خوردگی داغ مواد : مکانیزم جریان سیال ؟

  ?Hot corrosion of materials: a fluxing mechanism

خوردگی داغ مواد :  مکانیزم جریان سیال  ؟

ABSTRACT

Hot corrosion is the accelerated oxidation of a material at elevated temperature induced by a thin  film of fused salt deposit. Fused Na2 S04 , which is the dominant salt involved in hot corrosion, is an ionic conductor, so that the corrosion mechanism is certainly electrochemical in nature.  Further, the acid/base nature of this oxyanion salt offers the possibility for the disso- lution  (fluxing) of the normally protective oxide scale. Non-protective precipitated oxide par- ticles are  often observed in the corrosion products. In this paper, the status of knowledge for the  solubilities of oxides in fused Na2 S04 is reviewed, and the effects of various influences on a  fluxing mechanism are discussed. An evaluation of a negative solubility gradient as a cri- terion  for continuing hot corrosion is made. © 2001 Elsevier Science Ltd. All rights reserved.

مواد دما بالا برای توربین ها گازی

: High Temperature Materials for Gas Turbines
The Present and Future

مواد دما بالا برای توربین ها گازی:

حال و آینده

ABSTRACT

Increased operating temperatures and higher efficiency in gas turbines and jet engines can reduce CO2 emission, thus contributing to the prevention of global warming. To achieve this goal, it is essential to improve the properties of high temperature materials. Various Ni-base superalloys are used for high-temperature components, e.g., combustors and high-pressure turbine blades and vanes, that determine the power and efficiency of jet engines and industrial gas turbines. Among them, single crystal (SC) superalloys have the highest temperature capabilities. A third-generation SC alloy has been used practically in Jet engines and fourth-generation SC alloys with platinum group metals additions are being developed for the next generation Jet engines. In land-based gas turbines also SC superalloys have been introduced to increase their inlet gas temperatures, and thus thermal efficiencies. As for new materials, intermetallic alloys, refractory alloys, ceramics, etc., are also being developed as possible alternative materials. Some unique materials have recently been proposed in Japan and being evaluated.

توزیع ناهمگن نانوکریستال­ های فلزی در لعاب­ کاری سفالینه­ ها

Heterogeneous distribution of metal nanocrystals in glazes of historical pottery

توزیع ناهمگن نانوکریستال­ های فلزی در لعاب­ کاری سفالینه­ های باستانی

ABSTRACT

It has been recently shown that lustre decoration of medieval and renaissance pottery consists of silver and copper nanocrystals, dispersed within the glassy matrix of the ceramic glaze. Lustre surfaces show peculiar optical effects such as metallic reflection and iridescence. In many cases, lustre appears overlapped to colored drawings. Here we report the findings of a study on glazes, pigments and lustre of several shards belonging to Deruta and Gubbio pottery of XVI century. The components of glazes and pigments have been identified. Lustre is confirmed to be characterised by silver and copper metal nanocrystals inhomogeneously dispersed in the glassy matrix of the glaze. In the case of lustre overlapped to colored decorations, we found two contradictory cases. The first consists of a lustre surface successfully applied over a blue smalt geometrical drawing. The second consists of a lustre surface, unsuccessfully applied over a yellow lead-antimonate pigment. The yellow pigment hinders the formation of lustre and removes crystals of tin dioxide, normally present in the glaze as opacifier. # 2002 Elsevier Science B.V. All rights reserved.

درصد گاز در ریخته­ گری دایکاست فشار بالا

Gas content in high pressure die castings

درصد گاز در ریخته­ گری دایکاست فشار بالا

ABSTRACT

This paper presents the results of a quantitative study of the gas level in various types of castings from the high pressure die casting (HPDC) process using a vacuum fusion method. It was found that the major part of the gas was from the air entrapment during cavity filling. Other sources such as air entrapment during ladling, residual die lubricant and quenching water were also noticeable. Measurements of a large casting and castings from a multi-cavity die showed that the gas content was unevenly distributed. The modified vacuum fusion method has been proved to be a valuable tool for evaluating and quantifying the level of gas in castings as well as for an assessment of the influence of different process parameters on gas evolution in castings.

تغییر فاز در فولادهای میکرو آلیاژ کم آلیاژ استحکام بالا (HSLA)

Phase transformations in microalloyed high strength low alloy (HSLA) steels

تغییر فاز در فولادهای میکرو آلیاژ کم آلیاژ استحکام بالا (HSLA)

ABSTRACT

The addition of small amounts of Nb, Ti or V singly or in combination has been the key to  producing high strength low alloy (HSLA) steels with greatly improved mechanical properties. Such  microalloyed HSLA steels are now commonplace in a huge variety of applications. The combinations of  strength, toughness and formability that are possible are directly related to the way in which the steel is processed from the as- cast slab (usually) to the final plate or strip dimensions. The effects of microalloying result principally from three main effects on the austenite to ferrite  transformation. Firstly, the microalloying additions form precipitates in both austenite and ferrite; this factor alone can be exploited in various ways to control the microstructure, hence  mechanical properties, of the steel. Secondly, although present in relatively small quantities,there are discernible effects on ferrite transformations in terms of the hardenability of the steel. Finally, there are interactions between the various deformation sequences used in  steel production and the presence of microalloying elements as solutes or precipitates which can be  exploited to control microstructure. The interactions between processing, microstructural  development and the effect of microalloying additions on the austenite­ ferrite transformations are  the topic of this chapter. A brief history of the use of microalloying is also given as is some  indication of the range of mechanical properties that result from the current understanding of the role of microalloying additions.

