The reported study discusses the formation of micro- and nanocrystalline surface layers in alloys on the example of Ti-Y and Al-Si-Y systems irradiated by electron beams. The study has established a crystallization mechanism of molten layers in the micro-and nanodimensional range, which involves a variety of hydrodynamic instabilities developing on the plasma–melt interface. As suggested, micro- and nanostructures form due to the combination of thermocapillary, concentration and capillary, evaporation and capillary and thermoelectric instabilities. This mechanism has provided the foundation for a mathematical model to describe the development of structures in focus in the electron beam irradiation. The study has pointed out that thermoelectric field strength E ≥ 106 V/m is attributed to the occurring combination of instabilities in micro- and nanodimensional ranges. A full dispersion equation of perturbations on the melt surface was analyzed.
1. Formulation of the Problem. Predicting the residual service life of parts subjected to fatigue loading is a complex problem. For example, fatigue-limit data obtained by construction of the so-called Weller curve [1] only allow estimation of averaged characteristics of the material and provide no information on such an important indicator of product reliability as service life (fatigue life) [2]. Fatigue fracture usually occurs suddenly, without any noticeable outward signs of its impending occurrence. Microscopic studies [1, 3] show that microscopic damages gradually accumulate during fatigue. Then the fatigue crack undergoes slow latent growth that ends with the catastrophic growth of the main fracture crack. The existence of a long preparatory stage in the fatigue process suggests that it might be possible to find some suitable method of delaying the transition to the final stage. To do this, two problems must be solved:9 Selection and substantiation of a reliable and sufficiently informative indicator of the transition to the dangerous stage of structural changes for a specific product; 9 Creation of methods of delaying the development of dangerous defects whose growth could lead to fracture in a short period of time (these methods must be suited for use directly on machine parts or products).2. Informative Indicator of Fatigue Fracture. A convenient quantity that provides reliable information on the structure of metals and alloys and its changes is the ultrasound velocity (USV) in metals and alloys [4]. The fact that this quantity is determined by the elasticity modulus G (for transverse waves) and the density of the material p, i.e., VR = (G/p) 1/2, does not fully reflect the depth of the problem [5]. It was found in [4] that nearly all of the structural changes caused by heat treatment, alloying, or deformation lead to small but measurable changes in the USV. The measurement of USV has proved promising for diagnosing materials under fatigue loading. Measurements made by a method involving the automatic circulation of pulses of ultrasonic surface waves at a carrier frequency of 2.5 MHz with the use of an ISP-11 device [4] have shown that the dependence of the ultrasonic velocity on the number of loading cycles N is qualitatively the same for all specimens. Figure 1 shows data from the use of bending vibrations for the fatigue testing of specimens of steel St. 45 with a mean stress. Similar relations have been obtained also for specimens of rail steel M76. In all the cases, the curve of AV/VR(N) (AV is the decrease in the USV compared to the initial value of this quantity, i.e., VR) includes three successive stages. However, the level and rate of the quantitative Institute of the Physics of Strength and Materials Science, Siberian Division, Russian Academy of Sciences, Tomsk 634021.
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