Investigation Of Phase Transformations Research Articles

Effects of thermo-mechanical treatment and microalloying with Cr, Nb and Ti on phase transformation in C–Mn steel strips produced by compact strip production process

Abstract The C–Mn steel strips microalloyed with Cr, Nb, Ti was produced by compact strip production process and then heat-treated under different conditions. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy were used to investigate phase transformations in the steel after different treatments. It was revealed that the phase transformations upon quenching were greatly affected by the austenization temperature and time. When the steel was annealed at 950 °C, carbides of Cr, Mn and Fe were dissolved dramatically, while carbides of Nb and Ti are relatively stable at this temperature. When the temperature increases to 1100 °C, the carbides of Nb were dissolved rapidly, while the carbides of Ti still show somewhat stable (partial dissolution). Annealing time influences both the amount and the shapes of carbides in the steel, which leads to different phase transformations in the following air cooling processes. Grain growth in the steel in annealing process strongly depends on the dissolution of carbides on grain boundaries. Additionally, a subsequent rolling after annealing treatment at 950 °C lead to obvious precipitation of carbides of Ti and Nb in the steel.

Effect of tempering treatment on microstructure and fatigue life of TiC–Cr overlay, produced by plasma transferred arc alloying

In this work, TiC–Cr overlay formed by plasma transferred arc alloying was subjected to tempering treatment at 473, 673, and 873 K for 2 h, respectively. XRD was employed to investigate phase transformation, determine the austenite proportion and evaluate the crystallite size. Microstructure investigation by SEM and TEM, microhardness and fatigue test were performed on as-deposited condition and after tempering. Evolution of microstructure as well as its relationship with microhardness and fatigue property of coating was described. The results suggest that both microhardness and fatigue strength could be improved by tempering at 473 K; however, further elevation of the temperature directs degradation. Based on the Weibull distribution, tempering treatment increases the slope of fitting line, which suggests that fatigue life becomes more predictable.

Employment of a novel ultrasonic method to investigate high pressure phase transitions in oleic acid

In this work, the variation of sound velocity with hydrostatic pressure for oleic acid is evaluated up to 350 MPa. During the measurement, we identified the phase transformation of oleic acid and the presence of the hysteresis of the dependence of sound velocity on pressure. From the performed measurements, it can be seen that the dependence of sound velocity on pressure can be used to investigate phase transformations in natural oils. Ultrasonic waves were excited and detected using piezoelectric LiNbO3(Y-36 cut) 5 MHz transducers. The phase velocity of the longitudinal ultrasonic waves was measured using a cross-correlation method to evaluate the time of flight.

Phase transformation and sintering behaviour of mullite and mullite–zirconia composite materials

Mullite is one of the most promising engineering materials for applications at elevated temperatures, but has poor mechanical properties at ambient temperature; therefore, it is usually reinforced with particles, fibres or whiskers to improve its properties. Among particles added to mullite are ZrO2 particles which improve its fracture toughness through the well known process of phase transformation from tetragonal to monoclinic in zirconia particles. The aim of the present work is to explore the utilisation of Algerian kaolin, α-Al2O3 and ZrO2 to synthesise mullite–ZrO2 composites through reaction sintering and investigate phase transformation and sintering behaviour of the composites. The raw materials were mixed through planetary ball milling followed by attrition milling. Compacted samples were sintered at temperatures between 1100 and 1600°C for 2 h. The bulk density was measured by the water immersion method. X-ray diffraction (Rietveld method) was used to characterise phases present in the sintered samples. It was found that the zirconia phase retained its tetragonal structure with the addition of up to 16% zirconia. The formation of primary mullite in all samples was complete at 1250°C. The cristobalite started to form at 1150°C, and disappeared at 1300°C in the samples of mullite, and at 1250°C when ZrO2 was added. The zircon compound ZrSiO4 started to form at 1250°C and completely disappeared at 1400°C. The increase in ZrO2 ratio promoted the formation of grains with spherical shape.

Investigation of phase transformations by in-situ neutron diffraction in a Co–Re-based high temperature alloy

In-situ neutron diffraction at high temperatures was used to investigate phase transformation in a Co–Re-based alloy. Stability of carbides and transformation of Co matrix from ε (hcp) to γ (fcc) phase were studied. The ε ⇄ γ phase transformation exhibited a large hysteresis with temperature. The alloy has a complex microstructure with Cr 23C 6, TaC and σ phase stable over a wide temperature range. The hysteresis is the result of composition interplay between Co-matrix and other phases, e.g. Cr-carbide and σ. TaC is stable at high temperatures up to 1300 °C.

