Precipitation Of Secondary Phases Research Articles

Precipitate/vanadium interface and its strengthening on the vanadium alloys: A first-principles study

The precipitates of second phases with coherent or incoherent interfaces can interact with dislocations to act as slip obstacles that can significantly improve the mechanics properties of alloys. In this work, the first-principles calculations based on density functional theory were performed to characterize the precipitate/Vanadium interface in the vanadium alloys and uncovering how structure of precipitations and interfacial defects (vacancy or solutes) distribution influence on the effectiveness of precipitation strengthening. Based the Baker-Nutting orientation relationship, the equilibrium stable precipitate/matrix interface structure are obtained. It is found that the so-called layer buckling exists in the interfacial region. Meanwhile, considering the equilibrium interface structure, the compressive strain is taken up by the Ti-based precipitation in the interface. For V-based precipitations, on the contrary, the interfacial strain is reversed that the tensile strain is taken up by the V-based precipitations. Under the uniaxial tensile loading, the stress-strain relations of heterostructures with different kinds of precipitates MX (M = Ti, V and X = C, N) are obtained to characterize the precipitation strengthening in the Vanadium alloys. The ideal tensile strength for heterostructure is sensitive to the size of precipitation. Furthermore, it is interesting to note that Ti-based precipitations not only improve the tensile strength, but also meliorate the ductility of the V alloys, which pointing to the inference that Ti-based precipitations are more effective to strengthen the V alloys than V-based precipitations. Our findings from this study can be implemented into providing the theoretical strategy for further design of the new style alloys.

Ultrasonic assisted second phase transformations under severe plastic deformation in friction stir welding of AA2024

The study explores the effect of ultrasonic vibrations on the dissolution and precipitation of secondary phases in aluminum alloy 2024 under the action of severe plastic deformation in friction stir welding. A method for strain estimation in friction stir welding is proposed to assess the contribution of deformation to microstructural changes. The acoustoplastic effect is estimated which causes an increase in the degree of deformation. It is shown that ultrasonic vibrations give rise to two opposite effects that occur successively in the process of welding: enhancement of the strain-induced dissolution of second phase particles caused by the acoustoplastic effect, and enhancement of their repeated precipitation during ultrasonic aging. One or the other effect will dominate depending on the thermal and deformation conditions determined by the technological parameters of the welding process.

Metallographic, structural and mechanical characterization of low-density austenitic Fe-Mn-Al-C steels microalloyed with Ti/B and Ce/La in hot-rolling condition

ABSTRACTLow-density austenitic Fe-Mn-Al-C steels have high strength, high ductility and a significant weight reduction respect to other alloyed steels. However, this complex system exhibits second-phase precipitation, particularly κ-carbide. It is well-known that the microalloying elements addition to steel generates precipitation hardening, as well as grain refinement effect. It is worth noting that low-density steels can cause cracking during hot-rolling due to high Mn, Al and C contents and segregation in grain boundaries. Hot-rolling conditions play an important role in the dynamic recrystallization mechanisms, and therefore in the austenitic grain size. The main objective of this research work is the metallographic, structural and mechanical characterization of low-density steels microalloyed with Ti/B and Ce/La in hot-rolling condition. For this purpose Fe-(27-30)Mn-(7-8)Al-(1.2-1.8)C (wt.%) low density steels microalloyed with Ti/B and Ce/La were hot-rolled at 1200 °C in two stages. Metallographic, structural and mechanical characterization was carried out by optical (LOM) and scanning electron (SEM) microscopies, electron backscatter diffraction (EBSD) through quality images, inverse pole figures (IPF) and orientation distribution functions (ODF) maps, X-ray diffraction (XRD) and microhardness Vickers (HV) testing. In general, the first stage of hot-rolling exhibits a strongly bimodal microstructure of dynamically recrystallized austenitic grains, while the second stage shows more uniform recrystallized grain size. In the first stage of hot-rolling the austenite is the predominant phase, while in the second stage the α-ferrite phase is barely visible. Low-density steel microalloyed with Ti/B presented better grain size and microhardness values compared to steel microalloyed with Ce/La. Preferred crystallographic orientations were not found.

