Melt Oxide Research Articles

Microstructural characterization of 15-5PH stainless steel processed by laser powder-bed fusion

The 15-5PH (UNS S 15500) stainless steel combines high mechanical strength, ductility, and good corrosion resistance for aircraft and aerospace applications. This set of properties and its high added-value applications make it an excellent choice for additive manufacturing processes such as laser powder-bed fusion (LPBF). However, there is a need to understand the complex microstructure developed in LPBF-processed parts, which may show particularities such as columnar solidification, preferential orientation, different kinds of porosities, chemical segregation along the melt pools, metastable phases, and oxide nanoinclusions. We report the microstructural characterization of LPBF-processed 15-5PH stainless steel in the as-built condition and after aging. Low porosity (<1%), high hardness (420 HV0.1), and a large amount of retained austenite (15%) were found in the as-built samples. The microstructure shows the usual “fish scale”-like morphology. Grain size and hardness vary depending on the location within the melt pools. Reconstruction of the parent grains from EBSD maps indicates a microstructural refinement due to in situ reaustenitization of parts of the previous consolidated layers, where these heat-affected zones are harder and present larger amounts of retained austenite. After aging, as-built samples were harder and more resistant to overaging than annealed or wrought counterparts. Transmission electron microscopy reveals a large amount of nanometric crystalline silicon-oxide inclusions, indexed as cristobalite. The combination of fine-grained martensite, coherent Cu-rich clusters, nanometric oxide particles, and retained austenite makes LPBF-processed 15-5PH stainless steel a very promising material for high-end structural applications.

Structural characteristics and ablative behavior of C/C–ZrC–SiC composites reinforced with “Z-pins like” Zr–Si–B–C multiphase ceramic rods

In order to improve the ablation and oxidation resistance of C/C–ZrC–SiC composites in wide temperature domain, “Z-pins like” Zr–Si–B–C multiphase ceramic rods are prepared in the matrix. The influence of different sintering temperatures on the microstructure of ceramic rods and the ablative behavior of heterogeneous composites are studied. The results showed that the ZrB2 and SiC phases are formed in the sintered matrix, and the increase of sintering temperature is beneficial to improve the density of the ceramic rods. The ablation properties of samples have been greatly improved. The mass and linear ablation rate are 0.8 mg/s and 3.85 μm/s, respectively, at an ablation temperature of 3000 °C and an ablation time of 60 s. After ablation, the matrix surface is covered with SiO2 and ZrO2 mixed oxide films. This is due to the preferential oxidation of “Z-pins like” Zr–Si–B–C multiphase ceramic rods in the ablation process, and B2O3 melt, SiO2 melt, borosilicate glass, ZrSiO4 melt and ZrO2 oxide film can be generated successively from the low-temperature segment to the ultra-high temperature segment. These oxidation products can be used as compensation oxide melts for the healing of cracks and holes on the matrix surface in different temperature ranges and effectively prevent the external heat from spreading into the matrix. Therefore, C/C–ZrC–SiC composites with “Z-pins like” Zr–Si–B–C multiphase ceramic rods achieve ablation resistance in wide temperature domain.

