Some features of the formation and destruction of dislocation barriers in intermetallic compounds: IV. Thermoactivated straightening of dislocations along a preferred direction in TiAl

As a result of experiments which include plastic deformation and subsequent heating without stress, self-blocking of dislocations in TiAl has been revealed. The transformations of glissile dislocations into dislocation barriers has been observed both for superdislocations with Burgers vectors <101] and 1/2<112] and for single dislocations. The preliminary deformation was performed at room temperature; the heating without stress, at temperatures both below and above the peak temperature T _max in the temperature dependence of the yield stress σ_y(T). With a proper choice of the temperature of heating and its duration, it proved to be possible to fix the initial stages of the straightening of single dislocations along a preferred direction parallel to their Burgers vector and the subsequent formation of long blocked dislocations. As a result of TEM analysis, it has been shown that the barriers were formed during heating without stress but they were not destroyed. It has been revealed that the barriers with a total Burgers vector <101] in TiAl, in contrast to Ni₃Al, remain indestructible even when the experiments included a repeated deformation.     

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: I. Theory

A new concept of the possibility of thermoactivated blocking of superdislocations in the absence of external stresses is suggested. A factor that is related to the internal structure of a superdislocation and initiates its blocking due to the creation of a certain effective force has been revealed. Irrespective of the way in which the rearrangement of the superdislocation occurs and of the fine details of the barrier structure, the role of the above factor is determining. Conditions have been formulated under which the formation of barriers becomes possible without loading after a preliminary deformation while their destruction becomes impossible. Based on an analysis of slip on the cubic system, factors have been clarified that are responsible for the ambiguousness of the attempts of restoring the energy of antiphase boundaries from the width of superdislocations in some cases. The mechanisms of dislocation blocking in intermetallic compounds and semiconductors are compared.     

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: V. Single-valley and multivalley potential relief for dislocations

A sufficiently general problem of thermoactivated transitions of dislocations between valleys of the potential relief (single-valley and multivalley) has been considered. The possibility of straightening of dislocations along a preferred direction in the absence of an external stress has been investigated. The controlling factor of this process is a thermoactivated transition (flip) of a dislocation from a shallow valley into a deeper valley of the potential relief. The flip process involves the formation of a double kink and its subsequent reorientation resulting in the formation of an asymmetric kink. The driving force of the flip process has been calculated and shown to be determined by the difference in the depths of the valleys. It has been shown that in the absence of an external stress the flip process is allowed under specific conditions, whereas the escape from a deep valley is hindered with a result that self-blocking of dislocations becomes possible. An analysis of experiments on heating of intermetallic compounds without stress after a preliminary deformation have been analyzed (see parts II-IV of this work), and owing to the experiments, self-blocking of dislocations has been found. The observation or nonobservation of self-blocking in different materials is a tool that enables the character of the potential relief of dislocations to be restored; the self-blocking is possible only in the presence of two types of valleys of different depth. Moreover, the two-valley character of the relief is a prerequisite for the development of an anomalous behavior of the yield strength upon dynamic loading.     

Self-blocking of dislocations in intermetallic compound Ni<Subscript>3</Subscript>Ge: Cubic slip

Self-blocking of dislocations belonging to the cube plane has been detected in the intermetallic compound Ni₃Ge. The nature of cubic slip has been clarified. The experiments included deformation at a temperature above T _max (the temperature of the yield-stress peak) and subsequent heating without stress. In addition, changes in the dislocation structure upon slow cooling from the temperature of the preliminary deformation have been investigated; in doing so, self-blocking of dislocations has been revealed for the first time. Based on the totality of experimental data, a conclusion has been made on the two-valley potential relief for a dislocation motion in the cube plane. It is the difference in the depths between the Peierls valley (a diffuse core in the cube plane) and the deepest valley (a superpartial dislocation split in the octahedron plane) that provides for the driving force of self-blocking in the absence of an external stress. Different aspects of cubic slip have been considered: the disappearance of cubic slip at T < T _max and, correspondingly, the nonobservation of an anomalous behavior of the yield stress, as well as the disappearance of octahedral slip at T > T _max.     

