Sequence Of Phase Transformations Research Articles

The Effect of Ageing on Phase Transformations and Mechanical Behaviour in Ni-Rich NiTi Alloys.

In this article, the results of research on a NiTi alloy with a high nickel content (51.7 at.%), produced using the additive technology SLM method and subjected to isothermal ageing after solution annealing, are presented. The study involved the determination of the sequence of phase transformations occurring using differential scanning calorimetry (DSC) and the determination of the temperature range of these transformations. In parallel, the phase composition was determined using the XRD method; the hardness and the Young's modulus were also determined. The analysis of the DSC results obtained indicates the following characteristic features of the NiTi alloy, which change with ageing time: (1) During cooling (from +150 °C to -50 °C), the type of transformation changes from a one-step transformation after solution annealing to a two-step transformation after the ageing process over 1, 20, and 100 h at 500 °C; (2) during heating (from -50 °C to +150 °C) for all the samples, regardless of the ageing time, only a one-step transformation from martensite M(B19') to austenite A(B2) is observed; (3) the temperature at which the transformation starts increases with the ageing time; (4) the width of the total temperature range of the transformation M(B19') → A(B2) during heating changes from large (ΔT = 49.7 °C), after solution annealing, to narrow (ΔT = 19.3 °C and ΔT = 17.9 °C after 20 h and 100 h of ageing); and, most importantly, (5) a comparison with the literature data shows that, irrespective of the composition of the NiTi alloy and the manufacturing technology of the alloy samples (regardless of whether this was traditional or additive technology), a sufficiently long ageing process period leads to the occurrence of the martensite → austenite transformation in the same temperature range.

Simultaneous improvement of strength and ductility in near β Ti alloy with (α + β) hybrid structure via intermediate phases transformation

The influences of simultaneous precipitation of isothermal ω and O′ on deformation mechanisms and mechanical properties of near β Ti alloy Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe with (α + β) hybrid structure are revealed. During continuous heating, the phase transformation sequence in this alloy is β + αp + ω-embryos + O′ → β + αp + ωiso + O′ → β + αp + ωiso + O′ + αs → β + αp + αs. Low-temperature heat treatments from 200 °C to 400 °C are carried out to develop such intermediate phases. Upon aging at 200 °C and 300 °C, the isothermal ω and O′ domains are continuously generated. In comparison with solution state, the deformation behaviors of aged samples are altered, and the yield stress and ductility are simultaneously increased. When aged at 200 °C, the yield strength is increased by 13.8 %, and fracture elongation is increased by 63.2 %. When aged at 300 °C, the yield strength is increased by 18.2 %, and preserves large ductility same to solution state. This is due to the development of isothermal ω and O′ domains suppress long-range stress induced martensitic transformation during loading, and the deformation mechanisms evolve into dislocations slipping gradually. When the aging temperature increases to 400 °C, α precipitation takes place leading to an obvious decrement in ductility. Upon loading the primary α can also accommodate partly the strains and relax local stress concentration. The results in this work clearly proves that better mechanical properties can be achieved through tailoring intermediate phases generation.

The influence of boron and silicon on the phase transformations of fullerene C60 at high temperatures and pressure

The effect of doping with two carbide-forming nonmetals - Si or B - (7 at. % each) on the phase transformations of crystalline C60 fullerene at a pressure of 8 GPa in the temperature range of 400–950 °C was studied using the X-ray diffraction method. Samples were synthesized from powders in a high pressure toroidal chamber, and X-ray examination was carried out at room temperature after rapid cooling. It has been shown that doping does not change the sequence of phase transformations of C60 fullerene as the temperature increases, the following phases are still observed: fcc phase - rhombohedral phase – disordered (amorphous) graphite. Boron reduces the formation temperature of disordered graphite, but silicon does not. This influence of carbide-forming non-metallic elements is fundamentally different from the influence of carbide-forming metals – Al and Fe, the addition of which increases the stability of fullerene molecules and increases the temperature of its transformation into disordered (amorphous) graphite by hundreds of degrees (P. A. Borisova, M. S. Blanter, V. V. Brazhkin, and V. P. Filonenko, Phase Transformations in C60 Fullerene with Iron and Aluminum at High Pressures and Temperatures, Bulletin of the Russian Academy of Sciences: Physics, 84 (2020) 851–856).