On the thermodynamics and kinetics of scorodite dissolution

Scorodite (FeAsO4·2H2O)–water interaction is critical to As distribution and storage in surface environment but is inadequately understood due to ambiguities in the mineral’s stability and weathering rate at atmospheric conditions. In the present study we attempted to experimentally determine the thermodynamic and kinetic parameters needed to compute solubility and dissolution rate of scorodite at 25 °C. Experiments were carried out in low pH (1.15) solutions using specially synthesized large scorodite crystals (with Mean Diameter of 27.9 μm). Such experimental conditions ensured the results were not subject to the influence of secondary Fe-bearing phase precipitation and grain size effect. Measured equilibrium concentrations of Fe and As, along with newly published Fe-As complexes association constants, were first used to determine the solubility products Ksp and dissolution rates rn at 50–90 °C. The obtained Ksp ∼ T and rn ∼ T dependence was then used to derive the Gibbs free energy, enthalpy, entropy, heat capacity, and activation energy for the dissolution reaction. Finally, we extrapolated the measurements to 25 °C and obtained room temperature solubility and dissolution rate, scrutinized the pH effect on dissolution, and analyzed the ΔG ∼ rn relation of the dissolution reaction. Our results show that literature data are likely overestimated scorodite solubility at pH > 4 ∼ 4.5 due to neglecting the effect of ferric iron hydrolysis. Estimated ambient condition dissolution rate is an order magnitude lower than the earlier report, implicating the importance of size effect, but is one to two orders of magnitude higher than that of common rock-forming minerals, cautioning the proposed use of scorodite for As fixation and storage. The determined rn ∼ ΔG relation cannot be fully fitted by the transition state model, particularly at near equilibrium, suggesting dissolution in this study may be controlled by defect-assisted surface reactions.

Glucose is an active chemical agent on degradation of hydroxyapatite nanostructure

The mechanisms governing the glucose/bone mineral interface are still not fully described. By recognizing the multidisciplinary character of this problem, in this work we exclude any biological variable and provide insight with a pure materials science perspective. For that, hydroxyapatite nanoparticles were prepared in media with glucose concentrations analogous to those found in healthy and diabetic patients. We report that the influence of glucose over the nanoparticles depends on the stage in which it is added to the synthesis. First, nanoparticles precipitated in glucose-rich solutions present, as expected, decrease in crystallinity. However, this effect is driven by the action of glucose as an active chemical agent, rather than simply as a dispersant. This effect becomes more severe when hydroxyapatite nanoparticles are separately prepared and further allowed to interact with glucose. In this scenario, the deterioration of the nanoparticles’ bulk structure accompanies increase in surface crystallinity. In general, the effects of glucose over hydroxyapatite are concentration-dependent and associated with the precipitation of secondary phases - calcium hydroxide and calcium carbonate. Finally, we present illustrative data from bone minerals from one diabetic and one healthy rat and show that our methods and outcomes are employable in future biomedical investigations.

Unidirectional kaolinite dissolution rates at near-equilibrium and near-neutral pH conditions

