رسانایی گرمایی ZrB2 و HfB2

Thermal Conductivity of ZrB2 and HfB2

رسانایی گرمایی ZrB2 و HfB2 

ABSTRACT

ZrB2 and HfB2 are proposed for use in hypersonic vehicles and reentry spacecraft where they will be used as thermal management materials for extreme thermal environments. These applications typically include sharp control surfaces (leading/trailing edges) as well as scramjet engine components [1–5]. Hypersonic control surfaces need thermal conductivities (k) that are as high as possible to conduct heat generated from atmospheric friction through the material so that it can be dissipated elsewhere [1, 3]. In addition, higher k values increase thermal shock resistance in high heat flux situations. Because engine components cannot radiate to the atmosphere, lower k refractory materials could be beneficial, depending on the cooling capabilities of the vehicle [3]. From a materials engineering perspective, the effects of processing, composition (intentional additions or from impurities), and the resulting microstructure need to be characterized to design materials with appropriate properties. The goal of this chapter is to review the state-of-the-art with respect to thermal conductivity for single-phase and composite ZrB2 and HfB2 ceramics.

 

جوشکاری فلزات دیرگداز (نیوبیوم، تانتالیوم، مولیبدن و تنگستن) و لحیم کاری گرم آن ها

The welding and brazing of the refractory metals

niobium, tantalum, molybdenum and tungsten 

مروری بر جوشکاری و لحیم کاری گرم فلزات دیرگداز 

نیوبیوم، تانتالیوم، مولیبدن و تنگستن

ABSTRACT

This review covers the present and future problems in the development and welding of niobium, tantalum, molybdenum and tungsten and their alloys. Their metallurgical characteristics are discussed together with the influence of impurities and alloying elements on mechanical properties, alloy development and weldability. Other factors briefly discussed are availability, high temperature strength, ductility and oxidation resistance.

 

اثر فرایند متراکم سازی پودر بر روی خواص ترموالکتریکی سرامیک های کبالتیت

Effect of a powder compaction process on

the thermoelectric properties of Bi2Sr2Co1.8Ox ceramics

اثر فرایند متراکم سازی پودر بر روی خواص ترموالکتریکی

سرامیک های Bi2Sr2Co1.8Ox 

ABSTRACT

Bi–Sr–Co–O ceramics, as members of a thermoelectric cobaltite family, do not reach the quality of thermoelectric compounds used at low temperatures. Their thermoelectric performance can be, however, boosted by the microstructure tailoring or by the variation of cobalt valence. In this work, we compare Bi2Sr2Co1.82Ox polycrystalline samples that were compacted using different procedures. The precursor powder was prepared by a chelating sol–gel method. Three compaction methods were subsequently tested: (i) cold uniaxial pressing, (ii) hot uniaxial pressing on BELT equipment and (iii) spark plasma sintering, all three followed by additional annealing in air. The thermoelectric ceramics were characterized by XRD, SEM and electric transport measurement up to 950 K. The temperature dependence of the power factor and the figure of merit were also calculated. The spark plasma sintering process yielded a material with an improved value of Seebeck coefficient – 190 μV/K at 300 °C – the highest value reported for the polycrystalline Bi-222 material so far.

 

اثرات ترمودینامیکی بر سینتیک فرآیندهای تولیدکننده‌ی تهی‌جایی

Thermodynamic effects on the kinetics of

vacancy-generating processes

اثرات ترمودینامیکی بر سینتیک فرآیندهای تولیدکننده‌ی تهی‌جایی

ABSTRACT

he inhibiting effect of vacancies on the very process in which they are generated is considered from a thermodynamic viewpoint. Examples of such processes treated here in some detail are grain growth and pore dissolution. It is shown that these processes are inhibited due to vacancy generation. A particular scenario discussed implies intermittent “locking”. After a period of uninhibited kinetics the process comes to a halt due to a thermodynamic back force “locking” it. It can only re-start once the vacancies produced are removed by diffusion. This repetitive cycle leads to an overall reduction in the rate of the kinetic process in question. Specific predictions with regard to grain growth in fine-grained (particularly nanocrystalline) materials and void dissolution kinetics in sintering are made. A third example considered is vacancy drag on a moving individual grain boundary. The magnitude of the drag is re-assessed by taking into account the Gibbs free energy of the vacancies generated.

 

افزودن آهن بر سینترینگ فاز مایع کامپوزیت تنگستن- برنز

The effects of Fe additions on the liquid phase sintering of

W-bronze composites

اثرات افزودن آهن بر سینترینگ فاز مایع کامپوزیت تنگستن- برنز

ABSTRACT

In this investigation, experiments were conducted to evaluate the effects of Fe additives in the range of 1–5 wt.% on the densification of different compositions of W–pre-alloy bronze compacts sintered isothermally at temperatures ranging from 900 °C to 965 °C for 2.30 h. The results showed that substantial improvement in hardness by a factor of two folds and density by 10% was achieved for the W–pre-alloy bronze sintered compacts by the addition of 2–3 wt.% Fe as an activator.