Phase transformation sequence of pre-oxidized roast-leach ferrovanadium residue

The depletion of the primary metal sources has prompted the exploration of alternative avenues for metal recovery. In the case of titanium and iron, the ferrovanadium residue produced through roast-leach processing of titanomagnetite presents a viable option for accessing these metals. Titanomagnetite resources, which contain valuable elements, such as iron, vanadium, and titanium, possess significant valuable potential. Titanomagnetite deposits are normally treated via smelting for vanadium or vanadium and iron recovery; titanium is not commercially recoverable. Titanomagnetites have recently been processed through the roast-leach method for vanadium primary production, and iron and titanium are typically part of the waste stream in this process. This study proposes a novel approach to determine the characteristic mineralogy and to study the phase transformation sequence of the roasted-leached ferrovanadium residue during the pre-oxidation process. Leaching was also done to evaluate the extraction potential of Fe, V and Ti on the pre-oxidized residue in comparison to the raw residue The roasted-leached ferrovanadium residue was sampled using the cone and quartering method and then, dried in an oven at temperatures of between 30 and 40 °C, for an hour after which, the remaining moisture content was determined. The bond milling method was employed to reduce the sample size, while the particle size distribution (PSD) was verified by using the standard laboratory Tyler series. Thereafter, the roasted-leached ferrovanadium residue was characterized with XRD, SEM, ICP-OES, and XRF. The samples were pre-oxidized at temperatures ranging from 300 °C to 1000 °C with an aim of improving the grades of iron, vanadium, and titanium-bearing minerals prior leaching. The results revealed the moisture content to be ∼5.07%. The bond work index of typical slags was estimated to be 10.2 kwh/t, with a determined d80 value of 200 μm. According to the XRF analysis, the predominant compounds present are hematite, Fe2O3 (75.55%), titanium dioxide, TiO₂ (12.79%), silicon dioxide, SiO2 (3.03%), and alumina, Aℓ2O3 (2.62%), along with minor compounds. XRD patterns exhibited the presence of FeTiO3 and VO2 in the as-received samples, while pre-oxidation induced the evolution of new phases such as hematite, rutile, anatase, and pseudobrookite.

Adaptive Phase or Variant Formation at the Austenite/Twinned Martensite Interface in Modulated Ni–Mn–Ga Martensite

AbstractThe crystal structure and transformation path from austenite to 10M martensite in Ni–Mn–Ga single crystal are examined employing high‐energy synchrotron radiation, scanning, and transmission electron microscopy. Using temperature gradient, an austenite/twinned martensite interface is stabilized revealing the crystal structure and microstructure of both phases and the transformation sequence across the interface. Depending on the distance from the interface, three distinct types of martensite crystal lattice, namely simple tetragonal and two monoclinic modulated ones, that is, 10M′ and 10M are confirmed. In situ measurements show that lattice mismatch formed at the habit plane is compensated by the formation of micro‐twinned and branched martensite along with an elastic change in lattice parameters. It is shown that the characteristics for periodic shuffling twin boundaries, such as modulation twins or inverted stacking faults, are installed in later stages of transformation. In other words, large microstructure elements that most efficiently accommodate the strain are installed first, and then smaller elements are operable. Overall, the experimental results show that the crystal lattice does not adapt modulated phase at the habit plane and cannot be built up from simple non‐modulated tetragonal blocks but rather a specific sequence of phase transformations using shear/shuffling deformation takes place.

