X-ray diffraction methods for the evaluation of residual stresses in the surface layers with gradient structure
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X-RAY DIFFRACTION METHODS FOR THE EVALUATION OF RESIDUAL STRESSES IN THE SURFACE LAYERS WITH GRADIENT STRUCTURE S. Ya. Betsofen
UDC 621.785.53:539.22.26
We propose an X-ray diffraction method aimed at measuring residual stresses in the surface layers characterized by variable chemical compositions over the depth of the layer. The method is based on measuring the interplane distances for different diffraction reflexes with separation of the contributions of elastic stresses and the composition of a solid solution to these distances on the basis of specific features of the elastic anisotropy of the crystal lattices of metals with cubic and hexagonal structures. The efficiency of the proposed procedure is demonstrated by analyzing, as an example, ÉI961 and 12Kh18N10T steels, PT7M titanium alloy after ionic nitriding, and ÉP975ID nickel superalloy treated by electroerosion cutting.
The development of the methods aimed at the investigation and monitoring of the structure and properties of the surface is promoted by the extensive application of the technologies of surface engineering to structural materials. Moreover, almost all technologies of manufacturing of massive semifinished products are accompanied by the formation of gradient surface layers with different chemical and phase compositions, microstructure, crystallographic texture, and residual stresses (as compared with the internal layers). The residual stresses play an important role in the formation of modified surface layers but their their experimental evaluation is connected with serious difficulties. This is explained by the fact that the dilations of the lattice caused by the formation of a solid solution and by the action elastic stresses lead to similar diffraction effects and, in particular, to the displacements of locations of the reflexes. In the present work, we propose a procedure of separation of the contributions of residual stresses and heterogeneity of the chemical composition to the values of lattice constants based on the specific features of the elastic anisotropy of lattices of the alloys. By using this method, we determine the residual stresses and the compositions of the surface layers for titanium alloys and steels of different classes after ionic nitriding and for the ÉP975ID nickel superalloy after electroerosion cutting (EEC). Procedure of Evaluation of Residual Stresses In the sin2 ψ method, the asymmetric survey is performed so that the normal to reflecting surfaces is inclined to the surface of a specimen with polar angles of azimuthal rotation ϕ and slope ψ. In this case, the Xray strain ε p can also be represented via the components of the stress tensor for a quasiisotropic (textureless) material as follows: εφψ =
1+ ν ν σ φ sin 2 ψ − ⎛⎝ ⎞⎠ (σ11 + σ 22 ) . Ehkl E hkl
(1)
Equation (1) enables us to find σφ from the experimental dependence of dφψ on sin2 ψ. Indeed, “MATI” Tsiolkovskii Russian State Technological University, Moscow, Russia. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 77–84, May–Ju
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