Influence of heat treatment on microstructure and adhesion of Al 2O 3 fiber-reinforced electroless Ni–P coating on Al–Si casting alloy

Influence of heat treatment regime on microstructure, phase composition and adhesion of Al 2O 3 fiber-reinforced Ni–P electroless coating on an Al–10Si–0.3 Mg casting alloy is investigated in this work. The pre-treated substrate was plated using a bath containing nickel hypophosphite, nickel lactate and lactic acid. Al 2O 3 fibers pretreated with demineralised water were placed into the plating bath. Resulting Ni–P–Al 2O 3 coating thickness was about 12 μm. The coated samples were heat treated at 400–550 °C/1–8 h. LM, SEM, EDS and XRD were used to investigate phase transformations. Adhesion of coating was estimated using scratch test with an initial load of 8.80 N. It is found that annealing at high temperatures (450 °C and above) leads to the formation of hard intermetallic products (namely Al 3Ni and Al 3Ni 2 phases) at the substrate–coating interface. However, as determined by the light microscopy and by the scratch test, these phases reduce the coating adhesion (compared to coatings treated by the optimal annealing regime 400 °C/1 h). The analysis of scratch tracks proves that fiber reinforcement significantly reduces the coating scaling. However, due to the formed intermetallic sub-layers, partial coating delamination may occur on the samples annealed at 450 °C and above.

Phase transformations in yttrium –aluminium oxides in friction stir welded and recrystallised PM2000 alloys

PM2000 is an Fe –Cr – Al oxide dispersion strengthened (ODS) alloy containing 0.5wt% of fine, uniformly dispersed, yttrium oxide particles in a ferritic matrix. The alloys are attractive candidates for high temperature applications since nano-dispersoids improve the creep resistance of the alloys at high temperatures. Friction stir welding (FSW) has been used successfully for the joining of PM2000 sheet without oxide particle agglomeration and significant change in the microstructure. However, it has been reported that the initial Y2O3 particles may sometimes oxidise the aluminium from the surrounding matrix to form mixed Y–Al oxides. Hence, in this study, we have been using extraction replication plus high-spatial resolution scanning transmission electron microscopy (STEM) to investigate phase transformations and oxidation of Y–Al during FSW processing and recrystallisation treatments (1380°C, 1 hour in laboratory air).High-resolution SuperSTEM images indicate that the Y2O3 can transform to Y3Al5O12 garnet (YAG) and YAlO3 perovskite (YAP) particles even in the consolidated PM2000. These dispersoids appear to be stable during the FSW process, but most of the Y2O3 or YAG particles transform into YAP particles after the high temperature recrystallisation treatment at 1380°C. In some cases partially transformed particles were observed and these may enable the details of the oxidation/transformation mechanisms to be elucidated.

Phase transformations in yttrium –aluminium oxides in friction stir welded and recrystallised PM2000 alloys

PM2000 is an Fe –Cr – Al oxide dispersion strengthened (ODS) alloy containing 0.5wt% of fine, uniformly dispersed, yttrium oxide particles in a ferritic matrix. The alloys are attractive candidates for high temperature applications since nano-dispersoids improve the creep resistance of the alloys at high temperatures. Friction stir welding (FSW) has been used successfully for the joining of PM2000 sheet without oxide particle agglomeration and significant change in the microstructure. However, it has been reported that the initial Y2O3 particles may sometimes oxidise the aluminium from the surrounding matrix to form mixed Y–Al oxides. Hence, in this study, we have been using extraction replication plus high-spatial resolution scanning transmission electron microscopy (STEM) to investigate phase transformations and oxidation of Y–Al during FSW processing and recrystallisation treatments (1380°C, 1 hour in laboratory air).High-resolution SuperSTEM images indicate that the Y2O3 can transform to Y3Al5O12 garnet (YAG) and YAlO3 perovskite (YAP) particles even in the consolidated PM2000. These dispersoids appear to be stable during the FSW process, but most of the Y2O3 or YAG particles transform into YAP particles after the high temperature recrystallisation treatment at 1380°C. In some cases partially transformed particles were observed and these may enable the details of the oxidation/transformation mechanisms to be elucidated.