Self-blocking of dislocations in the intermetallic compound Ni<Subscript>3</Subscript>Ge: reconstruction of a two-valley potential relief

The possibility of the reconstruction of the shape of the potential relief (single-valley-two-valley) and its parameters using the effect of self-blocking has been demonstrated. Allowed and forbidden regions for self-blocking have been revealed. A relation has been established between the ratio of the dislocation energies in a deep (E _d) and shallow (E ₀) valleys of the potential relief and the limiting angle [(f)\tilde]\tilde \phi for self-blocking. A method is suggested for determining the ratio of the energy depths E _d/E ₀ of the valleys via a detailed analysis of TEM images of the dislocation structure. The dislocation structure of preliminarily deformed Ni₃Ge single crystals has been studied after heating without loading. The angles of the transition from curvilinear to rectilinear segments extended along preferred directions have been measured and the estimations of the limiting angle [(f)\tilde]\tilde \phi of self-blocking have been performed. Based on these estimations, it is shown that the ratio of the energies of dislocations in the deep and shallow valleys is E _d/E ₀ ≈ 0.7. This ratio between the energies of dislocations in the deep and shallow valleys of the potential relief has been obtained for the first time.     

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: II. Observation of blocked superidislocations upon heating without stress

Experiments have been performed which reveal that heating in the absence of an external stress after a preliminary both low-temperature and high-temperature deformation of intermetallic compounds leads to a fundamental change in their dislocation structure. For the investigation, [251] single crystals of Ni₃(Al, Nb) have been used. The low-temperature deformation was performed at ?196°C; the high-temperature deformation, at 800°C. It has been found that the initial dislocation structure consisting of curvilinear dislocations was changed upon heating without a load by a set of rectilinear blocked dislocations. It has been shown that upon heating after a preliminary low-temperature deformation the barriers present in the structure belong to the cubic cross-slip plane, whereas upon heating after high-temperature deformation, to primary cubic slip planes. It has been found that the decisive effect on the blocking of superdislocations upon heating without stress comes from one of the dislocations that compose the superdislocation, namely, either a superpartial dislocation in the case of low-temperature deformation or a simple partial dislocation in the case of high-temperature deformation. The concept of the possibility of thermoactivated blocking of superdislocations in the absence of external stresses suggested in part I of this work [Phys. Met. Metallogr. 102, 61–68 (2006)] has been confirmed experimentally.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Phase transformation and microstructural features of NiAl/Ni<Subscript>3</Subscript>Al two-phase alloys containing ternary elements

Various ternary elements were added to observe the effects on the microstructural features of β+γ′ two-phase alloys. The microstructural features of β+γ′ two-phase (Ni₆₆Al₃₄)_100-χX_χ(X=Ti, Si, Nb) depended on the A_s (austenite start) temperature of β-martensite with alloying elements. For A_s>250°C, a lamellar microstructure was found to form by the following phase transformation: Martensite→Ni₅Al₃→β+γ’. For A_s<250°C, two-type microstructures, mesh and Widmanstätten, were formed depending on the ternary element. When Ti or Nb was added as a ternary element, the β→Ni₅Al₃ transformation occurred very quickly. Conversely, this transformation proceeded very slowly in the case of Si addition, and the resultant microstructure assumed somewhat different features. Consequently, it could be suggested that the microstructures of NiAl/Ni₃Al two-phase alloys are determined by not only the A_s temperature but also by the β→Ni₅Al₃ transformation.     

Tribological Properties of Ni<Subscript>3</Subscript>Al Matrix Composite Sliding Against Si<Subscript>3</Subscript>N<Subscript>4</Subscript>, SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> at Elevated Temperatures

The Ni₃Al matrix self-lubricating composite was fabricated by powder metallurgy technique. The tribological behavior of the composite sliding against commercial Si₃N₄, SiC and Al₂O₃ ceramic balls was investigated from 20 to 1000 °C. It was found that the composite demonstrated excellent lubricating properties with different friction pairs at a wide temperature range, which can be attributed to the synergetic effect of Ag, fluorides, and molybdates formed by oxidations. The Ni₃Al matrix self-lubricating composite/Si₃N₄ couple possessed the stable friction coefficient and wear rate.     

Crystallographic Features of Phases Involved in the Oxidation of Ti<Subscript>3</Subscript>Ni<Subscript>3</Subscript>CrSi<Subscript>6</Subscript>

The oxidation behaviour of Ti₃(Ni,Cr)₃CrSi₆ and Ti₄Ni₄Si₇ was studied in air both at 1000 and 1100 °C. The formation of the oxidation products and the phase transformation were characterized by in situ X-ray diffraction and SEM-FEG post-mortem observations. The crystal structure of Ti₃(Ni,Cr)₃CrSi₆ was also determined using powder X-ray diffraction and Rietveld refinement in order to describe this phase as a pseudolamellar structure comparable to the one of Ti₄Ni₄Si₇. Results evidenced that diffusion in solid state governs the oxidation rate of these silicides. Ti₄Ni₄Si₇ oxidation rate was assessed as being one order of magnitude lower than the one of Ti₃(Ni,Cr)₃CrSi₆, while this latter readily transformed into Ti₄Ni₄Si₇ during the first time of oxidation. The understanding of this particular behaviour in which the oxidation rate of Ti₃(Ni,Cr)₃CrSi₆ was not affected by the phase transformation implied to consider the crystallographic lamellar features of these compounds that play a major role in the diffusion of the most oxidizable elements.     