Phase transformation sequence and kinetics of ω formation in Ti-3573 titanium alloy during continuous heating

The work explores the phase transformation sequence and the kinetics of ω formation in a novel metastable β titanium alloy of Ti–3Al–5Mo–7V–3Cr (wt%) through dilatometer test and phase/microstructure characterizations. The phase transformation sequence has been identified as β → β + ω, β + ω → α + β, β → β + α, and α → β during the continuous heating. The formation of ω phase has underwent nucleation and growth stages revealed by the S-shaped kinetic curve. The diffusion controlled mechanism has been analyzed via Avrami exponent. The average activation energy for β → β + ω phase transformation has been evaluated 90.42 kJ/mol via Kissinger–Akahira–Sunose equation. The activation energy difference between early and late stages of ω formation has been compared among metastable β titanium alloys. The small value of Ti-3573 alloy may imply a thermo-kinetic inconvenience of ω assisted α formation.

Unveiling the correlation between structural alterations and enhanced high‐voltage cyclability in Na‐deficient P3‐type layered cathode materials via Li incorporation

AbstractWith exceptional capacity during high‐voltage cycling, P3‐type Na‐deficient layered oxide cathodes have captured substantial attention. Nevertheless, they are plagued by severe capacity degradation over cycling. In this study, tuning and optimizing the phase composition in layered oxides through Li incorporation are proposed to enhance the high‐voltage stability. The structural dependence of layered Na2/3LixNi0.25Mn0.75O2+δ oxides on the lithium content (0.0 ≤ x ≤ 1.0) offered during synthesis is investigated systematically on an atomic scale. Surprisingly, increasing the Li content triggers the formation of mixed P2/O3‐type or P3/P2/O3‐type layered phases. As the voltage window is 1.5–4.5 V, P3‐type Na2/3Ni0.25Mn0.75O2 (NL0.0NMO, Rm) material exhibits a sequence of phase transformations throughout the process of (de)sodiation, that is, O3⇌P3⇌O3′⇌O3″. Such complicated phase transitions can be effectively suppressed in the Na2/3Li0.7Ni0.25Mn0.75O2.4 (NL0.7NMO) oxide with P2/P3/O3‐type mixed phases. Consequently, cathodes made of NL0.7NMO exhibit a substantially enhanced cyclic performance at high voltages compared to that of the P3‐type layered NL0.0NMO cathode. Specifically, NL0.7NMO demonstrates an outstanding capacity retention of 98% after 10 cycles at 1 C within 1.5–4.5 V, much higher than that of NL0.0NMO (83%). This work delves into the intricate realm of bolstering the high‐voltage durability of layered oxide cathodes, paving the way for advanced sodium‐ion battery technologies.

Thermochromic response of an individual Sn:VO2 strip with multiple coexisting phase domains

ABSTRACT The respective roles of Sn atoms and oxygen vacancies (Ovs) on phase transition in an isolated Sn:VO2 with multi-domains were investigated through the Temperature-dependent Raman spectra and the dynamical sequence of phase transformation. A Component-Temperature phase diagram depicting the dependence of phase transformation temperature on the content of Sn or Ovs is constructed. The increasing Sn content could elevate the tensile stress to sequentially induce the metallic R phase to finally form semi-conductive M1, T1, T2, and M2 phases. Meanwhile, the presence of oxygen vacancies caused by doped Sn exerted pressure stress to perturb the M2 phase to narrow its existing region slightly. In a certain order, the four co-existing phases M1, T1, T2, and M2 in an individual Sn:VO2 could be successively obtained by the distinct Semiconductor-Semiconductor Transition from M2, after undergoing Metal–Semiconductor Transition of R-M2 at 52°C, 62°C, 65°C, 67°C, respectively.

