Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900?C and stress rupture test was performed at 950?C with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ'phase and the size of γ/γ'eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ' eutectic decreased.In the cooling rate range of 1.42?C s~(-1)–0.84?C s~(-1),the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ' eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84?C s~(-1) showed the best comprehensive mechanical properties.     

Effect of filament and substrate temperatures on the structural and electrical properties of SiC thin films grown by the HWCVD technique

The effect of varying filament and substrate temperatures on the structure and electrical conductivity of crystalline SiC films prepared by HWCVD technique are described in this paper. At a constant filament temperature, the electrical conductivity of the SiC films increases with increasing substrate temperature. However, TEM studies show that there is no change in the size of the SiC columnar grains. On the other hand, a significant variation in filament temperature at constant substrate temperature leads to a variation of structure and conductivity. Raman spectroscopy and TEM studies reveal that crystallinity improves with increase in filament temperature. Furthermore, a μc-Si phase exists alongside SiC at low filament temperature (1750 °C).     

Effect of cooling rate on phase transformation in the low-carbon boron-treated steel

In this research, phase transformation process under different thermal cycles corresponding to a low and a high heat input welding in the heat-affected zone of low-carbon boron-treated steel is systematically investigated by a high temperature laser scanning confocal microscopy. The effect of thermal cycles on the phase transformation process is quantified by measuring the transformation start temperature of each transformation product and the average number of nucleation sites of intragranular acicular ferrite.     

Solidification pathway and phase transformation behavior in a beta-solidified gamma-TiAl based alloy

The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%) alloy after subsequent quenching from 1380 ℃ to 1000 ℃ was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM) and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→β_o+α_2+γ→β_o+γ+α_2/γ→β_o+γ+α_2/γ+β_(o,sec),with the phase transformation α→β temperature(T_β)=1311℃,phase transformation γ→β temperature of(T_(γsolv))=1231℃,phase transformation α_2→α or β_o→β temperature(T_(α2→α)/T_(β_o→β))=1168 C,eutectoid temperature(T_(eut))=1132 ℃ and T_(α_2/γ→βo,sec)≈1 120℃.In comparison with Ti-42 Al alloy,the T_(eut) and T_(γsolv)are slightly increased while both the Tp is decreased obviously by 5% Mn addition.When quenched from the temperature of 1380-1260 ℃,the martensitic transformation β→α' could occur to form the needlelike martensite structure in β area.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃ to 1380 ℃.When the temperature is below T_(γsolv)(1231℃),the γ grains would nucleate directly from the β phase.For the temperature slightly lower than T_(eut)(1132℃),the dotted β_(o,sec) phases could nucleate in the lamellar colonies besides the γ lamellae precipitated withinα_2 phase.Finally,at room-temperature(RT),the alloy exhibits(p_o+α_2+γ) triple phase with microstructure of β_o+lamellae+γ,of which the lamellar structure consists of α_2,γ and β_(o,sec) phases.The phase transformation mechanisms in this alloy,involving β→α',β→γ,α_2→α_2/γ and α_2→β_(o,sec) were discussed.     

Structural and microstructural phase evolution during mechano-synthesis of nanocrystalline/amorphous CuAlMn alloy powders

The formation mechanism of Cu–11.5Al–4Mn alloys by mechanical alloying (MA) of pure elemental powders was investigated. During milling, the powder sampling was conducted at predetermined intervals from 1 h to 96 h. The quantitative phase analyses were done by X-ray diffraction and the particles size and morphology were studied by scanning electron microscopy. Furthermore, the microstructure investigation and phase identification were done by transmission electron microscopy. Concerning the results, the nanocrystalline Cu solid solution were formed at short milling times and, by milling evolution, the austenite-to-martensite (2H) phase transformation occurred. Moreover, the formation of considerable amount of amorphous phase and its partial transformation to crystalline phases during the milling process were revealed. It was also found that, by milling development, the powder morphology changes from lamellar to semi-spherical and their size initially increases, then reduces and afterward re-increases.     

Effect of graphite addition on martensitic transformation and damping behavior of NiTi shape memory alloy

In this investigation, the effect of graphite addition on martensitic transformation and damping behavior of Ni₅₀Ti₅₀ (at.%) shape memory alloy has been studied. It is found that martensitic transformation temperature decreases obviously with the addition of graphite. Microstructural observation shows that TiC precipitates and forms whiskers when the carbon content is increased beyond ~ 0.6%. With the increase of graphite content, the damping capacity during reverse transformation increases initially and then decreases while the damping capacity of full martensite is remarkably improved by the addition of graphite particles. It is proposed that the enhancement of damping capacity can be ascribed to the high damping capacity of graphite itself, as well as, the increase of the amount of interface between martensite and austenite can be beneficial to the damping capacity.     

