Papers by Keyword: Heat Resistant Alloy

Abstract: The objects of the study were powder samples PV-NiCr16Al6Y and PV-Ni75Al23W, intended for the formation of a sublayer in heat-shielding coatings. The aim of this work is to develop and study a sublayer with a complexly organized structure of a functionally graded material of the superalloy-sublayer-ceramic system for operation under thermal cyclic loads and high-temperature gas corrosion. Experimental samples of binding materials for heat-protective coatings (HPC) were obtained by the method of spark plasma sintering (SPS) at T = 1050 °C / 15-30 MPa / 5 min. Methods of laser diffraction, X-ray structural analysis, optical and scanning electron microscopy, X-ray fluorescence spectroscopy, microdurometry were used to study powders and consolidated materials. It is shown that irregular powders contain individual particles of some components - in PV-NiCr16Al6Y powder these are particles of chromium and yttrium, in PV-Ni75Al23W - tungsten and chromium. Consolidated sublayers of such powders exhibit inhomogeneity and irregular porosity. To solve the problem of forming a homogeneous binder sublayer, mechanical alloying of the powders before sintering is proposed. It was found that MA increases the homogeneity of the structure, excludes local inclusions of unreacted particles, and increases the activation of consolidation. MA and subsequent sintering opens up wide possibilities for controlling the structure, including the porosity of the sublayers, and specifying various options for the HPC architecture. Experimental samples of HPC on an Inconel 625 substrate with a mechanically activated PV-Ni75Al23W sublayer and dioxides Zr/Y (YSZ) outer ceramics were obtained by the SPS method. A model of the architecture of a HPC made of a layered binder with different porosities is proposed.

Abstract: Different kinds of Al and Al alloys are used for all bare overhead energy transmission lines. In the present work, the effect of heat treatments on the conductivity of Al-0.25wt.%Zr alloy has been investigated. Al-0.25wt.%Zr samples were prepared in cold graphite crucibles by using a vacuum furnace and a continuous casting line. The samples were aged isothermally and isochronally in muffle and tube furnaces. Aging was carried out at a wide range of temperatures (350-600 °C) at constant aging times (75 h, 100 h) and a wide range of aging times (25-500 h) at constant temperatures (425 °C, 450 °C). According to the experimental results, conductivities of aged samples increase up to a certain aging temperature and aging time and reach peak values, then decrease with increasing aging temperature and aging time.

Abstract: The concept of the diffusion barrier coating system (DBC system) is summarized and the latest results are presented. The DBC system is comprised of alloy substrate/diffusion barrier/Al-reservoir/an external scale. Diffusion flux (JAl) of Al through the barrier layer will be given approximately by J_Al = D_Al x S_Al x (d C_Al/d x), where DAl and SAl are the diffusion coefficient and solubility limit of Al in the barrier layer, respectively as well as d CAl / d x is driving force given by the concentration difference across the barrier (d CAl) divided by the thickness of the barrier layer (d x). A slow diffusion flux can be obtained by using low values of DAl, SAl, or (d CAl /d x). Accordingly, a selection of a barrier layer with lower DAl and SAl is essential. A low driving force is also an important factor, and can be achieved by using lower C_Al with a constant barrier layer thickness dx. At higher temperatures, however, the barrier layer can react with the alloy substrate and Al-reservoir layer, resulting in gradual degradation of the barrier layer. This means that the thickness dx of the barrier layer tends to decrease and may finally disappear. With decreasing thickness of the diffusion barrier layer, the driving force (dCAl/dx) will increase, and the effectiveness of the barrier layer will be eliminated. Therefore, it is essential to maintain a constant thickness of the barrier layer for long exposure time. Several types of the DBC system are proposed, a single barrier layer and triple-layers with g + g’ and g’ inserted among these barrier layers.

Abstract: In order to improve the creep strength of the heat resistant steels at elevated temperatures over 700°C, a new attempt has been demonstrated using carbon and nitrogen free Fe-Ni martensitic and austenitic alloys strengthened by Laves phase such as Fe2W and Fe2Mo. It is important that these alloys are independent of any carbides and any carbo-nitrides as strengthening factors. The high temperature creep tests over 700°C exceed 36,000 hours and the test is continued. Creep behavior of alloys is found to be completely different from that of the conventional high-Cr ferritic steels. The alloys exhibit gradual change in the creep rate with strain both in the transient and acceleration creep regions, and give a larger strain for the minimum creep rate. Effect of Cr on the Fe-12Ni-9Co-10W alloys on the creep properties more over 700°C was investigated. It became clear that the value for 100,000 hours was exceeded at 700°C and 100MPa calculated from the Larson-Miller parameter at C=20. And surface appearance of crept specimen was investigated in detail.

Abstract: A molecular orbital approach to alloy design has recently made great progress. Single crystal Ni-based superalloys and high Cr ferritic steels have been developed following this approach. Some perspectives will also be described on the design of heat resistant alloys.