Microstructural changes and dry sliding wear behavior of Cu-15Ni-8Sn-0.5Co alloy during deformation and aging treatment

The microstructural evolution and dry friction wear performance of the Cu-15Ni-8Sn-0.5Co alloy during deformation and aging treatment were studied using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and three-dimensional surface profilometry. The results show that the peak hardness of the Cu-15Ni-8Sn-0.5Co alloy initially increases and then decreases with increasing cold deformation, reaching a maximum hardness of 440 HV at 70 % cold deformation. During the aging process at 400 ℃, the phase transformation sequence of the Cu-15Ni-8Sn-0.5Co alloy with 70 % cold deformation is as follows: modulated structure - D022 ordered phase - γ-D03 phase. The rapid formation of the D022 ordered phase, which is coherently dispersed within the matrix, is the main reason for the significant improvement in the mechanical properties of the alloy. However, the rapid nucleation and growth of the discontinuous precipitation structure are responsible for the reduced mechanical performance and limited resistance to over-aging in the alloy. Additionally, it also causes decreased friction and wear resistance in the alloy with 70 % cold deformation. At a load of 20 N, the primary wear mechanism of the peak-aged Cu-15Ni-8Sn-0.5Co alloy with 70 % cold deformation is abrasive wear. As the load increased to 120 N, the wear mechanism shifted to a combination of abrasive wear and fatigue spalling wear in the alloy.

Discontinuous phase separation, interstitial ordering and recrystallization during nitriding of FeNiCo medium entropy alloy

The sequence of phase transformations triggered as a consequence of inward diffusion of interstitial N during nitriding of FeNiCo medium entropy alloy was investigated. Recrystallized and cold rolled specimens of FeNiCo alloy were nitrided at 580 °C in a flowing ammonia gas atmosphere. Detailed microstructural characterization was carried out using scanning and transmission electron microscopies, X-ray and electron back scattered diffraction and atom probe tomography. Firstly, N supersaturation of about 2 at.% was observed to be established without any nanoscale chemical heterogeneities. Further increase in N-content triggered the occurrence of a discontinuous phase separation reaction, resulting in the formation of alternating Fe+N enriched and Co+Ni enriched phases with a characteristic lamellar morphology and cube-on-cube orientation relationship. The Fe+N enriched lamellae then undergo rapid interstitial N ordering, forming a γ′- Fe4N type nitride phase. The compositional partitioning however is only transient and is followed by a recrystallization of the lamellar microstructure into a compositionally homogeneous γ′-(FeCoNi)4N type matrix. Thermodynamic calculations show that this discontinuous phase separation reaction and subsequent composition homogenization via recrystallization results from isothermally forcing the alloy composition to first enter the metastable miscibility gap in the FeNiCo-N FCC solid solution (at N-supersaturation close to 2 at.%) and then moving out of the miscibility gap at high N contents, beyond 20 at.%. Role of volume misfits, the chemical partitioning and homogenization on the sequential occurrence of different transformations is discussed.

Effect of phase transformation sequence on the low cycle fatigue properties of bainitic/martensitic multiphase steels

The three microstructures of bainite-fresh martensite, pre-martensite-bainite and carbide-free bainite were obtained by modulating the phase transformation sequence by the heat treatment process in the medium-carbon multiphase steel. The effects of martensitic and bainitic phase transformation sequences on the low cycle deformation behavior of multiphase steels were investigated. The results showed that the sample of the fresh martensite transformation process (295 °C–15 min) exhibited the best fatigue life among all processes under strain control. The samples of the preformed martensite transformation process (250 °C–295 °C) and the carbide-free bainite transformation process (295 °C–120 min) exhibited similar fatigue life. The sample of 295 °C–15 min possesses good resistance to plastic destabilization, resulting in higher fatigue life. The results of the EBSD crack propagation path analysis demonstrate that high carbon M/A islands block crack propagation in the sample of 295 °C–15 min, and high angular grain boundaries skew the crack propagation path. Fracture scans observed that secondary cracks in the sample of 250 °C–295 °C extend inward with aggregation of voids, which are more likely to form crack sources.