Structure and mechanical properties of an Ni<Subscript>3</Subscript>Al single crystal upon high-temperature deformation

Structure and strength properties of single-crystal 〈001〉 samples of Ni₃Al have been studied in the as-grown and homogenized state during tensile tests in the temperature range of 1150–1250°C. At the strain rate of 1.32 mm/min (2 × 10⁻⁵ m/s), the samples are in the state of superplasticity. The basic mechanism of relaxation is dynamic recovery; in some regions of the sample, recrystallized grains are formed. At 1250°C, coarse twins are observed in the zone of fracture, which indicates the “switching on” of additional slip systems necessary to guarantee the relaxation process.     

Degradation behavior of Ni<Subscript>3</Subscript>Al plasma-sprayed boiler tube steels in an energy generation system

Boiler steels, namely, low-C steel, ASTM-SA210-Grade A1 (GrA1), 1Cr-0.5Mo steel, ASTM-SA213-T-11 (T11) and 2.25Cr-1Mo steel, ASTM-SA213-T-22 (T22) were plasma sprayed with Ni₃Al. The alloy powder was prepared by mixing Ni and Al in the stoichiometric ratio of 3 to 1. The Ni-22Cr-10Al-1Y alloy powder was used as a bond coat, with a 150 μm thick layer sprayed onto the surface before applying the 200 μm coating of Ni₃Al. Exposure studies have been performed in the platen superheater zone of a coal-fired boiler at around 755 °C for 10 cycles, each of 100 h duration. The protection to the base steel was minimal for the three steels. Scale spallation and the formation of a porous and nonadherent NiO scale were probably the main reasons for the lack of protection. In the case of T22-coated steel, cracks in the coatings have been observed after the first 100 h exposure cycle.     

Structural transformations in the deuterium-containing intermetallic compound (Ti<Subscript>3</Subscript>Al)D<Subscript>1.2</Subscript> induced by high-pressure torsion

The intermetallic compound Ti₃Al (which is a very brittle material) with a high deuterium concentration (x = 1.2) is fabricated in a monolithic state by high-pressure torsion at room temperature. The structure of the intermetallic samples is studied by X-ray diffraction and transmission electron microscopy after deformation at various degrees. Under certain conditions, the main volume of the material is found to transform into an amorphous state, and areas 1–2 nm in size with an atomic arrangement close to the initial arrangement are also present in the material. The possible causes of the deformation-induced amorphization of the material alloyed with interstitial atoms are discussed.     

Hydrogen in the Ti<Subscript>3</Subscript>Al intermetallic compound: Study by the NMR method

The times of nuclear spin–lattice relaxation have been measured and the spectra of NMR for ¹H and ²⁷Al in Ti₃AlH _x hydrogenated intermetallic compounds with hydrogen concentrations of х = 0, 0.31, 0.51, 1.0, 2.0, and 4.32 in a wide range of temperatures (10–500 K) and resonance frequencies (8.2–90 MHz) have been recorded. The analysis of low-temperature relaxation data has shown that the addition of hydrogen with a concentration of х ≥ 2 leads to a significant change in the density of electron states at the Fermi level. It has been found that the hydrogen mobility in Ti₃AlH _x strongly depends on its concentration. For solid solutions of hydrogen in Ti₃Al with х < 1, the mobility of hydrogen atoms on the scale of frequencies of NMR in the investigated temperature range has not been detected. In Ti₃AlH _x compounds with х ≥ 2, the fast diffusion of hydrogen is observed, which can be related to changes in the structure of these compounds.     

Microstructure and Functional Mechanism of Friction Layer in Ni<Subscript>3</Subscript>Al Matrix Composites with Graphene Nanoplatelets

Microstructure and functional mechanism of friction layer need to be further researched. In the present work, the friction coefficients and wear rates are analyzed through response surface methodology to obtain an empirical model for the best response. Fitting results show that the tribological performance of Ni₃Al matrix composites (NMCs) with graphene nanoplatelets (GNPs) is better than that of NMCs without GNPs, especially at high sliding velocities and high loads. Further research suggests that the formation of integrated friction layer, which consists of a soft microfilm on a hard coating, is the major reason to cause the differences. Of which, the wear debris layer (WDL) with a low shear strength can reduce the shear force. The ultrafine layer (UL), which is much harder and finer, can effectively avoid fracture and improve the load support capacity. Moreover, the GNPs in WDL and UL can be easily sheared and help to withstand the loads, trending to be parallel to the direction of shear force.     