CHARACTERISTICS OF DRY SLIDING ELECTRIC CONTACT OF METALS IN CONDITIONS OF CATASTROPHIC WEARING

The authors have studied the relation between wear intensity, average contact temperature and phase composition of the surface layers ofAISI 1020 steel, copper and NiTi alloy in dry sliding against the steel counterbody under electric current of density higher than 100 A/cm2 . These contact characteristics are considered carefully at the beginning of catastrophic wear, when the surface layers transit to the utmost state. It was noted that relaxation of stresses in the surface layers was due to the structural transformation in normal wear regime. It leads to tribolayers formation. The high strength of the copper tribolayer is first of all due to the formation of FeO oxide on the sliding surface, which prevents adhesion in contact. In addition, signs of a liquid phase were observed on the copper contact surface. It promoted the low rate of formation and accumulation of structural defects. Emergence of areas of melt and FeO oxide on the sliding surface provides high contact wear resistance. These factors, combined with the high thermal copper conductivity, have caused the tribolayer transition to the limit state at high current density and low contact temperature. The absence of oxides on the sliding surface of the NiTi alloy has caused strong adhesion in the contact, high rate of formation and accumulation of structural defects. Therefore, the tribolayer quickly deteriorates and high wear intensity and rapid increase in the contact temperature are observed with current density increase. Therefore, the catastrophic wear of the NiTi alloy begins at a temperature about 350 °C and at low current density. The sliding surface of AISI 1020 steel contained FeO oxide, therefore strong adhesion is not manifested. Formation of FCC-Fe in tribolayer of AISI steel 1020 is detected, that promotes its accelerated deterioration. Therefore, the tribolayer of AISI steel 1020 transites to the utmost state at a relatively low current density and at a higher temperature. The presented contact temperatures corresponding to the beginning stages of the utmost state of the tribolayer do not exceed 350 °С. Comparison of these temperatures with the known contact temperatures of other metals made it possible to assert that raising of the contact temperature of any metal higher than 400 °С leads to its utmost state. Therefore the characteristics of metals contact at temperatures of sliding contact higher than 500 °С is not of practical interest.

Effect of Superheat and Oxide Inclusions on the Fluidity of A356 Alloy

The effect of melt superheat and oxide inclusions on the fluidity of a commercial A356 alloy has been investigated. Fluidity measurements have been performed by means of Archimedean spiral in sand moulds. The specific testing method and the experimental apparatus show a good reproducibility. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes and the amount of defects occurring at the tip of the spirals. The results reveal that oxide films increase the variability in the fluidity results obtained at the same apparent experimental conditions. A long permanence in the holding furnace and the introduction of some turbulence during sampling increase the oxide formation and entrapment in the molten bath, thus decreasing the repeatability of the fluidity results. The fluidity increases linearly with superheat and it extrapolates to zero at the temperature corresponding to a fraction solid of about 23%. The initial Ti content in the alloy produces an independent crystallization during freezing of the fluidity spirals.

Phase equilibria in the system lamprophyllite-nepheline

Experimental studies of the melting diagram for the lamprophyllite-nepheline system and data on the phases crystallizing in this system indicate that lamprophyllite incongruently melts with the origin of melt and titanium oxide. The maximum temperature at which nepheline and lamprophyllite can occur in equilibrium is estimated at 833 ± 6°C.

Spontaneous Infiltration of Al Melts into SiC Preform

The infiltration process is one of the most useful fabrication methods for metal matrix composites (MMC). Nevertheless, the wettability between the molten metal and ceramics is usually so poor that pressured infiltration processes have been widely used. However, this process results in high production costs. Therefore, we have been developing a spontaneous infiltration process using an Al-Si-Mg melt and a SiC preform. The SiC preform, fabricated using a water glass binder mixed with Fe2O3 powder, causes a chemical reaction between the melt and the binder. The preform, dipped into the melt, was spontaneously infiltrated, and the infiltration process was confirmed by measurement of the preform weight change during the dipping. Nevertheless, the infiltration mechanism is still not clarified. To clarify the mechanism and further the development, additional experiments, using SiC particles with grain sizes of 30, 68, 220 and 420 μm, were carried out. In the preform, fabricated only with the fine particles, there is no infiltration. Nevertheless, if these fine particles were enclosed within the 420μm layers, spontaneous infiltration occurs. The onset of the infiltration occurred during the breaking of the melt oxide film along with a decrease in the gas pressure by the chemical reaction between the trapped gases in the preform and the Mg and Al in the melt.