Фазовые соотношения в системе SrO – GdO1.5 – FeO1.5

The lack of data systematization on the coexistence and stability of compounds formed in the SrO – GdO1.5 – FeO1.5 system at temperatures above 1100°C determines the interest in a more detailed study of this system. The research was focused on studying the stability of phases formed in the pseudobinary section (1-х)GdFeO3 – хSrO in the temperature range 1200- 1400°C in air. The interaction of components in the SrO – GdO1.5 – FeO1.5 system was studied by the method of annealing and quenching followed by physicochemical analysis. The phase composition and sequence of phase transformations were characterized by powder X-ray diffraction. Phase relations results in the SrO – GdO1.5 – FeO1.5 system in air were systematized. The formation of three complex perovskite-like oxides GdSr2FeO5, GdSrFeO4, Gd2SrFe2O7, located on the GdFeO3 – SrO binary section was established. Information on interplanar distances and reflection intensities of the GdSr2FeO5 compound has been supplemented and the existence of a miscibility gap for Gd1-xSrxFeO3-α solid solutions existing in the range 0.05≤x≤0.51 at 1400°C has been shown. Data on thermal stability of complex gadolinium ferrites based on GdO1.5 – SrO – FeO1.5 system has been expanded.

Elucidating the ball-milling-induced crystallization mechanism of amorphous NbCo1.1Sn via atomic-scale compositional analysis

NbCoSn-based half-Heusler is a promising thermoelectric material candidate due to its outstanding advantages. The recently introduced nanostructure tailoring via an amorphous precursor has greatly reduced thermal conductivity. Although the nanocrystallization of amorphous NbCo1.1Sn has been observed during ball milling, its mechanism remains unclear even though it affects the microstructure of the milled powder, which is important for optimizing the final microstructure after sintering. In this study, the crystallization mechanism of the amorphous matrix induced by ball milling was investigated and the phase transformation sequence of amorphous NbCo1.1Sn–NbCo2Sn full-Heusler was verified. In the ball milling process, the introduction of oxygen and the formation of niobium oxide were observed. Through the oxygen content comparison experiment and atomic-scale compositional research, i.e., atom probe tomography, it was proven that the introduced oxygen concentration determines whether the amorphous matrix crystallizes into NbCo2Sn full-Heusler or simply transforms into a Co-rich amorphous region. Furthermore, a ball milling-induced crystallization mechanism based on atomic diffusion and free volume theories in amorphous materials was suggested. In this mechanism, the free volumes become localized and some exceed the volume of the near constituent atom. Then, atomic diffusion occurs with different diffusivities depending on the atomic radius. In the Nb-depleted region originated by oxidation, the more dominant Co diffusion than Sn induces the Co-saturated region and relative Sn-depleted region. If this region reaches the chemical composition of NbCo2Sn, it transforms into NbCo2Sn full-Heusler via polymorphous crystallization through the ordering reaction in order to reach thermodynamical stability. The suggested crystallization mechanism deepens the understanding of the nature of the ball-milled feedstock and provides insight for optimizing the process to develop an ideal microstructure for realizing reduced thermal conductivity.

Research Progress on Cu-15Ni-8Sn Alloys: The Effect of Microalloying and Heat Treatment on Microstructure and Properties.

Cu-15Ni-8Sn alloy is the best choice to replace beryllium bronze alloy. This alloy has unparalleled application value in aerospace, ocean engineering, electronic information, equipment manufacturing, and other fields. However, the application of Cu-15Ni-8Sn alloy is challenged and limited because of a series of problems in its preparation and processing, such as easy segregation, difficult deformation, and discontinuous precipitation. It is an effective way to improve the comprehensive properties of Cu-15Ni-8Sn alloy using alloying design and process optimization to control the as-cast, deformed, and heat-treated microstructures. At present, it is a hot spot for scholars to study. In this paper, the grade generation, system evolution, and preparation technology development of Cu-15Ni-8Sn alloy are comprehensively reviewed. The phase transformation sequence of the Cu-15Ni-8Sn alloy is discussed. The influence of the type, amount, and existing form of alloying elements on the strength of Cu-15Ni-8Sn alloy and its mechanism are systematically summarized. Furthermore, the latest research progress on the effects of solid solution, cold deformation, and aging on the phase structure transformation and mechanical properties of Cu-15Ni-8Sn alloy is summarized. Finally, the future development trend of the Cu-15Ni-8Sn alloy is projected. The research results of this paper can provide a reference for the control of the microstructure and properties of high-performance Cu-15Ni-8Sn alloys used in key fields, as well as the optimization of the preparation process and alloy composition.