Structural evolution of electroless Ni–P coating on Al–12 wt.% Si alloy during heat treatment at high temperatures

The work is concerned with the high-temperature heat treatment of an Al–12wt.% Si alloy coated by an electroless Ni–P layer. The electroless deposition took place on a pre-treated substrate in a bath containing nickel hypophosphite, nickel lactate and lactic acid. Resulting Ni–P deposit showed a thickness of about 8μm. The coated samples were heat-treated at 200–550°C/1–24h. LM, SEM, EDS and XRD were used to investigate phase transformations. Adherence to the substrate was estimated from the scratch test and microhardness of the heat-treated layers was also measured. It is found that various phase transformations occur, as both temperature and annealing time increase. These include (1) amorphous Ni–P→Ni+Ni3P, (2) Al+Ni→Al3Ni, (3) Ni3P→Ni12P5+Ni, (4) Ni12P5→Ni2P+Ni, and (5) Al3Ni+Ni→Al3Ni2. The formation of intermetallic phases, particularly Al3Ni2, leads to significant surface hardening, however, too thick layers of intermetallics reduce the adherence to the substrate. Based on the growth kinetics of the intermetallic phases, diffusion coefficients of Ni in Al3Ni and Al3Ni2 at 450–550°C are estimated as follows: D(Al3Ni, 450°C)≈6×10−12cm2s−1, D(Al3Ni, 550°C)≈4×10−11cm2s−1, D(Al3Ni2, 450°C)≈1×10−12cm2s−1 and D(Al3Ni2, 550°C)≈1×10−11cm2s−1. Mechanisms of phase transformations are discussed in relation to the elemental diffusion.

Quantitative differential thermal analysis of polymorphic transformations in LaGe1.8 and SmSi2

The possibility of using quantitative differential thermal analysis to investigate phase transformations is examined. The temperature, enthalpy, and entropy of polymorphic transformations in LaGe1.8 and SmSi2 are determined: Ttr = 724 K, ΔtrH = 1635 ± 79 J · mole−1, ΔtrS = 2.3 ± 0.1 J · mole−1 · K−1 (LaGe1.8); Ttr = 658 K, ΔtrH = 1384 ± 69 J · mole−1, ΔtrS = 2.1 ± 0.1 J · mole−1 · K−1 (SmSi2).

Synthesis of nanopowders under high frequency electric discharge assisted mechanical milling

Electric discharge assisted mechanical milling can be applied to the synthesis of a range of fine powder products, including nanocrystalline agglomerates and individual nanoparticles and nanofragments. Processing variables include: starting powder sample size; electric arc parameters such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results of an initial experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a new pulsed power supply working at frequencies in the kHz range. The aims were to determine processing parameters required for the synthesis of potentially useful high surface area particles, nanostructural powders and nanoparticles and to compare products with those synthesised by Hz frequency discharge milling. Microstructural, morphological and phase changes induced by kHz discharge milling were characterised by X-ray diffractommetry and transmission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nanoparticles, agglomeration of powder particles, particle melting and/or sintering and chemical reaction induced by mechano-processing in the presence of a particular type of plasma. Discharge milling of graphite under Ar/4% H 2 resulted in a range of products including: graphite nanostructures, carbon nanotubes and other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical milling of graphite resulted in higher yields of carbon nanofragments. Discharge milling of hematite resulted in partial reduction to magnetite and FeO and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co–WC resulted in products including: micron and submicron fracture products, nanostructural regions of Co and WC and carbon rich nanorods.

Structure evolution and thermodynamics analysis of MoSi 2 during mechanical alloying

Mo–Si powders were milled using an attritor mill with a rotation speed of 300 rpm and ball to powder mass ratio of 24:1. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) were used to investigate phase transformation, microstructure, elemental constituents and morphology of the as-milled powders. Thermodynamics analysis was carried out using Miedema model. The results show that the structure evolution of Mo–Si binary system follows: dispersion of Si in Mo → solid solution in Si boundary → α-MoSi 2 and β-MoSi 2. There is thermodynamics driving force for Mo–Si binary to form solid solution and amorphous phase. Mo–Si binary system forms solid solution in the boundary system of Si. Si grain is further refined by the formation and detachment of MoSi 2 from the Si boundary, which leads to the disappearance of Si reflections in XRD patterns.