 

کامپوزیت های زمینه پلیمری (PMC)

polymer matrix composite (PMC)

کامپوزیت های زمینه پلیمری (PMC) 

ABSTRACT

A high-performance polymer matrix composite (PMC) consists of a thermoset or thermoplastic resin matrix reinforced by fibers that are much stronger and stiffer than the matrix.1 Structural fibers that may be used as the reinforcement phase include carbon, aramid, glass, and many others. PMCs are attractive because they are lighter, stronger, and stiffer than conventional materials, with the additional advantage that their properties and form can be tailored to meet the needs of a specific application. Depending on the characteristics of the resin matrix and fiber reinforcements, PMCs may also be tailored to exhibit such properties as high thermal or electrical conductivity, stealth characteristics, and sensor capabilities.

 

الزامات پاکیزگی برای فولادهای X80 و X65 خط لوله جوشکاری‌شده

Steel cleanness requirements for X65 to X80

electric resistancewelded linepipe steels

الزامات پاکیزگی برای فولادهای X80 و X65 خط لوله جوشکاری‌شده

به روش مقاومت الکتریکی 

ABSTRACT

CONTROL OF OXIDES AND SULPHIDES : Clean steel practices at the BHP Steel Ltd, Flat Products plant have always been related to customers’ requirements and expectations. Monitors were developed for critical cleanness grades from either in-house tests or from customer supplied data. Some of these monitors have been in existence for nearly 20 years and have successfully guided process improvements both in steelmaking and in continuous slab casting, such as ladle opening with submerged shroud, vibration ladle slag detection, argon shielding of delivery systems, weir wall design, and quick melting  ux addition to the tundish on start up.

 

ریزساختار و خواص مکانیکی Ti-5Al-2Sn-2Zr-4Mo-4Crعملیات حرارتی شده

Microstructure and mechanical properties of

heat-treated Ti–5Al–2Sn–2Zr–4Mo–4Cr

ریزساختار و خواص مکانیکی  Ti-5Al-2Sn-2Zr-4Mo-4Crعملیات حرارتی شده

ABSTRACT

The effects of heat treatment parameters on the microstructure, and mechanical properties and fractured morphology of Ti-5Al-2Sn-2Zr-4Mo-4Cr with the equiaxed, bi-modal and Widmanstätten microstructures were investigated. The heating temperatures for obtaining the equiaxed, bi-modal and Widmanstätten microstructures were 830, 890 and 920 °C, respectively, followed by furnace cooling at a holding time of 30 min. The volume fraction of primary α phase decreased with increasing the heating temperature, which was 45.8% at 830 °C, and decreased to 15.5% at 890 °C, and then the primary α phase disappeared at 920 °C during furnace cooling. The variation of volume fraction of primary α phase in air cooling is similar to that in furnace cooling. The increase in heating temperature and furnace cooling benefited the precipitation and growth of the secondary α phase. The equiaxed microstructure exhibited excellent mechanical properties, in which the ultimate strength, yield strength, elongation and reduction in area were 1035 MPa, 1011 MPa, 20.8% and 58.7%, respectively. The yield strength and elongation for the bi-modal microstructure were slightly lower than those of the equiaxed microstructure. The Widmanstätten microstructure exhibited poor ductility and low yield strength, while the ultimate strength reached 1078 MPa. The dimple fractured mechanism for the equiaxed and bi-modal microstructures proved excellent ductility. The coexistence of dimple and intercrystalline fractured mechanisms for the Widmanstätten microstructure resulted in the poor ductility.

 

تاثیر انجماد بر تحول ریزساختاری فلز جوش پایه نیکل

Influence of solidification on the microstructural evolution

of nickel base weld metal

تاثیر انجماد بر تحول ریزساختاری فلز جوش پایه نیکل 

ABSTRACT

The effect of segregation resulting from solidification on the microstructural evolution of nickel base alloys was investigated. Two different groups of alloys were produced to simulate either the Hastelloy C type or the Inconel 625/718 type of alloys. Gravitational thermal analysis welding was performed on these experimental alloys and the extent of microsegregation in the weld metal was determined. Using composition profiles obtained from solidified microstructures, predictions of the phase stability of these microstructures were made based on metal d-level (Md) calculations. The maximum Md values obtained in these microstructures were compared with those critical to the formation of topologically close-packed (TCP) phases such as sigma, P, and Laves. These profiles showed an increase in the Md level in the interdendritic regions which was correlated to the formation of TCP phases.

 

اثر عملیات حرارتی بر ریزساختار و خواص مکانیکی فولاد زنگ‌نزن دوپلکس

Effect of heat treatment on microstructure and

mechanical properties of duplex stainless steel

اثر عملیات حرارتی بر ریزساختار و خواص مکانیکی فولاد زنگ‌نزن دوپلکس

ABSTRACT

Present study concerns the effect of deformation and heat treatment on the microstructure and mechanical properties of a duplex stainless steel. While hot rolling causes the coarse distribution of the constituent phases (ferrite and austenite), 50% cold rolling results into the elongated and splintered two — phase structure. Supersaturated ferrite structure established by water quenching from 1300°C results into the strengthening due to the formation of fine dispersed austenite precipitates within ferrite grain after isothermal heat treatment (1000°C, 0.5 hour). Duplex structure consisting of ferrite and austenite in a fine-grained form is obtained after isothermal heat treatment of cold rolled sample. Cold deformed and heat treated steel exhibits best combination of strength and ductility among all the investigated steel samples.