厚条带Mg-Al-Zn基合金的显微组织研究

利用单辊低速旋铸方法制备出Mg-3%Al-i.2%Zn-0.2%Mn合金厚条带,估算了不同转速下条带的冷却速度,并重点研究了条带正面及其横截面的显微组织.结果表明:单辊低速旋铸条带(300转/分和500转/分)的冷却速度在104K/s和105K/s之间,厚条带的晶粒较铸态组织品粒细小.500转/分的镁合金条带的断面组织晶粒沿横截面分布比较均匀,并基本均为等轴晶区.300转/分条带的自由面出现部分柱状晶,而贴辊面及贴辊面和自由面之间的过度区都为等轴晶.     

Effects of cooling rate on latent heat released mode of near pure aluminium and aluminium–silicon alloys

Abstract

The information concerning solid fraction with respect to temperature in the mushy range is very important to solidification models which employ the effective specific heat method. The computer aided cooling curve analysis (CA–CCA) method is used in this study to measure the relationships between solid fraction (f_s) and temperature/time for aluminium alloys of different composition for various cooling rates. The present study examined near pure aluminium, A356.2, A390, and A413 (near eutectic) alloys. The results of the measurements and analyses show that a rather large temperature range is observed near the end of solidification for both near pure aluminium and A413 alloys. This mushy range becomes longer as cooling rate increases. For the A356.2 and A390 alloys, the solidus temperature, liquidus temperature, eutectic temperature, and maximum solid fraction at the eutectic temperature decrease as cooling rate increases. This is not true, however, for cooling rates higher than 9·5 K S^?1. It is also known that afunctional relationship of f_s with temperature is very convenient when it is applied to a solidification model. Two non-linearity factors n_e and n_p are required to define the function; n_e and n_p are found to increase as cooling rate increases. The relationship between n_e and n_p and cooling rate can also be obtained. A reasonable estimation of the solid fraction datafor the cooling rate not measured can then be made.

MST/3262     

Transient heat transfer and fluid dynamics during the melt spinning process of Fe78Si9B12Mo amorphous alloy

A numerical model of heat transfer and fluid dynamics in the melt spinning process is developed based on the coupled Navier-Stokes and heat conduction equations. The thermal and velocity fields inside the melt puddle are calculated using the SIMPLE algorithm. The average cooling rates across the whole thickness of the puddle are characterized for different wheel velocity. The experimental investigations of thermal field and quenching rate of Fe₇₈Si₉B₁₂Mo amorphous alloy are conducted with the help of an infrared thermovision technique. It is shown that the calculated average cooling rate is in good agreement with experimental results over the greater part of the puddle, except for that near to the up-meniscus and down-meniscus.     

Transient heat transfer and fluid dynamics during the melt spinning process of Fe78Si9B12Mo amorphous alloy

A numerical model of heat transfer and fluid dynamics in the melt spinning process is developed based on the coupled Navier–Stokes and heat conduction equations. The thermal and velocity fields inside the melt puddle are calculated using the SIMPLE algorithm. The average cooling rates across the whole thickness of the puddle are characterized for different wheel velocity. The experimental investigations of thermal field and quenching rate of Fe₇₈Si₉B₁₂Mo amorphous alloy are conducted with the help of an infrared thermovision technique. It is shown that the calculated average cooling rate is in good agreement with experimental results over the greater part of the puddle, except for that near to the up-meniscus and down-meniscus.     

