Abstract
The modern approach to the design of heat-resistant metal alloys (HRAs) is analyzed, according to which the creep rupture characteristics of an alloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of a matrix phase. The main attention is paid to the concept of “low alloying additions” to polycrystalline alloys with transition metals, because of which the cohesive strength of the GBs and the cohesion energy of the alloy matrix are increased. This approach is especially important in relation to alloys obtained by powder metallurgy, which, in the compacted state, are fine-grained polycrystals. The methodology for calculating the key parameters of the theory (the energy of impurity segregation to the grain boundaries Egb and to the free surface Efs, as well as the values of the partial molar energy of the cohesion of the alloys) from the first principles is given. The results of applying the theory to the study of Ni-, Cr- and Ti-based alloys and the development of new HRAs based on them are presented. Typical defects in the microstructures of objects obtained using additive technologies (AT) and the application efficiency of standard methods of processing powder alloys (Hot Isostatic Pressing (HIP), heat treatment (HT)) to improve the microstructure and increase the mechanical properties are considered.
Highlights
It is known that doping of titanium heat-resistant alloys (HRAs) with a significant amount of W (5–7.5 wt.%) ensures the achievement of high heat resistance and the preservation of the working capacity of alloys up to a temperature of 750 ◦ C when tested for durability and up to 800 ◦ C under short-term loading [24]
Segregation, the effect of strengthening of grain boundaries (GBs), and the increase in the cohesion energy of the matrix, the authors [19,23] distinguish Zr, Nb, Hf, and Ta among refractory metals, which were recommended for introduction as “low-alloying” additions in nickel HRAs obtained by powder metallurgy (P/M) and having a fine-grained polycrystalline structure
It is assumed that the creep rupture characteristics of HRAs are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of the matrix
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. One of the weakest elements of the microstructure of polycrystalline parts is the high-angle grain boundaries (GBs), which, in comparison with the bulk, are characterized by increased diffusion permeability [5,6], which leads to the acceleration of the creep of parts at high temperatures and loads This is why one of the main achievements of the last few decades in the field of the production of blades made of Ni-based HRAs is the development of technology for casting single-crystal parts, which allows one to eliminate high-angle GBs. single crystals are inferior to polycrystalline objects in another key characteristic, namely fatigue resistance, which is especially important for the operating conditions of gas turbine disks. GBs and increase their cohesive strength in several classes of alloys
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