Effect on microstructure and plastic deformation behavior of a Zr-based amorphous alloy by cooling rate control

In this study,Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloys samples with the same diameter(8 mm)were prepared by using self-designed molds(viz.refractory steel,pure graphite,and copper molds)with dif-ferent cooling capacities.Moreover,by eliminating the size effect,the effect of the cooling rate on the microstructure and compression deformation behavior of Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloys was investigated.Differentiation of the cooling curves revealed that the instantaneous cooling rates of the alloy melt at the glass transition temperature(Tg)are 45,52,and 64K·s-1 for refractory steel,pure graphite,and copper molds,respectively.X-ray diffraction,differential scanning calorimetry,and high-resolution transmission electron microscopy analysis revealed that with the decrease in the cooling rate,trace icosahedral-like atomic clusters and nanocrystals appear in local areas of the amorphous alloy and that the amount of free volume decreases with the increase in the amount of icosahedra-like atomic clusters and nanocrystals.Compression test results revealed that the elastic strain,yield strength,and compressive strength of the amorphous alloy marginally change with the decrease in the cooling rate,while the plastic strain gradually increases.By fitting,the effective size of the vein-like pattern was lin-early related to the enthalpy released during structural relaxation and plastic strain,indicating that at a low cooling rate,the trace nanocrystals in the amorphous alloy could not effectively improve its plasticity and that the amount of free volume mainly affects its plasticity.     

Cooling rate effects on the microstructure and phase formation in Zr51Cu20.7Ni12Al16.3 bulk metallic glass

A detailed transmission electron microscope (TEM) study has been conducted to investigate the microstructures of the Zr₅₁Cu_20.7Ni₁₂Al_16.3 metallic glass formed at different cooling rates. It has been found that the most competitive crystalline phase to the amorphous structure is an oxygen-stabilized FCC NiZr₂-type phase, which in turn acts as the leading phase to trigger the formation of other crystalline phases in the slow-cooled alloy.     

Electrical properties of Schottky diode Pt/SiC and Pt/porous SiC performed on highly resistif p-type 6H-SiC

We investigated the electrical characteristics of two different Schottky diode as Pt/SiC and Pt/porous SiC, elaborated on highly resistif hot-pressed p-type 6H-SiC supplied by Goodfellow. The Schottky diode was characterized in air ambient and in vacuum, this latter could be used for exhaust gas monitoring as gas sensors for different gas (O₂, H₂, CO, CO₂ and hydrocarbure). The result shows an ideality factor in range 1.1-1.5 with a barrier height varying between 0.780 and 0.950 eV function of the ambient characterization. The result indicated clearly the dependence of electrical parameters on the surface whose Schottky contact was realized (Pt) and on the ambient where the electrical tests were performed.     

Effect of heating rate on the hysteresis in the phase transition in silver telluride thin films

The electrical resistance of vacuum deposited silver telluride thin films was measured in the temperature range from 300 to 430 K at different heating rates. It is found that silver telluride films undergo a structural phase transition, with a hysteresis. The phase transition occurs over a wide temperature range of about 30 K and the transition temperature as well as the hysteresis width are found to be influenced by the heating rate. The effect of heating rate on the phase transition temperature and the hysteresis are discussed.     

Influence of composition on mechanical behaviour of porcelain tile. Part III: Effect of the cooling rate of the firing cycle

This paper is the third part of a study focusing on determining the influence of the porcelain tile composition on mechanical behaviour of sintered bodies. Tile compositions were prepared according to a simplex-centroid mixture design set out in Part I of this research, in which the microstructural characterisation of sintered specimens was carried out. In Part II the influence of the starting composition on the mechanical properties of sintered porcelain tile was evaluated on the basis of the linear elastic fracture mechanics. Finally, in this last Part, ceramic bodies from seven compositions were subjected to fast cooling after firing, in order to reproduce the industrial cooling rates. The main objective was to analyze the influence of the mineralogical composition of the starting mixture on the development of macroscopic residual stress and growth of flaw size. When the pieces were subjected to fast cooling, flaw size was the main factor determining the variation of the mechanical strength. This increase in flaw size can be interpreted from the Weibull modulus, from 6 to 8 in those mixtures, with high deterioration of mechanical properties. The mullite hypothesis as a strengthening mechanism in triaxial porcelains was clearly manifested when the samples are fast cooled. This mechanism was the main responsible for the strengthening, what contrasts with the increase in flaw size. The microscopic residual stress caused by the thermal expansion mismatch of the phases also acted as a reinforcement mechanism.     

A review on the synthesis of SiC from plant-based biomasses

Substantial research efforts had been put in developing new ways to produce SiC and its nanostructures. SiC nanostructures showcase excellent hardness, chemical inertness, thermal and electrical properties. With this in mind, many methods have been reported in synthesizing SiC nanostructures. The use of biomass in producing SiC is one of these methods that is promising since it provides an alternative in converting these wastes into something useful and they are a cheap source of silicon and carbon for SiC formation. Thus, this paper will cover the definition of biomass, pyrolysis of biomass in terms of kinetics and processes, effect of different reaction parameters, pre-treatments available and finally SiC recovery process.     

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb-V microalloyed steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb-V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.     