Microstructure — cooling rate correlations in melt-spun alloys

Photocalorimetric techniques have been used to measure top surface temperatures during melt spinning of Ni-Al and 316L stainless steel ribbons, in order to investigate the effect of cooling rate on the melt-spun alloy microstructures. Cooling conditions during melt-spinning are found to be near-Newtonian, with mean cooling rates, heat transfer coefficients and Nusselt numbers in the range 4×10⁴ to 5×10⁵ K sec^–1, 5×10⁴ to 3× 10⁵ Wm^–2K^–1 and 0.07 to 0.22, respectively, for wheel speeds in the range 4 to 36 m sec^–1. The cooling rate during melt-spinning is directly proportional to the wheel speed and inversely proportional to the square of the ribbon thickness. Melt-spun Ni-Al and 316L stainless steel ribbons exhibit a columnar through-thickness solidification microstructure, with a segregation-free region adjacent to the wheel surface. Solidification takes place by heterogeneous nucleation of the undercooled liquid on the wheel surface, followed by partitionless solidification during recalescence, and finally cellular breakdown and segregated solidification. The columnar grain size decreases and the fractional segregation-free thickness increases with increasing wheel speed and cooling rate, indicating that the nucleation undercooling in the liquid is proportional to the cooling rate.     

Effect of modification melt treatment and chilling on eutectic arrest temperature and time during solidification of A357 alloy

Abstract

Thermal analysis technique has been recognised as an efficient non-destructive tool to assess the degree of modification in Al–Si alloys. Apart from chemical modification, chilling refines the microstructure. This is particularly significant as majority of Al–Si alloys are cast in metallic moulds. In the present study, the interaction between chilling and modification melt treatment is investigated to assess their effect on thermal analysis parameters using computer aided cooling curve analysis. For modified alloys, the depression of the eutectic arrest temperature was significant at higher cooling rates. The eutectic arrest temperature and time were correlated with the cooling rate using a power law. High cooling regime in thermal analysis plots was attributed to the combined effect of chilling and modification melt treatment on heat transfer.     

The effect of the main processing parameters on the geometry of amorphous metal ribbons during planar flow casting (PFC)

Experimental work on PFC performed with some amorphous alloys is reported. The subject of the investigation is the relationship between the main processing parameters and the resulting ribbon thickness. Calculations applying Bernoulli's equation reveal the relationship between the ribbon thickness and substrate velocity, ejection pressure, nozzle slot breadth and crucible wheel gap distance. Empirical data from numerous experiments are in good agreement with the calculations. Ribbons with smooth surfaces and precise cross-sections can be obtained by melt spinning with a small gap distance. The efficiency of a constrained melt puddle on the ribbon surface was exhibited by topographic measurements and high-speed motion pictures. Results of surface tension measurements have been used to explain the effect of the melt temperature on the ribbon thickness. Calculations and experimental data are a base for practical use of the PFC process in the production of ribbons with predetermined precise cross-section.     

Microstructure and properties of monotectic Cr—Cu cast irons

Abstract

A metal quasi-composite, containing copper inclusions within a chromium cast iron matrix was investigated. The dispersion of Cu particles was produced directly from the melt by cooling through the miscibility gap. The effect of casting parameters (pouring temperature and cooling rate) on the size and distribution of Cu particles has been studied using optical metallography, SEM, and TEM. It has been shown that Cu particles are responsible for improved machineability and wear resistance of Cr cast irons with 10 -12 wt-%Cu, since Cu particles break up the network of primary carbides and also act as a solid lubricant.     

Studies on solid dispersions of nifedipine

Abstract

Nifedipine-Polyethylene glycol solid dispersions were prepared by melting or fusion method in order to improve nifedipine solubility in the aqueous body fluids. The dissolution rate of the drug was markedly increased in these solid dispersion systems. The increase in dissolution was a function of the ratio of drug to polyethylene glycol used and the molecular weight of polyethylene glycol. The dissolution rate was compared with a 10% w/w physical mixture of drug with polyethylene glycol.

The physical state of nifedipine after fusion was determined by X-ray crystallography on the pure drug and on the solidified melts. The X-ray diffraction studies indicated that nifedipine in the solid dispersion which was obtained by sudden cooling of the melt, was in the thermodynamically unstable metastable form. It was established that the slow cooling of the melt as well as powdering of solid dispersion resulted in the emergence of crystallinity.

