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NDRM

As known, the functional properties of NbTi – based superconductors become formed, mainly, by its structural and phase state, i.e. structure type, grain (subgrain) size, content by volume of secondary phase precipitations, their size and morphology [1]. To increase the current-carrying capacity of the superconductor, different methods are used at stages of ingot processing to wire making, they are alloying, cold deformation, thermal treatment, radiation, etc., as well as optimization thereof. The conventional method of NbTi superconducting alloy processing is based on formation of the fine-disperse nonuniformities of the structure and on the fact that the α-Ti precipitates are the most effective pinning centers [2-4]. That’s why, the whole of processing stages should be aimed at the formation of a uniform submicrocrystalline or nanostructure across final diameter of the wire.

Cold deformation is considered to be the main tool for the attaining of such a structure. However, the conventional procedures, such as extrusion, rolling and drawing, which are peculiar from technological viewpoint, do not always allow the accumulation of strain needed for reaching a desired result. At the same time, there exist methods of plastic metal working with which the result can be reached with no changes in the initial shape and dimensions of a billet [5]. The equal-channel angular pressing (ECAP) is among them [6].

This method of severe plastic deformation consists in pressing a billet through two deforming channels of the same cross-section equal to that of the billet and intersecting with each other at angle 2θ=90º. It provides high value of unit degree of deformation (e > 1), high power level and thermal effect of billet self-heating. New method of the equal-channel multiple-angle pressing (ECMAP) was created at Donetsk Physics and Technology Institute of the NAS of Ukraine. This method is effective for transformation of the alloy structure, it has several technological advantages as compared to ECAP method. The essence of the ECMAP is in fractional deformation conditions realized with a few angles θ of 60º and higher, at which the deforming channels intersect each other.

In the paper, the results of the ECMAP method application in combination with traditional deformation methods and thermal treatment with a view to improve current-carrying capacity of superconductive products has been presented. 

II. EXPERIMENT

Original billets were bimetallic rods from Nb+60 at.% Ti (60T) in a copper matrix prepared by hot pressing at 750ºC. The alloy is of two-phase composition representing a β-solid solution with bcc lattice and with a small volume fraction   (~ 1 %) of hexagonal α-phase.

The rods, 60 mm in diameter and 80 mm long were subjected to the equal-channel multiple-angle pressing with no change in cross-section of the billet. The laboratory plant was mounted on a hydraulic press of 2,5 MN force. The deforming block (fig. 1) was a container having a set of thick-walled operation and sizing bushes located and secured in the casing and held in position by studs. The container and the studs formed four intersecting operation channels of equal cross-section and with intersecting half-angles θ₁ = θ₃ = 80º and θ₂ = 70º.

Illustration 1. a set of thick-walled operation and sizing bushes.

The ECMAP was realized at room temperature according to the „billet after billet” or „false-billet after billet” scheme with deformation rate per a pass e₁= 0,82. Value of deformation per a cycle was determined by the formula:

,

where θ – half-angle of channels intersection, the accumulated strain was determined as e = N·e₁, where N – number of cycles. The sizing bush created the possibility of multi-cycle treatment realization by the above scheme and provided with a backpressure to improve deformation uniformity and structure processing. The pressure of pressing was in the range of 600-800 MPa.

         The ECMAP involved in the scheme of processing the superconducting alloy of 60T quality results in the formation of a uniform enough nanocrystalline structure with a high-disperse and uniformly distributed α-phase precipitations. As a result, the mechanical properties and critical current density of the superconducting wire produced from this alloy become much improved.