Microstructure formation during laser powder bed fusion of Ti-22Al-25Nb with low and high pre-heating temperatures

We compare microstructure formation during laser powder bed fusion (LPBF) of a Ti-22Al-25Nb alloy applying low and high pre-heating build plate temperatures. Fast cooling rates during low-temperature LPBF lead to metastable weakly ordered β phase, i.e., bcc–(Ti,Al,Nb) with pronounced 〈100〉 texture in build direction and nanosized segregations within grains elongated in the build direction. For high-temperature LPBF a Widmanstätten microstructure was observed with lenticular O phase precipitates within the β matrix. Microscopical and in situ high-energy synchrotron diffraction investigations demonstrate that the microstructure formation can be widely explained by the precipitation of O phase from supersaturated β rather than by a sequence of solid-state phase transformations, as it would be expected under thermodynamic equilibrium conditions. A detailed analysis of the microstructural gradient in the subsurface region, however, demonstrates that precipitation from metastable β cannot fully explain the observed microstructures, and that the process energy density, i.e., the intensity of the intrinsic heat treatment of LPBF, plays a relevant role. The investigation of the graded area near the surface of the LPBF materials produced with high pre-heating temperature is particularly interesting as it reveals preferred nucleation of the O phase at subgrain boundaries and dislocations.

Deciphering the multiple deformation mechanisms responsible for sustained work hardening in a FeCrCoNi medium entropy alloy

Two important and desirable properties of materials for most structural applications are high tensile strength and ductility, which typically require high work hardening to delay necking. Here, we designed and tensile tested a face-centered cubic (fcc) Fe-Cr-Co-Ni medium-entropy alloy in which multiple deformation mechanisms are triggered during tensile loading at different temperatures to induce sustained work hardening. Our strategy involved control of the relative stabilities of the fcc, hcp (hexagonal close-packed), and bcc (body-centered cubic) phases in this quaternary system via high-throughput thermodynamic calculations. This alloy not only exhibits extensive deformation-induced nanotwinning at room temperature, but also displays a two-step sequential phase transformation [γ (fcc) → ε (hcp) martensite → α’ (bcc) martensite] at 77 K, which contrasts with the single-step phase transformation [γ → ε martensite] observed in many other fcc high/medium entropy alloys with a low stacking fault energy. The sequence of phase transformation at 77K was supported by first-principles density functional theory calculations. This work provides new templates for the design of alloys capable of multiple deformation mechanisms for sustained work hardening.

Influence of sulfur addition on the glass formation, phase transformation and mechanical properties of Cu50Zr50 alloy