The effect of aging on nifedipine-polyethylene glycol 6000 solid dispersions has been investigated. After storage at room temperature for six months, solid dispersions showed no change in the dissolution rate and the X-ray diffraction pattern showed slight enhancement in crystallinity.     

Microstructural characterization of rapidly solidified nickel-base superalloys

Several Ni-Al-Mo-based eutectic superalloys were rapidly solidified using a chilled block melt spinning process. The effects of rapid solidification on the microstructure were studied using optical microscopy, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). Results showed, except for the alloy containing chromium, that the microstructure varied as a function of ribbon thickness from segregationless solidification at the wheel side of the ribbon to dendritic solidification at the free side. In addition, alloys with the same solidification rate showed a large variation in microstructure depending upon the solid state cooling rate. The rapidly solidified eutectic Ni-Al-Mo alloy with a small amount of rhenium and vanadium did not show any improvement on delaying or prohibiting the formation of the embrittling-NiMo phase on ageing at 1000 C. This was determined from microstructural as well as chemical analysis using STEM. Differential thermal analysis was used to obtain melting temperature,-Ni₃Al solvus, and heat of formation for the alloys.     

Effects of lithium addition and wheel velocity on the microstructure of aluminium-lithium alloy ribbons

The effects of Li addition and wheel velocity on the microstructure of as-cast AI-Li ribbons were studied by optical metallography, scanning and transmission electron microscopy and by X-ray diffraction. Li addition has a marked effect on the ribbon solidification mechanism. Ribbons spun at 12.5 ms^–1 and which contained up to 1.79 wt% Li experienced a cooling rate of about 10⁴ Ks^–1 and solidified under the melt jet with a growth rate greater than 0.3 m s^–1. However, when Li content exceeded about 3 wt%, the ribbons were undercooled (cooling rate 10⁶ Ks^–1) and formed far from the melt puddle effect. Under these conditions, unusual secondary featureless zones occurred through the ribbon thickness and the detected phases corresponded to those expected to be formed under equilibrium conditions. Nevertheless, (AI₃Li) phase did not occur in ribbons containing up to 1.79 wt % Li or in the featureless zones of the most Li-rich ribbons (Li >about 3 wt%). Changing of the wheel velocity from 12.5 to 22.2 or 35.7 ms^–1 did not affect the ribbon formation mechanism, but it favoured the precipitation and decreased slightly the grain size.     

Effect of Ti-Zr addition on the microstructure and the arc chopping current of melt-spun CuCr ribbon

The microstructure and the arc chopping current of melt-spun Cu₇₁Cr₂₉ ribbon added by Ti-Zr are studied in the article. The results reveal that the melt spinning could partly restrain the liquid phase separation of CuCr alloys because it has a high cooling rate (about 10⁶ K/s), the size of the Cr rich phase from liquid phase separation in the Cu₇₁Cr₂₉ microstructure can be decreased from the micron-scale to about 250 nm by using melt spinning. On the melt-spun base, alloying by Ti-Zr could further decrease the size of the Cr rich phase from 250 nm to about 100 nm. For nano-grained CuCr alloys, its lower arc chopping current is advantageous to the use of contact and the circuitry protect, its long arc trace route and high velocity of spot direction motion could mitigate the partial ablate of cathode surface and the lifetime of contact could be prolonged.     

Effect of Processing Parameters on Microstructures of Melt—Spun Al—In Immiscible Alloys

Melt spinning rapid solidification technique was employed to fabricate homogeneous Al-In immiscible alloys and their final microstructures and morphologies were investigated.There existed a transition of columnar Al grain→equiaxed grain for the thicker ribbon,but only columnar Al grain for the thinner ribbon throughout the thickness.In the columnar grain field,most of the fine In particles was distributed within the cells,but a minority of bigger In particles or short rods was perpendicularly distributed at the grain boundaries.In the equiaxed grain field,the fine In particles were located in Al grains and coarser particles were situated at the boundaries.The average particle size increased with increasing distance from the chilled surface throughout the ribbon.At the same wheel speed (same cooling rate),the average particle size increased with raising In content.At the same composition condition,the average particle size decreased with increasing wheel speed and/or decreasing ejection temperature.     