Kaolinite dissolution rates at ambient temperature and pH 7.1–8.5 were measured with the isotope tracer method. A rare Si isotope 29Si was introduced to the experimental solutions, which reacted with Georgia kaolinite (KGa-1b) composed of mostly 28Si. Reaction rates were tracked by 29Si/28Si ratios of reacted solutions. The 16 batch experiments were designed with a grid of solutions that ranged from near saturation to supersaturation with respect to kaolinite. An average dissolution rate (unidirectional) of 5.4 ± 1.6 × 10−14 mol (kaol) s−1 m−2 consistently fitted the 29Si/28Si ratios for all 16 experiments, indicating the dissolution rates were independent of pH in near-neutral pH waters and independent from the levels of departure from equilibrium. In other words, it appears that the dissolution reaction mechanisms do not change across from the kaolinite-undersaturated to kaolinite-supersaturated solutions near-equilibrium.The near-equilibrium kaolinite dissolution rates in this study are a new type of rates—unidirectional rates (from the isotope tracer method), which differ from all near-equilibrium kaolinite dissolution rates in the literature that are based upon Si or Al concentrations and are net rates (dissolution minus precipitation rates). Kaolinite dissolution was non-stoichiometric in all experiments. The Si and Al concentrations were sometimes systematic but more often erratic, resulting from the precipitation of AlSi secondary phases. The experimental solutions were grossly supersaturated with respect to gibbsite, allophanes, and imogolites. This confirms our hypothesis that the scatter and conflicts of near-equilibrium data are caused by unaccounted-for secondary phase precipitation, but the isotope tracer method successfully circumvents this experimental pitfall. Most natural waters are supersaturated with clays and are near-neutral pH. Our experimental rates are more applicable to the studies of natural waters than the majority rates available in the literature, which have mostly been measured at far-from-equilibrium, acidic pH, and high temperatures.

Investigation of Localized Corrosion Phenomena in Selective Laser Melted 316L Stainless Steel

Selective laser melting (SLM) refers to the additive manufacturing of bulk material from powder on a layer by layer basis through melting via laser and rapid solidification. In recent years, additively manufactured metallic alloys produced by SLM have garnered attention due to opportunities in waste reduction, time and cost efficiency, and fabrication of geometrically complex structures. However, limited work has been performed to investigate the corrosion properties of these alloys. Understanding these mechanisms is crucial in assessing the attendant risks associated with the use of AM alloys in engineering design. Stainless steels such as 316L produced by SLM have presented advantages due to the corrosion resistant properties of these alloys. However, the non-equilibrium microstructure in these SLM materials, significantly different from that of the wrought counterpart, result in heterogeneities that may lead to localized corrosion such as melt pool boundary attack. A point of concern in stainless steels is intergranular corrosion due to the formation of secondary phases such as chromium carbides that deplete chromium from grain boundaries when the material is exposed to temperature ranges of 450 – 800 °C. While low-carbon steels such as 316L have great resistance to deleterious chromium carbide precipitation, this phenomenon has not been investigated in great detail in the case of selective laser melted 316L. However, it has been shown that precipitation of secondary phases is accelerated in SLM alloy systems such as Inconel 625 with respect to the wrought counterpart. Intergranular corrosion can ultimately lead to intergranular stress corrosion cracking, a form of brittle failure. This study investigates localized corrosion phenomena such as melt-pool boundary attack and intergranular corrosion and the associated microstructure-based mechanisms in the corrosion of these SLM 316L alloys. Galvanostatic etching, potentiodynamic scans through the double-loop potentiokinetic reactivation method, and mass-loss testing of wrought and SLM 316L are utilized to reveal the electrochemical behavior of these alloys. Additionally, focused ion beam and transmission electron microscopy are utilized to understand the microstructural origins of the observed corrosion phenomena.

Creep behavior of a 10%Cr heat-resistant martensitic steel with low nitrogen and high boron contents at 650 °C

The purpose of this study is to identify the relationships between the creep behavior of the 10% Cr martensitic steel with 0.008% B and 0.003% N and changes in microstructure and precipitations of secondary phases. Creep tests were carried out at 650 °C under an applied stress ranging from 180 to 120 MPa. The 10% Cr steel exhibits a linear stress vs. time to rupture dependence without creep strength breakdown up to approximately 40 000 h. The 100 000 h creep rupture strength at 650 °C is predicted to be 110 MPa. The high creep strength of the 10% Cr steel is attributed to high threshold stress of 111.5 MPa. Threshold stress is associated with the detachment stress, i.e. the stress required for detachment of dislocations from M23C6 carbides after finishing the climb, mainly. Analysis of the pinning pressures exerted from different precipitates shows that M23C6 carbides also play the main role in preventing the lath boundary migration and therefore, provide high stability of the tempered martensite lath structure under creep conditions. Coarsening of the lath structure under long-term creep conditions is associated with growth of Laves phase particles, mainly.