Nanotube and Nanorod Synthesis under High Frequency Electric Discharge Assisted Mechanical Milling

Electric discharge assisted mechanical milling using a 50 Hz power supply has been used to produce a range of fine and nanostructural products, including nanocrystalline aglomerates and individual nano-particles and nano-fragments. Processing variables include; starting powder sample size; electric arc parameters such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results of an experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a new pulsed power supply working at frequencies in the kHz range. The aims of this preliminary investigation were to determine processing parameters required for the synthesis of potentially useful high surface area particles, nanostructural powders and nanoparticles, and to compare products with those synthesised by Hz frequency discharge milling. Microstructural, morphological, and phase changes induced by kHz discharge milling were characterised by x-ray diffractommetry and transmission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nano-particles, agglomeration of powder particles, particle melting and/or sintering, and chemical reaction induced by mechanoprocessing in the presence of a particular type of plasma. Discharge milling of graphite under Ar/4%H2 resulted in a range of products including; graphite nanostructures, carbon nanotubes and other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical milling of graphite resulted in higher yields of carbon nanotubes. hematite resulted in partial reduction to magnetite and FeO,and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co-WC resulted in products including; micron and submicron fracture products, nanostructural regions of Co and WC, and carbon rich nanorods and nanotubes.

In situ FTIR investigation on phase transformations in BN nanoparticles

Fourier transformation infrared spectrum (FTIR) was employed to investigate phase transformation of nanoparticle BN, containing eBN, rBN, wBN and cBN, in which eBN and rBN are the main components. We found that part of eBN was converted into rBN at 200–350°C. At and above 400°C, eBN transformed into wBN, and some of wBN were converted into cBN simultaneously. This conclusion is supported by the results from high-resolution transmission electron microscopic (HRTEM) and Raman spectroscopic measurements. In this work, the converting processes between different BN phases were continuously monitored by in situ FTIR spectroscopy. The results will be helpful for synthesis of cBN at moderate conditions.

In situ Transmission Electron Microscopy and Nanoindentation Studies of Phase Transformation and Pseudoelasticity of Shape-memory Titanium-nickel Films

Transmission electron microscopy (TEM) and nanoindentation, both with in situ heating capability, and electrical resistivity measurements were used to investigate phase transformation phenomena and thermomechanical behavior of shape-memory titanium-nickel (TiNi) films. The mechanisms responsible for phase transformation in the nearly equiatomic TiNi films were revealed by heating and cooling the samples inside the TEM vacuum chamber. Insight into the deformation behavior of the TiNi films was obtained from the nanoindentation response at different temperatures. A transition from elastic-plastic to pseudoelastic deformation of the martensitic TiNi films was encountered during indentation and heating. In contrast to the traditional belief, the martensitic TiNi films exhibited a pseudoelastic behavior during nanoindentation within a specific temperature range. This unexpected behavior is interpreted in terms of the evolution of martensitic variants and changes in the mobility of the twinned structures in the martensitic TiNi films, observed with the TEM during in situ heating.

Ti-8 mass%Mo合金におけるα"マルテンサイト組織の時効に伴う相変態

The orthorhombic martensite was formed in Ti-8 mass%Mo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the martensite to α and β phases was observed near 780 K, so that isothermal aging was performed at 723 K and 923 K for 9.0 ks. Optical microscopy, X-ray diffraction and transmission electron microscopy were performed to these specimens. Band-like products that were composed of the single variant of ω phase were observed in the quenched martensite structure. On the other hand, (111)α twins were observed in the 723 K-aged martensite structure. The quenched martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged martensite structure indicated high yield stress and brittleness. It was pointed out that the ω products were formed to relax the volume expansion from the β phase to the martensite, and the (111)α twins were formed during the isothermal aging at 723 K with the extinction of the ω products.