Solvothermal synthesis and thermoelectric properties of skutterudite compound Fe（0.25）Ni（0.25）Co（0.5）Sb3

Nanostructured skutterudite-related compound Fe_0.25Ni_0.25Co_0.5Sb₃ was synthesized by a solvothermal method using FeCl₃, NiCl₂, CoCl₂, and SbCl₃ as the precursors and NaBH₄ as the reductant. The solvothermally synthesized powders consisted of fine granules with an average particle size of tens of nanometers. The bulk material was prepared by hot pressing the powders. Transport property measurements indicated a heavily doped semiconductor behavior with n-type conduction. The thermal conductivity is about 1.83 W·m⁻¹·K⁻¹ at room temperature and decreases to 1.57 W·m⁻¹·K⁻¹ at 673 K. The low thermal conductivity is attributed to small grain size and high porosity. A maximum dimensionless figure of merit of 0.15 is obtained at 673 K.     

Tribological Performance of Ni<Subscript>3</Subscript>Al Matrix Self-Lubricating Composites Containing Multilayer Graphene Prepared by Additive Manufacturing

In order to improve tribological performance of Ni₃Al-based alloy, Ni₃Al matrix composites containing 1.5 wt.% multilayer graphene (MLG) are prepared through additive manufacturing (AM) and spark plasma sintering (SPS), which are denoted as NMAM and NMSPS, respectively. Tribological behaviors of NMAM and NMSPS against Si₃N₄ balls are researched under constant speed (0.2 m/s) and varied loads (from 4 to 16 N) for evaluating the tribological properties of NMAM and NMSPS. The results present that NMAM exhibits the excellent tribological properties [low friction coefficients (0.26-0.40) and considerable wear resistance (2.8-4.6 × 10^?5 mm³ N^?1 m^?1)] as compared to NMSPS, which attributes to the uniform enrichment of MLG with properties of high tensile strength and being easily sheared off on the worn surfaces. Owing to the use of spherical prealloyed powder containing multilayer graphene and the characteristics of layer by layer depositing in the AM process, NMAM has a more compact and uniform substrate, which persistently provides a source of the formation of continuous and stable frictional layer. Due to the characteristics of AM rapid solidification, NMAM has the small grain size and well-compacted microstructure, which can effectively reduce the probability of spalling wear and lead to the increase in wear resistance of materials. The research can offer the reference for self-lubricating materials prepared by AM technology.     

Phase Diagram of the Quaternary Al(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>-Cu(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-Zn(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-H<Subscript>2</Subscript>O System

The phase diagram of the H₂O-Zn(NO₃)₂-Al(NO₃)₃-Cu(NO₃)₂ quaternary system at 30 °C has been established by using the conductivity measurements. The solid-liquid equilibria of the H₂O-Zn(NO₃)₂-Al(NO₃)₃, H₂O-Zn(NO₃)₂-Cu(NO₃)₂, H₂O-Al(NO₃)₃-Cu(NO₃)₂ ternary systems and two isoplethic sections were determined experimentally. The solid phases in equilibrium with the saturated solution are the tri- and hemipentahydrate of copper nitrate, the hexahydrate α and β of the zinc nitrate and the nonahydrate of aluminum nitrate. The copper and zinc nitrates are relatively soluble in opposition to the aluminum nitrate which presents some important precipitation domains.     

Analysis of the Deformation Process in Ni<Subscript>3</Subscript>(Al,Fe) Intermetallic under Longitudinal Bending

The structure and magnetic properties of an Ni₃(Al, Fe) single crystal after high-temperature rolling deformation have been studied. It has been shown that high-temperature rolling deformation induces longitudinal bending in Ni₃(Al, Fe), which is accompanied by the nonuniform distribution of stresses along the length of the sample. It has been found that longitudinal bending leads to change in either the mechanical or physical properties of the metal. Dynamic recrystallization has been found to occur at high degrees of deformation (starting at 40%). Local change in the distance between nickel and aluminum atoms, as well as the chemical composition, that takes place in the bending zones (distortions) of a crystal lattice of Ni3Al intermetallic compound is accompanied by change in the saturation magnetization and Curie temperature.