Evaluación del comportamiento mecánico de la aleación con memoria de forma Cu-Zn-Al con adiciones de Boro y Hierro y tratadas térmicamente

Se estudiaron las propiedades mecánicas de la aleación Cu-Zn-Al modificada y sin modificar con 0,05% en peso B y Fe fabricada siguiendo la ruta de estado líquido. La aleación se sometió a tratamientos térmicos de envejecimiento a 200 °C y 450 °C. Los ensayos mecánicos y de microscopía óptica se realizaron para evaluar la respuesta al tratamiento térmico. Los resultados mostraron la presencia de microestructuras con diferentes características estructurales en la aleación Cu-Zn-Al sin modificar y modificada con B y Fe, tanto en las condiciones sin envejecer como en las envejecidas. La dureza de la aleación Cu-Zn-Al sin modificar y modificada con B aumentó significativamente con los tratamientos a 200 °C y 450 °C, mientras que la aleación Cu-Zn-Al modificada con Fe solo mostró pequeños cambios en la dureza. Adicionalmente, la resistencia a la tracción final (RTF) de la aleación Cu-Zn-Al sin modificar fue la más sensible al tratamiento térmico a 200 °C, ya que el aumento de la RTF llegó hasta un 18,%, comparado con aumentos de 6,8 y 6,1% obtenidos con la aleación Cu-Zn-Al modificada con Fe y B, respectivamente. El porcentaje de alargamiento de todas las composiciones de la aleación Cu-Zn-Al mejoró significativamente con el tratamiento térmico, con valores máximos obtenidos con el envejecimiento a 200 °C. Se interpretó que los cambios, generalmente pequeños, en las propiedades mecánicas de la composición de la aleación Cu-Zn-Al modificada, se debieron al efecto estabilizador del B y del Fe en la fase primaria de Cu-Zn, que redujo en cierta medida la tendencia a la precipitación de las fases secundarias.

Evaluation of Pitting Susceptibility and Secondary Phase Formation in Newly Designed Ni-Free (Fe–16Cr–1Mo–1Si–0.2N–xMn–xCu) Duplex Stainless Steel Alloys

The electrochemical properties of three Ni-free duplex stainless steel alloys, designed in-house, were investigated in chloride (0.2M NaCl) solution at room temperature. The corrosion performance was measured in terms of pitting potential (Epit), passive current density (ip), polarization resistance (Rp) and susceptibility to metastable pitting. The results obtained showed that alloy 2 (Fe–16Cr–5Mn–1Mo–1Si–1Cu–0.22N) exhibited the highest corrosion resistance (highest Epit, lowest ip, highest Rp and lowest metastable pitting susceptibility) among the designed alloys. Alloy 2 was also found to be free of any secondary phase precipitations (sigma phase, Cr nitrides, chi phase) when aged at 600–900 °C for 3–50 h owing to its optimized composition.

Effect of Microstructure on the Corrosion Resistance of TIG Welded 1579 Alloy.

The paper studies microstructure, chemical composition and corrosion activity of the tungsten inert gas welded joint of the Al-Mg-Sc alloy. An intensive corrosion attack of the heat affected zone (HAZ) was found due to precipitation of secondary phases at recrystallized grain boundaries. The ccorrosion process initiated along the boundary of α-Al grains, where a high concentration of anodic (Mg2Si and Mg2Al3) and cathodic phases ((MnFe)Al6) was observed. Increased temperatures during welding led to coalescence of the anodic phases in HAZ. Additionally, HAZ was found to be enriched with hard intermetallic compounds (Mg2Si and (MnFe)Al6). This area had a higher microhardness (930 MPa) compared to base metal (BM, 895 MPa) and fusion zone (FZ, 810 MPa). The volume fraction of secondary phases was 26% in BM, 28% in FZ and 38% in HAZ. The average grain size increased in the following order: (9 ± 3) µm (BM) < (16 ± 3) µm (HAZ) < (21 ± 5) µm (FZ). A plasma electrolytic oxidation (PEO) coating of aluminum-based material was applied to protect the weld from oxidation. The PEO-coating provided a high corrosion protection in the aggressive Cl−-containing environment.