Phase Transformation of α″ Martensite Structure by Aging in Ti-8 mass%Mo Alloy

The orthorhombic a martensite was formed in Ti-8 mass%Mo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the a martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the a martensite to and β phases was observed near 780K. so that isothermal aging Was performed at 723 K and 923 K for 9.0 ks. Optical microscopy. X-ray diffraction and transmission electron microscopy were performed to these specimens. Band-like products that were composed of the single variant of ω phase were observed in the quenched α martensite structure. On the other hand, twins were observed in the 723 K-aged α martensite structure. The quenched a martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged α martensite stricture indicated high yield stress and brittleness. It was pointed out that the ω products were formed to relax the volume expansion from the β phase to the α martensite, and the (111)α twins were formed during the isothermal aging at 723 K with the extinction of the ω products.

Methods of Increasing the Measurement Accuracy during the Experimental Determination of the Melting Point of Graphite

The experimental procedure involving pulsed laser heating is realized, which enables one to investigate phase transformations in graphite and eliminates the homogeneous condensation of carbon vapor over a sample. Data are obtained on the melting point of graphite and the temperature of crystallization of liquid carbon at a pressure of 15 MPa. The absence of a difference between the melting point of graphite at a heating rate of 100 MK/s and the temperature of crystallization of liquid carbon at the cooling rate of the melt bath of 1.6 MK/s is revealed, which is indicative of the quasi-equilibrium of the melting process. In view of this, the temperature data are averaged by a unified value of 4800±100 K. It is shown that, during the crystallization of carbon melt, highly ordered graphite with crystallites up to 250 #x00B5;m in size is formed in a liquid film 5 #x00B5;m thick on a graphite substrate.

Temperature-modulated DSC provides new insight about nickel-titanium wire transformations

Differential scanning calorimetry (DSC) is a well-known method for investigating phase transformations in nickel-titanium orthodontic wires; the microstructural phases and phase transformations in these wires have central importance for their clinical performance. The purpose of this study was to use the more recently developed technique of temperature-modulated DSC (TMDSC) to gain insight into transformations in 3 nickel-titanium orthodontic wires: Neo Sentalloy (GAC International, Islandia, NY), 35°C Copper Ni-Ti (Ormco, Glendora, Calif) and Nitinol SE (3M Unitek, Monrovia, Calif). In the oral environment, the first 2 superelastic wires have shape memory, and the third wire has superelastic behavior but not shape memory. All wires had cross-section dimensions of 0.016 × 0.022 in. Archwires in the as-received condition and after bending 135° were cut into 5 or 6 segments for test specimens. TMDSC analyses (Model 2910 DSC, TA Instruments, Wilmington, Del) were conducted between −125°C and 100°C, using a linear heating and cooling rate of 2°C per min, an oscillation amplitude of 0.318°C with a period of 60 seconds, and helium as the purge gas. For all 3 wire alloys, strong low-temperature martensitic transformations, resolved on the nonreversing heat-flow curves, were not present on the reversing heat-flow curves, and bending appeared to increase the enthalpy change for these peaks in some cases. For Neo Sentalloy, TMDSC showed that transformation between martensitic and austenitic nickel-titanium, suggested as occurring directly in the forward and reverse directions by conventional DSC, was instead a 2-step process involving the R-phase. Two-step transformations in the forward and reverse directions were also found for 35°C Copper Ni-Ti and Nitinol SE. The TMDSC results show that structural transformations in these wires are complex. Some possible clinical implications of these observations are discussed.

Phase Transformation and Domain Switching Behaviors during Superplastic Deformation in 3Y-TZP

Test pieces of a tetragonal zirconia polycrystal containing 3mol% yttria were deformed in tension under superplastic conditions in air to pre-determined nominal strains ranging from 0.0 to 200%. The X-ray diffraction experiments were conducted on surface regions of gauge sections of the deformed test pieces to investigate phase transformation and domain switching behaviors that might occur during the superplastic deformation. The measured X-ray patterns were analyzed using a Rietveld refinement method. As a result: (i) Reflections from monoclinic (m) phase were not observed in the X-ray patterns, indicating that tetragonal (t) to m phase transformation might not occur during superplastic deformation under the above condition. Meanwhile, the fraction of cubic (c) phase seemed to decrease with increasing strain. (ii) Traces of ferroelastic domain switching were observed in the X-ray patterns of the deformed specimens. The switching amount was found to depend on flow stress and deformation amount. The tetragonal axis ratio (c/a) along the scattering vector used in this study was found to decrease with increasing the switching amount.