Effect of Cr, Hf and temperature on interface reaction between nickel melt and silicon oxide core

Effects of Cr, Al and Hf, varied from 0–32 wt.%, 1–7.5 wt.% and 0.3–1.5 wt.% respectively, on reactions between nickel base alloy melt and silicon oxide core were investigated in this paper. Role of temperature also was studied. Morphology and structures of reaction products were analyzed by scanning electron microscope (SEM) with the energy dispersive spectrometry (EDS) and the X-ray diffraction (XRD) technique. Amount of alumina in interface increased with Cr level, and formed a layer with thickness up to several microns in alloy of 32 wt.% Cr, and 1 wt.% Al. Reaction products in alloy without Hf were mainly alumina, while in alloy with Hf were mainly hafnium oxides and alumina. More hafnium oxides were formed in alloy with 1.5 wt.%Hf and 9 wt.% Cr, while more alumina formed in alloy with 0.3 wt.%Hf and 15 wt.%Cr, a double layer of reaction products formed at the temperature of 1540 °C, which were composed of a hafnium oxide layer near the metal and a alumina layer above it. The result shows that Cr, Hf levels in the alloy and reaction temperature have the effect of accelerating interface reaction.

Effects of the furnace pressure on oxygen and silicon oxide distributions during the growth of multicrystalline silicon ingots by the directional solidification process

Oxygen impurities can reduce the carrier lifetime of mc-Si solar cells. In this study, simulations of the transient temperature, velocity and concentrations of oxygen and silicon oxide are carried out in order to clarify the transport mechanism of oxygen impurities in the silicon melt and silicon oxide through argon gas, in a directional solidification system (DSS) furnace. As the solidification fraction enlarges, the oxygen concentration in the melt diminishes, because of the reduction in the amount of crucible surface immersed below the silicon melt. When the solidification fraction is small, two pairs of vortices appear in the melt. Oxygen originating from the crucible is carried towards the free surface by the upper vortex. Oxygen concentration is higher with a higher furnace pressure rather than with a lower one due to the low SiO evaporation at the free surface. When the solidification fraction increases, the upper vortex gradually disappears. The lower vortex occupies almost the whole of the melt, with the exception of a small upper central region where a small vortex forms because of the cooling effect of the argon gas. Oxygen impurities carried by the lower vortex along the crystallization front towards the central region are obstructed by this small vortex. The size of the small vortex increases as the solidification fraction increases. Since the small vortex is stronger when the furnace pressure is higher, the concentration is lower around the central region. This means that the oxygen concentration is smaller when the furnace pressure is higher rather than lower. The simulation results agree well with the experimental results.

Growth of Optical Crystals by the Micro-Pulling-Down Method

AbstractThe micro-pulling-down technique is a crystal growth method that has been mostly developed since 1992. The general scheme of the growth system is relatively simple: the melt (oxide, fluoride, metal) residing in a crucible is transported in downward through microcapillary channel(s) made in the bottom of the crucible. Two driving forces (capillary action and gravity) support the delivery of the melt to the liquid/solid growth interface formed under the crucible due to a properly established temperature gradient. Appropriate configuration of the crucible bottom allows for controlling of the crystal shape (fibers, rods, tubes, plates) and the dimensions of the crystals' cross sections that range approximately from 0.1 to 10 mm. A great number of scientifically and industrially important optical crystal fibers have been successfully produced using this method.

Preparation and hydrogen-storage properties of 90(Mg–23.5Ni)–10Ta2O5 alloy by melt spinning and oxide addition

The 90 wt.% (Mg–23.5 wt.% Ni)–10 wt.% Ta 2 O 5 alloy (designated as (Mg–23.5Ni)–10Ta 2 O 5 ) was prepared by pulverizing the melt spun and heat treated Mg–23.5 wt.% Ni alloy with the Ta 2 O 5 prepared by spray-conversion in a planetary ball mill. The activated (Mg–23.5Ni)–10Ta 2 O 5 alloy had a higher hydriding rate than the activated Mg–23.5Ni alloy but a similar dehydriding rate. The activated (Mg–23.5Ni)–10Ta 2 O 5 alloy absorbed 4.64 wt.% H for 5 min (i.e., a hydriding rate of 0.93 wt.% H/min) at 573 K, and desorbed 5.02 wt.% H for 5 min (i.e., a dehydriding rate of 1.00 wt.% H/min) at 623 K. The activated (Mg–23.5Ni)–10Ta 2 O 5 alloy exhibited a hydrogen-storage capacity of 6.68 wt.% H.

