of Annealing in a Ferromagnetic State on the Structure of an Fe–18 at % Ga Alloy

Physics of the Solid State - The atomic structure of the iron–gallium alloy containing 18 at % Ga has been studied by X-ray diffraction. The samples were annealed in the paramagnetic (T...     

Microstructure,creep, and tensile behaviour of a Ti–15Al–33Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep and tensile deformation behaviour of a Ti–15Al–33Nb (at.%) alloy was studied. Monolithic sheet material was produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Electron microscopy studies showed that depending on the heat-treatment schedule, this alloy may contain three constituent phases including:?β?(disordered body-centred cubic), α₂ (ordered hexagonal close-packed based on Ti₃Al) and O (ordered orthorhombic based on Ti₂AlNb). Heat treatments at all temperatures above 990°C, followed by water quenching, resulted in fully-β microstructures. Below 990°C, Widmanstätten O-phase or α₂-phase precipitated within the?β?grains. The fine-grained as-processed microstructure, which exhibited 90?vol.% β-phase, exhibited excellent strength (UTS?=?916?MPa) and ductility (?_f>12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower ? _f values with UTS values ranging between 836–920?MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63?vol.% O-phase. Specimens subjected to 650°C tensile experiments tended to exhibit lower strength values while maintaining higher elongation-to-failure. Tensile creep tests were conducted in the temperature range 650–710°C and stress range 49–275?MPa. The measured creep exponents and activation energies suggested that grain boundary sliding operates at intermediate stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behaviour are discussed in comparison to a Ti–12Al–38Nb O?+?β alloy.     

Nucleation of the Al6(Fe,Mn)-to-α-Al–(Fe,Mn)–Si transformation in 3XXX aluminium alloys. I. Roll-bonded diffusion couples

Roll-bonded diffusion couples are used to investigate a transformation of intermetallic particles from Al₆(Fe,?Mn) to α-Al–(Fe,?Mn)–Si that occurs upon homogenization of 3XXX aluminium alloys. By diffusing silicon into an Al–Fe–Mn alloy, the couples permit a progressive increase in the driving force for this 6-to-α transformation, thus allowing study of the nucleation of the transformation. Initially, the aluminium matrix is highly defected from rolling. This microstructure gives frequent (yet stochastic) nucleation of a eutectoid 6-to-α transformation expected from study of direct-chill-cast 3XXX alloys. However, once the matrix has recrystallized, nucleation is restricted to particles that lie on the matrix grain boundaries. The remaining particles, unable to transform eutectoidally, dissolve and supply growth of these α-phase particles, producing marked coarsening.     

Precipitation structure in an Al-2·0 at. % Zn-1·0 at. % Mg alloy within and close to the grain boundaries and its effects on mechanical properties

During ageing the Al-2·0 at. %Zn-1·0 at. % Mg alloy at elevated temperatures the width of the precipitation free zone (PFZ) at both sides of the grain boundaries (GB) is identical with that free of quenched-in dislocation loops (DL). After sufficiently long ageing a band of precipitates is growing inside the former PFZ and the DLs in the midgrain regions act as sites for the heterogeneous nucleation of the-phase. Two precipitation processes significantly affect the mechanical properties. TEM correlates grain boundary precipitate density and morphology with grain boundary misorientation.     

Interface dislocation structures at the onset of coherency loss in nanoscale Ni–Cu bilayer films

We have performed a transmission electron microscopy study, using weak beam imaging, of the interface dislocation arrays that form initially at the (001) Ni–Cu interface during coherency loss. Interface dislocations were absent in the 2.5?nm Ni/100?nm Cu bilayers, but were present in the 3.0?nm Ni samples, indicating that the critical Ni film thickness for coherency loss is between 2.5 and 3?nm. The key features of the interface dislocation structure at the onset of coherency loss are: (i) the majority of interface dislocations are 60° dislocations, presumably formed by glide of threading dislocations in the coherently stressed Ni layer, and have Burgers vector in the {111} glide plane; (ii) the interface contained approximately 5% Lomer edge dislocations, with Burgers vector in the {001} interface plane, and an occasional Shockley partial dislocation and (iii) isolated segments of interface dislocations terminating at the surface are regularly observed. Possible mechanisms that lead to these dislocation configurations at the interface are discussed. This experimental study shows that near the critical thickness, accumulation of interface dislocations occurs in a somewhat stochastic fashion with favourable regions where coherency is first lost.     

Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.     

Substructural Transformations in Cu – 12 at.% Al Single Crystals with [001] Deformation Axis

Russian Physics Journal - The paper presents a detailed analysis of the relation between the substructural transformations and plastic deformation stages in [001]-oriented Cu – 12 at.% Al...     