Study on the microstructural characteristics of adiabatic shear band in solid-solution treated ZK60 magnesium alloy

The phase transformation during high strain rate deformation was generally considered to be martensite transformation and diffusion transformation could not happen due to the limited time for diffusion. The motivation of this paper is to study the mechanism of dynamic precipitation of second phases and the grain refinement in the adiabatic shear band (ASB) of solid-solution treated ZK60 magnesium alloy. The specimens of the solid-solution treated ZK60 magnesium alloy were loaded by means of radial collapse of thick-walled cylinders (TWC). The microstructural characteristics within ASB of sample were studied by optical metallography (OM) and transmission electron microscopy (TEM). OM observations showed that ASB with a width of ~3 μm was formed after dynamic loading. It was found for the first time that fine second phase particles were precipitated in the central region of ASB. The second phases were instantaneously precipitated under the combined effect of a large number of crystal defects and adiabatic temperature rise during the adiabatic shearing process. TEM observation revealed that elongated grains appeared at the inner edge of ASB, while fine equiaxed grains were formed in the central region of ASB. The fine equiaxed grains were the result of rotational dynamic recrystallization of subgrains induced by a combined effect of shear stress and the adiabatic temperature rise. The findings provided new data about dynamic transformation for aging alloy.

Effect of austenitisation temperature on corrosion resistance properties of dual-phase high-carbon steel

Austenitisation is one of the most important factors in heat treatment process of dual-phase high-carbon steel, as it can affect the grain size, production of secondary phase precipitation, size of martensite laths and distribution of the phases. Despite the importance of this heat treatment on mechanical behaviour, its correlation on corrosion behaviour and electrochemical properties of high-carbon steel required in-depth investigation. The aim of this study is to investigate the effect of different austenitising temperatures on microstructure and corrosion behaviour of dual-phase high-carbon steel. Microstructural evolution was observed in situ using ultra-high-temperature laser microscope, and the steels have been characterised further using optical microscope, electron backscatter diffraction, 3D laser scanning confocal microscope, scanning electron microscope and energy energy-dispersive spectroscope and electron probe microanalysis. The powerful electrochemical corrosion test was employed by using Tafel polarisation method to measure its corrosion rate. Results have indicated that higher austenitising temperature increased grain size of retained austenite and martensite, which reduced the grain boundary length, but at the same time increased the size and amount of carbide precipitations. As the main corrosion mechanisms in dual-phase high-carbon steel were pitting corrosion and intergranular corrosion, effect of generated precipitations has overcome grain boundary corrosion caused by influence of size and shape of microstructures and, thus, reduced the corrosion resistance around 9.68% as temperature increased. These findings are crucial for designing new applications from high-carbon steels for mining and automotive industries.

Influence of composition of nuclear waste glasses on vapor phase hydration

For the first time, the influence of glass composition on the vapor hydration kinetics of the French AVM nuclear waste glass simulants was investigated. Three complex borosilicate glasses (>20 oxides) along with three simplified Na/(Ca, Na)/(Mg, Na)-alumino-borosilicate glasses with four or five oxides were altered at 50 °C in water vapor (95% RH) for up to 557 days. The solid characterization of the altered samples (by SEM, TEM, XRD, ToF-SIMS, SAXS) revealed that the rate-controlling vapor hydration mechanism is composition dependent. The vapor hydration rate of the more durable glasses, whose molar ratio of Al2O3/(CaO or MgO) is ≥ 1, seems to be limited by network-hydrolysis, whereas the overall glass alteration rate of the less durable glasses, whose Al2O3/MgO ratio is < 1, seems to be driven by the precipitation of Mg-rich secondary phases. All the vapor hydrated glasses show the presence of a homogeneous gel layer of a few tens of nm thickness. The more durable glasses have a smaller quantity of secondary phases precipitated on their surface. The less durable glasses have a relatively significant quantity of secondary phases precipitated above the gel layer and irregularly shaped, scattered and highly porous altered zones formed beneath the gel layer. The overall alteration rate of the less durable glasses was 10–20 times faster than the more durable glasses. For all glasses, FTIR spectroscopy indicated an inflexion in the vapor hydration rate after 120–200 days of alteration, likely due to a passivating effect of the altered layer formed under unsaturated conditions. The average pore size of the altered layer in certain vapor hydrated glasses measured by SAXS is similar to the pore sizes of the gel layer formed in aqueous medium and varies very slightly with glass composition.

