Formation of residual stresses in the surface layers of targets from heat-resistant titanium alloys by irradiation of high-current pulsed electron beams

The present paper reviews the experimental results dedicated by the effect of the irradiation with high-current pulsed electron beams under the melting regime on residual stresses creation taking place on the surface of titanium alloy targets (VT6, VT8, VT9, VT18U). Investigations of physical and chemical state in the surface layers before and after irradiation were made with the transmission electron microscopy, optical metallography, X-rays analysis and microhardness measurements. It was showed that irradiation with high-current pulsed electron beams under the melting stage of VT6 and VT8 alloys leads to formation the stress texture into the surface layer with thickness up to 20 μm. Irradiation of VT9 and VT18U alloy targets with 18-45 J/cm2 leads to formation of tensile residual stresses. It effects on the fatigue strength of titanium alloy parts.


Introduction
In [1], was studied the influence of the electron-beam and finishing heat treatment stages on the operational properties of blades from α + β-titanium BT6 alloys, as a result of which it was found that, using irradiation of the HCPEB in the Geza-1 accelerator [2] at an electrons energy of 115-120 keV, with an energy density of 18-20 J/cm 2 and a number of pulses greater than 2 it is possible to increase the following characteristics of the blades: endurance limit -from 10 to 40%; erosion resistance -more than 2 times, heat resistance -more than 3 times; resistance to hot salt corrosion -more than 4 times [1].
In the work [3], the question of the reasons for the increase in the fatigue strength of the VT6 alloy blades treated with a high-current pulsed electron beam (HPEB) was considered. Irradiated at an energy density of 18-20 J/cm 2 and a number of pulses of more than 2, VT6 alloy blades characterized by an increase in the endurance limit of σ-1 by 20% immediately after the treatment with HCPEB and by 40% after finishing heat treatment at 520-560°C in a vacuum of 10-6 mm Hg within 2 hours. The optical metallography (figure 1) and the transmission electron microscopy showed that irradiation of the HCPEB in the melting stage leads to the formation in a surface layer of up to 20 μm in thickness of a finely dispersed globular lamellar microstructure with an orientation of the plates almost parallel to the surface (zone 1, quenching from liquid). The latter can lead to an increase in the endurance limit. Another reason for this may be texture formation in the surface layer when irradiating with HCPEB. In this regard, the aim of this work was to study the residual stresses formed in the recrystallized material of the surface layer of parts made from titanium alloys when they are irradiated with HCPEB in the melting stage, since it is the residual stresses that largely determine the strength characteristics of the product-parts.

Materials, equipment and research methods
As objects of investigation, samples of 24 mm in diameter and 5 mm in thickness from alloys VT6, VT8 and VT9 [3] were used in the present work, manufactured using the standard technology for producing compressor blades of the RD33 GTE compressor used at the MMB named after V.V. Chernysheva. In addition, plate samples (50×20×3 mm 3 ) from alloy VT18U after grinding and polishing were investigated. The state of the material in the surface layers of the blades was studied by the following methods: X-ray structural analysis, transmission electron microscopy, and optical metallography. The processing of HCPEB targets [1] was realized in the Geza-MMP accelerator in the melting stage: electron energy of 120 keV; pulse duration of 30 μs; energy density in the beam of 18-20 J/cm 2 ; crosssectional area of the beam of 80 cm 2 ; non-uniformity density over the beam cross section of less than 10%.

Experimental data and discussion
Some results of the study [4] are presented in figures 1-3. With the latest data, the results of microhardness measurements on the targets surface after irradiation correlate well (figure 2). It follows directly from this data that irradiation of targets from VT8 and VT6 alloys with energy densities less than 20 J/cm 2 leads to the formation of residual compressive stresses and an increase in microhardness, and hence to hardening of the material in the surface layer. Irradiation of samples from VT9 alloy ensures the formation of residual tensile stresses and tensile textures, which leads to a decrease in microhardness.     A mixed type of textures [4] is formed during irradiation of BT18U alloy plates. X-ray measurements were performed on an automated BRUKER diffractometer using CuKα radiation. The spectra obtained were processed using Bruker software: DIFFRAC.EVA and TOPAS. Evaluation of the texture of the samples by the method of direct (DPF) and inverse pole figures (IPF) construction. It can be seen from the obtained X-ray data that as a result of irradiation, the size of the coherent scattering regions decreases, and the value of microarrays increases, which indicates an increase in the number of defects in the crystal lattice of the investigated alloy. Results of the textural studies (figures 6 and 7). Figure 6 shows the partial (incomplete) direct pole figures (IDPF) {11 0}, {10 0} and {10 2}, which were subsequently used to reconstruct the full RPF (0001), which allows us to estimate the orientation of the basis normals responsible for the anisotropy of the product's properties. Analysis of the obtained IDPF allowed us to conclude that the plates are cut from a hot-rolled sheet, their axis being oriented along the rolling direction of the sheet, and the plane is characterized by a change in the orientation of the base normals when moving along its complex surface.  According to the orientation of the investigated BT18U alloy plates and the results obtained, the targets were cut from hot-rolled sheets, for which the surfaces are approximately parallel to the rolling plane of the sheet. The texture of the irradiated sample is similar to the result of phase transitions as a result of melting and subsequent solidification on a crystalline substrate. The residual macrostresses were determined by the bending of the plates before and after irradiation (according to Davidenkov) and were stretching, due to the melting of the surface layer on one side of the plate (figure 8).  It is undoubtedly interesting the processing by a high-current pulsed electron beam simultaneously from both sides of the plates, in which case, the residual stresses will be determined only by structural changes in the surface layer.

Conclusion
It has been shown by X-ray diffraction analysis, optical metallography and transmission electron microscopy methods that irradiation with HCPEB in the melting stage leads to the formation of residual compressive stresses in the surface layer up to 20 μm in thickness, compression texture and a finely dispersed globular-lamellar microstructure, with predominant orientation α-α'-α'' plates parallel or nearly parallel to the surface of BT6 and BT8 alloy samples. All this should lead to an increase in fatigue strength in bending tests.