Global heat and mass transfer in vapor pressure controlled Czochralski growth of GaAs crystals

We present a numerical study of vapor pressure controlled Czochralski growth of GaAs bulk crystals, using a global two-dimensional (2D) model of conjugated heat transfer and flows in the GaAs melt, argon gas, and boron oxide encapsulant. At first, the model is examined with respect to the prediction of melt/crystal interface shape, whose accuracy is closely related to a correct simulation of the heat transfer near the crystallization front. A qualitative agreement between the computed and measured interface geometries exhibits, nevertheless, some overestimation of the interface convexity, provided by the 2D model. Then, an analysis of three-dimensional unsteady melt convection has been performed to clarify whether the 2D model is sufficient to describe correctly convective heat transfer. It has been shown that in spite of strong spatial and time variations of the melt flow, the 2D approximation is capable of providing reliable information on averaged melt characteristics, but does not reproduce properly the convection intensity in the under-crystal region. Finally, the 2D model is applied to study the specificities of the melt turbulent convection and transport mechanisms for various growth parameters.

Modeling of Oxidation During Plasma Spraying of Iron Particles

The plasma-sprayed metallic powder deposits contain usually some amounts of oxides originating from a gas–liquid reaction after melting the particles flying in the plasma plume, and a gas–solid reaction after the particle impact and solidification. In the present paper, a model of the in-flight oxidation of molten metallic paricles is described. Only the growth of a continuous oxide layer on the molten particle surface is assumed, whereas melt convection and oxide nodule formation inside the particles are not taken into consideration. The model is applied to spraying of pure iron powder by a water-stabilized plasma gun. After the presumed flight time, the calculated and measured values of the oxide thickness are of the same order of magnitude.

Kinetics and mechanism of reactions between iron oxides and iron‐carbon melts

The reaction between iron oxides and iron‐carbon melts was studied in the temperature range 1523–1973 K. Pellets made from three different sources of iron oxides were added onto the melt surface, and the reaction time was measured through the constant volume pressure increase (CVPI) method. The effects of reaction temperature, oxide and melt surface areas, oxide type and weight on reaction time were evaluated. Analysis of partially reduced pellets through SEM was also performed. It has been determined that the reaction time increases as the pellet weight increases, and decreases as the temperature, the contact area between the oxide and the melt and the surface area of the melt increase. Examination of partially reduced pellets had shown that the reduction occurs topochemically. Based on the results, it is proposed that the overall reaction rate is determined by the consecutive reactions of dissociation of FeO and formation of CO.

The Use of Phosgene for the Removal of Oxide Impurities from a Sodium Chloroaluminate Melt Saturated with Sodium Chloride

A method based on the use of phosgene to remove oxide impurities from sodium chloroaluminate melts saturated with sodium chloride was investigated using infrared spectroscopy as a probe for melt oxide species. The efficiency of phosgene to remove oxide impurities was found to be affected by temperature. The practical value of this method for oxide removal was demonstrated by the preparation of a solution of pure transition metal chloride complex in a phosgene‐purified melt.

The wetting of aluminum oxide by chromium-nickel melts

A study was made of the effect of time, temperature, and concentration on the angle of contact between aluminum oxide and chromium-nickel melts. The wetting isotherm has a minimum. The presence of the minimum is attributed to reaction between the melt and solid aluminum oxide. The best adhesion to aluminum oxide is exhibited by nickel-chromium alloys containing up to 30 at.% chromium.