In this work, a thorough study on the effects of sulfur-addition in the Cu50Zr50 glass-forming system was conducted using laboratory-based X-ray diffraction (XRD), synchrotron-based high-energy XRD (HESXRD), differential scanning calorimetry, and uniaxial compression testing. We report that the glass-forming ability (GFA), thermal stability, phase transformation sequence during heating and cooling, as well as the mechanical properties of the Cu50Zr50 system are highly sensitive to the sulfur concentration. Proper addition of sulfur can not only stabilize the B2-CuZr phase, but also enhances the GFA of the system. This enables the formation of BMG composites that solely contain a B2 phase, which results in a combination of high strength and substantial plastic strain under compressive loading. The total strains of the newly developed BMG composites with sulfur contents of 0.5 and 1.0 at.% are as large as ∼10 at.%, exceeding the previously reported strains of the Cu50Zr50 BMG by far. These superior properties make the Cu-Zr-S BMG composites a promising candidate material for industrial applications. In addition, it is found that excessive addition of sulfur leads to the precipitation of the Cu10Zr7 phase upon cooling from the alloy melt and a sulfide phase during heating from the glassy state. HESXRD experiments suggest that the preferential bonding between Zr and S atoms could be the determining structural mechanism on the atomic scale, leading to the changing primary crystallization sequence for heating and cooling. The findings in this work further strengthen the high relevance of sulfur in alloy development and tuning of properties in BMGs, especially considering the low cost, good availability, and non-toxicity of sulfur in the relevant modifications.

Decomposition of γ-Fe in 0.4C–1.8Si-2.8Mn-0.5Al steel during a continuous cooling process: A comparative study using in-situ HT-LSCM, DSC and dilatometry

Continuous cooling transformation (CCT) diagrams represent roadmaps for producing all heat-treatable steels. CCT curves provide valuable information on the solid-state phase transformation sequence, depending on the defined cooling strategies, the alloying concept of the steel and previous processing steps. The experimental characterization of CCT diagrams is usually done on a laboratory scale applying thermal analysis of dilatometry. In current research studies, however, also other in-situ methods such as high-temperature laser scanning confocal microscopy (HT-LSCM) or differential scanning calorimetry (DSC) are frequently used to investigate phase transformations during thermal cycling. In the present study, HT-LSCM observations and DSC analysis are critically compared with dilatometry results by investigating the CCT diagram of a 0.4%C-1.8%Si-2.8%Mn-0.5%Al (in mass pct.) advanced steel grade. Furthermore, classical examinations by optical microscopy and hardness measurements were performed to support the analysis. In general, very good consistencies between all experimental techniques were identified in determining the transformation start temperature for pearlite, bainite and martensite. The optical microscopy confirmed the observed phase transformations and the results correlated with the measured hardness response. Based on the results, coupling of HT-LSCM and DSC is considered as a valuable novel approach to plot CCT diagrams in future research.

Exothermic Reactions in AlNi Foils Produced by Rolling Powders

The possibility of manufacturing multilayer reaction foils of the Al-Ni system by rolling a powder mixture is considered. The sequence and temperatures of phase transformations during heating was established by the differential thermal analysis and X-ray phase analysis. The formation of the intermetallic compounds in the foils fabricated via rolling just mixed powder occurs only when a liquid phase appears (660°C). However, the foils fabricated from mechanically activated powders exhibit significant exothermic effect at temperature of 320°C unnecessarily for a liquid phase formation and can be proposed as a multilayer reactional foils for local heating in the technology of joining of various materials.

Sequence of phase transformations in metastable β Zr–12Nb alloy studied in situ by HEXRD and complementary techniques

Phase transformations in a metastable beta Zr–12Nb alloy were investigated by high-energy X-ray diffraction (HEXRD) measured simultaneously with thermal expansion in situ during linear heating from room temperature to 800 °C. Complementary in-situ methods of electrical resistance and differential scanning calorimetry, which were performed using the same heating conditions as in the HEXRD experiment, provided additional information on the transformation sequence occurring in the Zr–12Nb alloy. Two bcc phases with a different lattice parameter, βZr and βNb, were observed in the investigated temperature range and identified using the phase diagram of the Zr–Nb system. In the initial solution-treated condition, metastable βZr phase and athermal ω particles are present in the material. At about 300 °C, Nb-rich βNb phase starts to form in the material and the original βZr phase gradually disappears. Ex-situ observations of the microstructure using transmission electron microscopy revealed a cuboidal shape of the ω particles, which is related to a relatively large misfit between the ω and β phases. At 560 °C, ω solvus was observed, identified by an abrupt dissolution of ω particles which was followed by growth of the α phase.