Integrated Simulation Framework for Additively Manufactured Ti-6Al-4V: Melt Pool Dynamics, Microstructure, Solid-State Phase Transformation, and Microelastic Response

To accelerate the establishment of fundamental understanding of the additive manufacturing (AM) process and its influence on microstructural evolution and related properties, we develop a multiphysics and multiscale modeling framework that integrates: (1) a high-fidelity powder-scale three-dimensional simulation of transient heat transfer and melt flow dynamics, (2) cellular automaton simulation of solidification grain structure and texture, (3) phase-field modeling of precipitation and dissolution of second-phase precipitate during repeated thermal cycles, and (4) microstructure-based micro- and mesoscopic elastic response calculation. Using Ti-6Al-4V as a model system, we demonstrate the application of the integrated framework to simulate complex microstructure evolution during a single-track laser powder bed fusion process and the associated mechanical response. Our modeling framework successfully captures the solidification β grain structure as a function of laser power and scanning speed, α precipitation upon subsequent cooling with different rates, and elastic response of the resulting (α + β) two-phase microstructure. The key features of solidification and second-phase precipitate microstructures, and their dependence on processing parameters, agree well with existing experimental observations. The established modeling framework is generally applicable to other metallic materials fabricated by AM.

Effect of pH on the stability of passivating gel layers formed on International Simple Glass

It has been well established that borosilicate glass dissolves much more slowly in saturated silica solutions than in deionized water. The present study assesses this assertion for the specific case of International Simple Glass, which is a 6-oxide borosilicate glass of nuclear interest, and was altered between pH = 1 and 10.7 at 90 °C. Depending on the reaction stage, aqueous silica can promote either the formation of a passivating gel layer on the glass surface or the precipitation of certain secondary phases at the expense of the passivating gel. In this study, a negligible effect of aqueous silica on glass dissolution at acidic pH is demonstrated, while a marked effect above pH90 °C = 7 is observed, ensuring better chemical durability of the glass. Conversely, at higher reaction progress and above pH90 °C = 9.5, the chemical durability of the glass decreases owing to the formation of secondary phases such as hydroxides or zeolites.

Microstructure response of ferritic/martensitic steel HT9 after neutron irradiation: effect of dose

A ferritic/martensitic steel, HT9, was irradiated in the BOR-60 reactor to ∼17.1 and ∼35.1 displacements per atom (dpa) at 650 ± 23 K (377 ± 23 °C). Irradiated samples were comprehensively characterized using analytical scanning/transmission electron microscopy and atom probe tomography, with emphasis on the role of irradiation dose on the microstructure evolution. Radiation-induced Mn/Ni/Si-rich (G-phase) and radiation-enhanced Cr-rich (αʹ) precipitates were observed within the martensitic laths at all doses. In addition, the G-phase was also observed to precipitate heterogeneously at various defect sinks. The number density for these second-phase precipitates decreases while the size increases with increasing dose, resulting in an increase of the volume fraction. Both a <100> and a/2 <111> type loops were observed with the a <100> type being the predominant type at both doses. The proportion of a <100> loops is consistent with that previously observed in HT9 ion irradiated to similar doses at ∼693–743 K (∼420-470 °C). Only small cavities (diameter < 2 nm) were observed at ∼17.1 dpa whereas both small and large cavities were observed at ∼35.1 dpa, resulting in a bi-modal cavity size distribution at this dose. Alloying elements, Ni and Si, were observed to segregate to the cavity surface, forming Ni/Si-rich shells around the cavities. The swelling at ∼17.1 dpa is evaluated at ∼0.02% while the swelling at ∼35.1 dpa is found to be ∼0.07% with variations from grain to grain. attributed to the spatial variation of the density of large cavities (in different grains). The swelling data obtained in this study was compared with the neutron data of F/M steels available in the literature.

