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Tube High-Pressure Shearing
Tube high-pressure shearing (t-HPS) is a deformation processing, in which a tubular sample is subjected to azimuthal shearing under a hydrostatic pressure. The shear plane is in parallel to the cylindrical surface of the tube, and the shear direction is in the azimuthal direction.
2. Experimental ResultsFigure 1 shows the microstructures of the Bi–Sn alloy (a) in the as-cast condition and after t-HPS processing for (b) 0.25, (c) 1, (d) 5 and (e) 20 turns, where the SEM observations were conducted after storage at RT for 8 h following t-HPS processing. Inspection showed the as-cast Bi–Sn alloy has a typical eutectic structure where the Bi phase and the Sn phase are complementary and packed together. As the etchant preferentially attacked the Sn phase, large amounts of very small Bi precipitate particles became visible within the Sn-rich areas after etching. After t-HPS processing for 0.25 and 1 turn, no significant microstructural change was observed despite some slight rotation of the lamellar structure, and the overall eutectic structure was not destroyed. As the number of turns increased to 5 and 20, it was observed that each phase started to fracture. The edges of both phases became sharper and grains with equiaxed shapes became visible.
|Strain Rate||1.0 × 10−2 s−1||1.0 × 10−3 s−1||1.0 × 10−4 s−1|
|UTS, MPa||Elongation||UTS, MPa||Elongation||UTS, MPa||Elongation|
3. Principle of tube high-pressure shearing
The principle of t-HPS is depicted schematically in Figure 1 where the sample, in the form of a tube, is radially confined between a central mandrel and an outer cylinder. The principle of the process is that a sufficiently high hydrostatic pressure is introduced in the tube wall so that the frictional forces at the interfaces between the sample-mandrel and the sample-cylinder are high enough to prevent any localized slip. By fixing the mandrel and rotating the outer cylinder (or vice versa), a simple shear strain is then produced in the tube wall.
Figure 1. Schematic illustration of the principles of t-HPS
A critical factor determining the success of t-HPS is to obtain a sufficiently high hydrostatic pressure in the tube wall confined between the central mandrel and the outer cylinder. Different procedures may be adopted for introducing a hydrostatic pressure into the tube wall. A radial force may be applied at the cylinder surface by, for example, compressing the mandrel within the elastic regime. An alternative and attractive procedure is to apply an axial force at the two ends of the tube. This may be accomplished by fully confining the tube through the use of pressure rings at both ends of the sample and then compressing the rings directly to build up a high hydrostatic pressure in the tube wall as illustrated in Figure 1.
The average strain, γ, introduced into the tube wall is:
where Ri and R are the inner and outer radii of the tube respectively.
The strain distribution in fact in tube wall is not uniform and a strain gradient exists. For a constitutional equation in the formwhere τ0 and A are material constants and n the strain hardening exponent, the shear strain dependence on radical coordinate r is:
where C is integration constant. This leads to a simpler expression of shear strain γ for the special case of τ0 =0.
The average equivalent strain:
The equivalent strain distribution for materials with constitutional relation τ = τ0+Aγ n:
Or for the materials with simple constitutional relation τ = Aγ n:
It is clear that the strain distribution is materials dependent.
t-HPS has been utilized to synthesis multilayered structures in one single step of t-HPS rotation .
The following is recent application t-HPS to process eutectic Bi–Sn (57/43) alloy for superplasticity .
This entry is adapted from 10.3390/cryst11101229
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