INTRODUCTION

The amount of the second component (Al) in Cu–Al alloys allows us to vary the stacking fault energy (SFE) and the degree of short-range order in an alloys, which alters the resistance to dislocation motion in the material. The stacking fault energy falls rapidly as the content of Al rises. The plastic deformation of metals and alloys allows us to increase the accumulation of different structural defects characterized by such parameters as mean scalar density of dislocation 〈ρ〉, the density of statically stored dislocations ρS, the density of geometrically necessary dislocations ρG, the curvature and torsion of the crystal lattice χ, the density of microtwins ρT, the density of deformation microstripes, and the density of dangling subboundaries. The formation of a defective substructure in a material is possible when such factors as plastic deformation and its rate, test temperature, mean grain size 〈d〉, and SFE γSF are considered; the value of γSF can affect the formation of a dislocation substructure. Substructural strengthening alters the initial stress of dislocations and friction forces (which can be especially noticeable at the start of a plastic flow), and thus the resistance to deformation. This is an important way of strengthening metals and alloys [1–3].

In this work, we analyze the effect the SFE has on the formation and accumulation of different substructural defects in polycrystalline materials. There are few such quantitative studies in the literature.

For metals without impurities (e.g., Cu, Al, and Ni), mean density of dislocation 〈ρ〉 rises at low γSF [4]. It should be noted that the densities of dislocation measured via X-ray diffraction analysis do not agree with data obtained by structural means in studies of defect structure. This was observed in studying pure copper and alloys with additions of zinc Cu–Zn (the content of zinc in the alloy was 10 and 30 wt %) [5]. The stacking fault energy in these materials was 41, 22, and 7 MJ m−2, respectively The effect γSF has on measuring the density of dislocation under shock loading was studied for Cu–Al alloys. The amounts of second component Al were 0.2, 2, 4, and 6 wt % [6]. Results showed the relationship between the SFE and the accumulation of dislocations [6]. There are currently few studies on the effect γSF has on the rate of accumulation of various defects.

Detailed study of the fine microstructure of alloys is now required when creating materials with predetermined strength properties. It is known that varying the SFE can create alloys with different types of dislocation substructures and thus determine their strength characteristics. Cu–Al alloys are models for solving these problems. Many types of dislocation substructures are observed when the SFE changes from 10 to 60 mJ m−2 [2, 7–9].

The aim of this work was to study qualitative and quantitative parameters and determine the relationship between them and how they change along with the SFE in Cu–Al alloys at different degrees of deformation.

EXPERIMENTAL

We studied the defect substructure in Cu–Al alloys. The content of the second element Al was varied from 0.5 to 14 at %. The defect structure was considered using mean grain sizes of 20 and 240 μm. Samples 100 × 12 × 2 mm3 in size were deformed using an Instron machine at a rate of 2 × 10−2 s−1. The samples were analyzed at a temperature of 293 K. The dislocation substructure was studied via transmission electron microscopy (TEM). The true strain of the samples εtrue = 0.02–0.90. Images in the microscope column were enlarged up to 30 000×. Micrographs were used to measure such parameters of the dislocation substructure (DSS) as its components, the density of dislocations, the density of microtwins, and the curvature and torsion of the crystal lattice. Ways of determining the parameters of a DSS were described in [10].

RESULTS AND DISCUSSION

As noted above, the amount of the second element Al in Cu–Al alloys has a substantial effect on the stacking fault energy and the formation of different DSSes. In alloys of Cu + 0.5, 3, 5 at % Al, in which γSF ∼ 4 × 104 J m−2, the sequence of DSS evolution at low degrees of deformation (εtrue = 0.05–0.10) is a chaotic distribution of dislocations, a coiled substructure, a cellular substructure, and a fragmented substructure. When the content of the second element Al is raised to 14 at % at the same degrees of deformation, the sequence of DSS evolution is chaotic distribution of dislocations; accumulation of dislocations; long rectilinear dislocations mainly along directions of close packing; with clusters of several dislocations that have meshes whose dislocations intersect in two directions. Any increase in the degree of deformation is accompanied by misoriented substructures. DSSes with misorientations of more than 0.5° (cellular DSSes, microstrip DSSes, cellular-mesh DSSes, and microtwinned DSSes) form upon raising the second aluminum component to 14 at % at high degrees of deformation (εtrue = 0.20–0.90). One system of microtwins forms upon deformations of εtrue = 0.10; several systems of microtwins form upon deformations of εtrue = 0.30−90.