Investigation of the nucleation and growth of SiC nanostructures on Si

Using in situ reflection high energy electron diffraction (RHEED), ex situ atomic force microscopy (AFM) and transmission electron microscopy (TEM) the early stages of SiC growth on Si during the carbonisation were investigated in a solid source molecular beam epitaxy equipment. Different mechanisms of SiC precipitate growth by SSMBE were found. The SiC growth during carbonisation of Si(111) at 600°C is controlled by diffusion and at higher temperatures by a two-dimensional nucleation process, which is mononuclear at 660°C and polynuclear above 750°C. At temperatures greater than 750°C and 850°C three-dimensional nucleation occurs at (111) and (100) surfaces, respectively.     

Improved prediction of the grain size of aluminum alloys that includes the effect of cooling rate

A comprehensive study of the effect of cooling rate on the grain size of a range of grain refined wrought aluminum alloys was carried out under quiescent solidification conditions where nucleation occurs predominantly by a constitutional undercooling mechanism. Increasing the cooling rate reduced the grain size by increasing the number of particles that nucleate grains and by affecting the development of constitutional undercooling. Both effects are represented using simple analytical relationships. By coupling these results with earlier work, an empirical relationship is developed between grain size, density of nucleant particles, cooling rate, nucleant potency and alloy composition that allows prediction of grain size across a wide range of alloys and cooling rates.     

Effect of high-temperature annealing on the optical and photoluminescence properties of nanocrystalline SiC films

The optical and photoluminescence (PL) properties of nanocrystalline 3C-SiC films and the effect of the boundary regions between the nanocrystals were studied for two sets of films: (a) films with 10-15 nm nanocrystal size obtained by direct ion deposition method and (b) similar films annealed in oxygen at 850-950 °C. It was shown that annealing of the nanocrystalline SiC films resulted in weaker absorption in a broad spectral range, and to the increase of the optical band gap from 1.8 to 2.2 eV. On the contrary, the edge PL bands in the UV range (2.2 to 2.4 eV) remained similar. In the IR range, three maxima absent in the as-grown films, appeared at 1.52 eV, 1.56 eV and 1.63 eV. Measurement of the intensity of PL maxima as a function of the excitation power showed a nonlinear dependence that was attributed to the onset of stimulated emission.     

Effect of gas Mach number on the flow field of close-coupled gas atomization,particle size and cooling rate of as-atomized powder: Simulation and experiment

Gas velocity is a key parameter regulating the particle size and the cooling rate of the gas atomized powder applied in additive manufacturing, metal injection molding, thermal spraying, and soft magnetic composites. In this paper, on basis of the well-designed close-coupled nozzles with different gas Mach numbers at the outlet, the gas field structure was simulated by Computational Fluid Dynamics (CFD) software, and the process of cooling and solidification of Fe-6.5 wt% Si metal droplets was calculated by finite difference method. The results show that with the increase of Mach number, both the gas velocity downstream and the pressure at the base of melt delivery tube tip rise, whereas the mass flow rate of the melt decreases. The nozzles with high Mach number can produce finer powder with higher cooling rate. The median diameter of the powder prepared by the nozzle with Mach numbers of 1.0, 1.5, 2.0, and 2.5 is 44.9, 39.0, 32.5, and 29.1 μm, respectively, and the corresponding cooling rate of the metal droplet with a diameter of 80 μm is 2.85 × 10⁴, 2.98 × 10⁴, 3.32 × 10⁴, and 3.50 × 10⁴ K/s, respectively. This work provides new ideas and suggestions for the preparation of metal powder with small particle size at high cooling rate.     

The influence of trace magnesium content on the phase composition of silicon-stabilized calcium phosphate powders

A novel silicon-stabilized calcium phosphate phase mixture possesses a characteristic phase composition of ∼ 75 wt.% silicon-stabilized α-tricalcium phosphate (Si-α-TCP) with the balance being calcium hydroxyapatite (HA) and traces of β-tricalcium phosphate (β-TCP). Variability in the phase composition has been shown to be caused by trace magnesium (Mg) contained in the calcium nitrate tetrahydrate used to prepare the sol gel. Mg contents between 250 and 300 ppm are sufficient to form significant quantities of β-TCP at the expense of the Si-α-TCP phase.     

The effect of cooling rate on the wear performance of a ZrCuAlAg bulk metallic glass

In the present work, the local atomic ordering and the wear performance of ZrCuAlAg bulk metallic glass (BMG) samples with different diameters have been studied using transmission electron microscopy (TEM) plus autocorrelation function analysis, and pin-on-disc dry sliding wear experiments. Differential scanning calorimetry and TEM studies show that smaller diameter BMG sample has higher free volume and less local atomic ordering. The wear experiments demonstrate that with the same chemical composition, the smaller BMG sample exhibits higher coefficient of friction, higher wear rate, and rougher worn surface than those of the larger ones. Compared with larger BMG sample, the faster cooling rate of the smaller sample results in looser atomic configuration with more free volume, which facilitates the formation of the shear bands, and thus leads to larger plasticity and lower wear resistance. The results provide more quantitative understanding on the relationship among the cooling rate, the local atomic ordering, and the wear performance of BMGs.