Tailoring the microstructure and mechanical properties of metastable Ti–29Nb–13Ta-4.6Zr alloy for self-expansible stent applications

In this paper, thin-struts of β-type titanium alloy (Ti–29Nb–13Ta-4.6Zr, wt%) were successfully fabricated by cold rolling, and their elastic behavior and mechanical properties are discussed for different processing conditions. Before cold rolling, the extremely low yield and ultimate tensile strength of the as-cast and solution-treated specimens, result of a notable “double yielding” behavior, make them inadequate to be applied as structural devices. However, after cold rolling plus short time aging, a nonlinear elasticity with a substantial recoverable strain, considered as the intermediate stage between “double yielding” and normal linear elasticity, is obtained and ascribed to a moderate precipitation of nanometer-sized secondary phases. These precipitates strengthen the material by hindering the motion of dislocations while the β-matrix with relatively low content of β-stabilizers can give rise to a low elastic modulus. The ultimate tensile strength, yield strength, Young's modulus and elastic recoverable strain of the cold rolled plus aged specimens were measured as being between 967 and 1192 MPa, 920–1180 MPa, 48–83 GPa and 1.5–2.0%, respectively. As such, the alloy is anticipated to be promising metallic self-expansible stent material by virtue of the unique combination of extraordinarily high elastic strain, high mechanical strength, and excellent biocompatibility [1, 2].

Thermodynamic modeling of elastic mismatch strain energy on epitaxial growth of GaInN thin films

The effect of mismatch elastic strain energy between GaInN epitaxial film and GaN (0001) substrate is investigated by classical thermodynamics combining with classical elastic mechanics and first-principles calculations. The mismatch elastic strain energies in different indium content are calculated by classical mechanics with stiffness coefficients determined by the first-principles calculations. A negative excess term is discovered in the mismatch strain energy of GaInN epitaxial films. The negative excess term largely counteracts the positive mixing enthalpy of GaInN solution compound, suppressing the large miscibility gap of GaInN alloy. The calculations well explain the successful growth of GaInN epitaxial films with high indium content. This work provides an easy and logical approach to evaluate the thermodynamics of lattice mismatch in various material systems, such as heteroepitaxial growth, second-phase precipitation, etc.

Research on AZ80 + 0.4%Ce (wt %) Ultra-Thin-Walled Tubes of Magnesium Alloys: The Forming Process, Microstructure Evolution and Mechanical Properties

Ultra-thin-walled tubes of magnesium alloys have received more and more attention in producing precision components for medical devices. Therefore, thin-walled tubes with high quality are desperately needed. In this study, the process of multi-pass variable wall thickness extrusion was carried out on an AZ80 + 0.4%Ce Mg alloy with up to five passes—one-pass backward extrusion and four-pass extension—to fabricate the seamless thin-walled tube with an inside diameter of 6.0 mm and a wall thickness of 0.6 mm. The average grain size decreased from 46.3 μm to 8.9 μm at the appropriate deformation temperature of 350 °C with the punch speed of 0.1 mm/s. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM), and the Vickers hardness (HV) tester were utilized to study the phases, microstructure, and hardness evolution. It can be observed that low deformation temperatures (240 °C and 270 °C) and low strain (1 pass extrusion and 1 pass extension) lead to twins that occupy the grains to coordinate deformation, and a slip system was activated with the accumulation of strain. The results of the Vickers hardness test showed that twinning, precipitation of second phases, twinning dynamic recrystallization (TDRX), and work hardening were combined to change the hardness of tubes at 240 °C and 270 °C. The hardness reached 93 HV after the third pass extension without annealing at 350 °C.