مواد عایق برق Electrical insulating materials

Electrical insulating materials

مواد عایق برق

ABSTRACT

Electrical insulating materials ensure the integrity of desired paths of electromagnetic power flow in electrical systems and equipment . They are materials at very high resistivity and can thus be used as isolators or separator between conductors having different potential (voltage) levels . In its use such as in isolators an insulating materials has the basic function of confining the current flow within the conductive circuit of a given device or part of equipment thus protecting the latter from short circuit current leakage and similar undesirable malfunctions . insulating materials may be solid liquid or gaseous substances , they may be monolithic ( discrete ) materials or may be constituted by physically cohesive composites structured with multiple constituents . Based on the primary functions electric insulation can be categorized as follows :

• Separation-type insulation
• Barrier insulation
• Creepege insulation

چقرمگی شکست اپوکسی اصلاح شده با لاستیک هیبریدی

Fracture Toughness of a Hybrid Rubber Modified
Epoxy. II. Effect of Loading Rate

چقرمگی شکست اپوکسی اصلاح شده با لاستیک هیبریدی.

II. اثر نرخ بارگذاری

ABSTRACT

Effect of loading rate on toughness characteristics of hybrid rubber-modified epoxy was investigated. Epoxy was modified by amine-terminated butadiene acrylonitrile (ATBN) and recycled tire. Samples were tested at various loading rates of 1–1000 mm/ min. Fracture toughness measurements revealed synergistic toughening in hybrid system at low loading rates (1–10 mm/min); hybrid system exhibited higher fracture toughness value in comparison with the ATBN-modified resin with same modifier content. However, synergistic toughening was eliminated by increasing the loading rate. At higher loading rates (10–1000), the fracture toughness of hybrid system decreased gradually to the level lower than that of ATBN-modified epoxy. Fractography of the damage zones showed the toughening mechanisms of ATBN-modified system was less affected by increasing the loading rate compared to that of hybrid system.

اثر ترکیب روی بافت کریستالوگرافی در آلیاژهای AL-Li-Cu

Effect of composition on crystallographic texture in hot-rolled Al-Li-Cu alloys

اثر ترکیب روی بافت کریستالوگرافی در آلیاژهای AL-Li-Cu نورد گرم شده

ABSTRACT

The hot-rolling textures of ternary Al-Li-Cu alloys have been characterized to determine their effects on the yield strength anisotropy. The alloys used in the study had (Li/Cu) ratios varying from 0 to co and were hot-rolled in the 520-400 “C temperature range. All the alloys showed well developed p-fibers, with a maximum intensity at Brass, and a weak background of a-fiber. The Brass components were very strong for the alloys with intermediate (Li/Cu) ratios that had relatively more T, precipitates in the microstructure compared with other alloys. In addition, all compositions showed a weak Cube or rotated Cube recrystallization texture components. The observed textures are attributed to the effects of constrained deformation during hot deformation. The texture results were used to predict the yield strength anisotropies in the alloys. The predictions were in agreemenwt ith the experimentalr esultsf or the high (Li/Cu) alloys. However, in the low (Li/Cu) alloys, the discrepancies between the predicted and the measured yield strength anisotropies can be attributed to a modifying effect of the directional platelet precipitates on deformation.

مواد دی­ الکتریک Dielectric Materials

Dielectric Materials

مواد دی­ الکتریک

ABSTRACT

Dielectric materials refer to those having the basic electric Property Of being Polarized in the Presence of an electric Field and having an electrostatic field within Them under the state of polarisation .(polarisation here refers to the molecular aligment along the direction of the applied electric field . ) Also synosymusly referred  to as an electrical insulation material the dielectric has the ability to prevent the leakage of electrical charges from the body on which it is deployed .In general electrical insulation materials which possess  dielectric properties offer a very hight resistance to the passage of electric current under the action of an applied direct - current voltage .Hence such materials differ distincty in basic electric properties from those classified as electric conductors such as metals .

خوردگی سرب و آلیاژهای سربی

Corrosion of Lead and Lead Alloys

خوردگی سرب و آلیاژهای سربی

ABSTRACT

LEAD has such a successful record of service in exposure to the atmosphere and to water that its resistance to corrosion by these media is often taken for granted. Underground, thousands of kilometers of lead-sheathed cable and lead pipe give reliable long-term performance all over the world. In the chemical industry, lead is used in the corrosion-resistant equipment necessary for handling many chemicals. Batteries account for the largest use of lead and are the source of most recycled lead. General information on compositions, properties, and applications can be found in the article “Lead and Lead Alloys” in Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Volume 2 of ASM .The rate of corrosion is a function of the current flowing between the anodes and cathodes of the corrosion cell. Many factors and conditions can initiate or influence this flow of current. In the corrosion of a single metal, such as lead, local anodes and cathodes may be set up as a result of inclusions, inhomogeneities, stress variations, and differences in temperature. In galvanic corrosion, the anodic and cathodic sites are ondifferent metals, with the less noble metal (anode) corroding in preference to the more noble metal (cathode).