The role of melt superheat in splat-quenching

The influence of experimental parameters in the gun splat-quenching technique is briefly discussed. The effect of initial melt temperature on cooling rate as manifested by lamellar spacing measurements in eutectic aluminium-copper alloy is small. Foils quenched from just above the melting point are generally thicker than those quenched from higher temperatures and thus there is a slight decrease in cooling rate with decrease in melt superheat. Increase in the temperature of quenching increases the proportion of degenerate eutectic microstructure obtained.     

Al—Mn二十面体准晶相的形态观察

用扫描电镜和透射电镜以熔态单辊急冷的Ａｌ－Ｍｎ条带炽的二十面体准晶相的形态进行观察，发现在冷却速度高的与辊筒面接触的区域，准晶相形态呈“花”状，而在远离接触面的区域，则呈较规则的枝晶状，两种形态的准晶似乎都是在量亚晶粒的相互聚集由于先形成的准晶中心核处向外生长，由于成核密度的差异，枝晶状准晶的尺寸是“花”状准晶的数倍。     

The Influence of Various Dispersing Agents on the Dissolution Rate of Hydrochlorothiazide

Abstract

The solvent and melt methods were employed to prepare solid dispersions with various water soluble carriers and sparingly soluble drug, hydrochlorothiazide. The carriers investigated included dextrose, sorbitol, tartaric acid and urea. Dispersion with urea was superior to other carriers in releasing the drug into solution. Dextrose - melt showed decomposition of the drug. Sorbitol drug physical mixture produced a faster rate of dissolution of the drug than the melt dispersions. The eutectic mixture of urea and drug cooled at room temperature (28°) produced a faster dissolution rate of the drug than the mixture cooled at -2°.     

The Control of Drug Release from Conventional Melt Granulation Matrices

Abstract

Sustained release potassium chloride tablets were prepared using a melt granulation formulation in a Baker Perkins Granulator. Parts of the validation for this manufacturing process are highlighted in this paper including granulation end point temperature, incorporation of extragranular excipients, amount of wax in the formulation, granule cooling rate and scale of the operation. A number of other factors have been studied which are not Included here although they are no less important. The release of potassium chloride from tablets was found to be dependent on the wax level and the amount of extragranular excipients (“wicklng agent”). Within the controlled production process, any variation in granulation end point temperature and granule cooling rate should not have any significant effect.     

Effect of Ageing on Progesterone-Polyoxyethylene Glycol 6000 Dispersions. X – Ray Study

Abstract

Progesterone solid dispersions in polyoxyéthylene glycol 6000 were prepared by fusion method in order to improve the progesterone wettability and solubility in aqueous body fluids. Its crystalline state after fusion was determined by radiocrystallography on the pure drug and on the solidified melts. It has been established that a slow cooling speed of the melt induced the stable polymorph emergence in the solidified melt (α form). Moreover, X-ray diffraction patterns investigation showed a good physical stability of the preparations after one year storage at room temperature.     

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract

The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T _NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.     

Simulation studies of liquid phase epitaxial growth of In1-xGaxAsyP1-y

Liquid phase epitaxial growth of In_1-xGa_xAs_yP_1-y, a well known quaternary compound semiconductor, has been studied systematically. A numerical simulation technique has been employed to construct the concentration profiles of Ga, As, and P atoms in an In-rich melt at successive equally spaced layers in front of an InGaAsP crystal growing by LPE. The growth rate has been calculated using the concentration gradients established at the solid-liquid interface. The composition and the thickness of the grown InGaAsP solid layer have been studied as a function of growth parameters, such as cooling rate, system temperature, and growth time. It is observed that the thickness of the grown solid depends on the cooling rate, whereas the solid compositions do not show any dependence on the cooling rate. Our theoretical findings have been compared with experimentally reported values and the results are discussed in detail.