Raising the density of such defects as dislocations, microstripes, and microtwins results in misorientations characterized by contours of extinction deformation in micrographs of the studied alloys at high degrees of deformation (Fig. 1). The contours of deformation indicate the curvature and torsion of the crystal lattice in the material [11, 12].

Fig. 1.
figure 1

Micrographs of extinction deformation contours (C) in the studied alloys at high degrees of deformation (εtrue = 0.30–0.90).

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The values ​​of 〈ρ〉, ρS, and ρG depending on γSF were measured using micrographs [13, 14]. Experimental data for grain sizes of 20 and 240 μm deformed to εtrue = 0.30 are presented in Fig. 2. We can see that the density of dislocation and its components fall as the SFE rises. The density of statistically stored dislocations is higher in alloys with a weak SFE. It can be seen from Fig. 2 that the grain size affects the values of ρS and ρG at a weak SFE. The values ​​of ρS and ρG are higher in alloys with a grain size of 20 μm. The values ​​of ρS and ρG differ negligibly at high values ​​of the SFE and the considered grain sizes.

Fig. 2.
figure 2

Dependences of mean scalar density of dislo cation 〈ρ〉, density of statistically stored dislocations ρS, and density of geometrically necessary dislocations ρG on stacking fault energy γSF for grain sizes of 〈d〉 = (a) 20 and (b) 240 μm. Degree of deformation εtrue = 0.30.

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Figure 3 presents dependences of microtwin density ρT on stacking fault energy γSF for the considered grain sizes. Analyzing the dependences, we may conclude that the density of microtwins falls linearly at all of the given degrees of deformation as the stacking fault energy rises. The density of microtwins falls in alloys with smaller grain sizes at all of the studied degrees of deformation.

Fig. 3.
figure 3

The dependences of microtwin density ρT on stacking fault energy γSF at the degree of deformation of εtrue = 0.10 (1), 0.20 (2), 0.30 (3), 0.40 (4), and 0.50 (5). Grain size is 〈d〉 = 20 (a) and 240 μm (b).

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As noted above, the formation of extinction deformation contours in a material indicates the curvature and torsion of crystal lattice χ. The dependences of the curvature and torsion of crystal lattice χ on stacking fault energy γSF are shown in Fig. 4. As the SFE falls, the curvature-and-torsion value of crystal lattice χ grows along with deformation for the two grain sizes. Analysis of the dependences shows that the maximum value of χ is observed for alloys with weak SFEs. For alloys with a grain size of 20 μm, the value of χ differs by as much as 100% at high and low SFE values.

Fig. 4.
figure 4

Dependences of the curvature and torsion of crystal lattice χ on stacking fault energy γSF at degrees of deformation εtrue = (1) 0.10, (2) 0.30, and (3) 0.50. Grain size 〈d〉 = (a) 20 and (b) 240 μm.

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CONCLUSIONS

We studied the evolution of the dislocation substructure in Cu–Al alloys upon changes in the value of the stacking fault energy. The stacking fault energy affects the parameters of the defect substructure. Reducing the stacking fault energy in alloys results in new types of DSSs and raises the mean scalar density of dislocation and its components ρS and ρG. The density of microtwins falls as the stacking fault energy rises. Extinction deformation contours appear in the material at higher degrees of deformation. They testify to the presence of misorientations in the material and indicate the curvature and torsion of the crystal lattice. The value of χ falls as γSF rises.