Intralamellar dislocation networks formed by glide in γ-TiAl I. The mechanism of formation

In a lamellar TiAl alloy deformed at room temperature under an orientation that activates slip parallel to the interfaces, the nphase exhibits intralamellar dislocation networks parallel to the primary slip plane and entirely glissile in their habit plane. Their meshes are mainly rectangular with branches all coplanar, screw or near-screw in character and with Burgers vector of and types. Dislocation organization at and in the near-vicinity of these intralamellar networks suggests a reaction between a family of primary coplanar d011] dislocations that slip in the network habit plane and a family of dislocations that cross-slip from a plane inclined to the lamellae into the network. The reactions result in junctions with Burgers vector that subsequently transform into rectangular units. The presence of these networks is consistent with that of a residual elastic twist between adjacent nlamellae.     

In-situ transmission electron microscopy study of dislocation processes at precipitate-free zones in a γ ′-strengthened superalloy

In precipitation-strengthened polycrystals, precipitate-free zones (PFZs) often form along grain boundaries. These PFZs lower the yield strength. In this investigation, thin foils of the commercial γ′-strengthened nickel-based superalloy Nimonic PE16 have been strained inside a transmission electron microscope and the relevant dislocation processes in the PFZs and in the γ′-strengthened material next to them have been observed under load. Since the PFZs are only solid solution strengthened, they are softer than the interior of the γ′-strengthened grains. Many different slip systems are activated in the PFZs even at relatively low external stresses. Multiple slip allows for compatible deformation of neighbouring grains. Extensive cross-slip and double cross-slip in the PFZs lead to a high dislocation multiplication rate. Easy creation of dislocations in the PFZs and pile-ups at the border between the PFZs and the γ′-strengthened interior of the grains enhance the propagation of slip across grain boundaries and thus lower the yield strength of the material.     

Magnetic losses at high flux densities in nonoriented Fe–Si alloys

We present and discuss power loss measurements performed in Fe–(3.5 wt%)Si nonoriented laminations up to very high flux densities. The results are obtained on disk samples using a 1D/2D single-sheet tester, where the fieldmetric and the thermometric methods are applied upon overlapping polarization ranges. The power loss in the highest polarization regimes (e.g. J_p>1.8 T) is measured, in particular, by the rate of rise of temperature method, both under controlled and uncontrolled flux density waveform, the latter case emulating the conditions met in practical unsophisticated experiments. Lack of control at such extreme J_p levels is conducive to strong flux distortion, but the correspondingly measured loss figure can eventually be converted to the one pertaining to sinusoidal induction at the same J_p values. This is demonstrated as a specific application of the statistical theory of magnetic losses, where the usual formulation for the energy losses in magnetic sheets under distorted induction is exploited in reverse fashion.     

Electron temperature diagnostics of aluminium plasma in a z-pinch experiment at the ＂QiangGuang-1＂ facility

<正>Two curved crystal spectrometers are set up on the "QiangGuang-1" generator to measure the z-pinch plasma spectra emitted from planar aluminum wire array loads.Kodak Biomax-MS film and an IRD AXUVHS5# array are employed to record time-integrated and time-resolved free-bound radiation,respectively.The photon energy recorded by each detector is ascertained by using the L-shell lines of molybdenum plasma.Based on the exponential relation between the continuum power and photon energies,the aluminum plasma electron temperatures are measured.For the time-integrated diagnosis,several "bright spots" indicate electron temperatures between(450 eV～520 eV)±35%.And for the time-resolved ones,the result shows that the electron temperature reaches about 800 eV±30%at peak power. The system satisfies the demand of z-pinch plasma electron temperature diagnosis on a～1 MA facility.     

Comparison of the deformation behaviour of commercially pure titanium and Ti–5Al–2.5Sn(wt.%) at 296 and 728 K

The tension and tensile-creep deformation behaviours of a fully-α phase commercially pure (CP) Ti and a near-α Ti–5Al–2.5Sn(wt.%) alloy deformed in situ inside a scanning electron microscope were compared. Tensile tests were performed at 296 and 728?K, while tensile-creep tests were performed at 728?K. The yield stress of CP Ti decreased dramatically with increasing temperature. In contrast, temperature had much smaller effect on the yield stress of Ti–5Al–2.5Sn(wt.%). Electron backscattered diffraction was performed both before and after the deformation, and slip trace analysis was used to determine the active slip and twinning systems, as well as the associated global stress state Schmid factors. In tension tests of CP Ti, prismatic slip was the most likely slip system to be activated when the Schmid factor exceeded 0.4. Prismatic slip was observed over the largest Schmid factor range, indicating that the local stress tensor varies significantly from the global stress state of uniaxial tension. The basal slip activity in Ti–5Al–2.5Sn(wt.%) was observed in a larger faction of grains than in CP Ti. Pyramidal ?c?+?a? slip was more prevalent in CP Ti. Although twinning was an active deformation mode in tension tests of the CP Ti, it was rare in Ti–5Al–2.5Sn(wt.%). During creep, dislocation slip was the primary apparent deformation mechanism in CP Ti, while evidence for dislocation slip was much less apparent in Ti–5Al–2.5Sn(wt.%), where grain boundary sliding was dominant. A robust statistical analysis was carried out to assess the significance of the comparative activity of the different slip systems under the variety of experimental conditions examined.