for zirconia coated specimens are lower than for uncoated specimens. In addition, there is no peaks in the curve of Fig. 2, and the coated specimens do not break when the reduction of area is 60%. Of special interest are the results from zirconia-coated specimens on which zirconia was overlaid after drawing. For the reduction of area from 6 to 28%, the yield stresses in the coated specimens differ little from similar values in zirconia-coated Set B specimens. For the reduction of area larger than 28%, zirconia overlaying produces a substantial increment in the yield stress as compared both to the uncoated specimens and to the Set B specimens. The same trend is observed in the ultimate tensile strengths for the Set C specimens. In this way, the application of zirconia to Ni–Cr tubes promotes hardening in work, and overlaying after each drawing operation considerably strengthens this effect. Characteristically, all deposited films gradually increase the alloy plasticity (Fig. 3). While the total elongation at ultimate strain hardening in drawing of the uncoated alloy is within 1%, its value for a coated alloy is substantially higher. Specifically, the elongation amounts to 4% for the specimens coated by zirconia before being repeatedly drawn. And for specimens on which zirconia films were overlaid each time they were drawn, the elongation is 6%. The following questions arise: What is the reason for this? And which is the expression of this in terms of materials science? Prior research, specifically, metallographic evidence, implies that the effects of amorphous deposited films are a consequence of the same physical processes in materials that were discovered earlier in mechanical tests [1]. The only distinction is that the role of the surface in working and under the contact action of the extrusion device becomes more significant. For example, under the contact action during drawing, films (that naturally possess a certain set of properties) can efficiently improve the uniformity of micro plastic strain in polycrystalline materials. Their effect in contact action becomes much stronger. As a result of the increased uniformity of plastic strain in every grain of the polycrystalline material (it was noted that additional crystallographic directions were involved in this [1]), the deformation defect structure becomes far more homogeneous and energy-balanced. A situation appears where grain boundaries; segregations (inclusion phases); the second, higher strength phase; texture; and other structural details of the initial material substantially loss their tendency to induce various local processes during deformation. Most likely, the weaker tendency of the polycrystalline material to locally accumulate structure defects is precisely the reason for the increment in the integral strength simultaneously with a relative increase in plasticity at ultimate strain hardening. The results of the tests and studies on this set of specimens should be regarded as extraordinary, even if the effect of films on the mechanical properties [1] is taken into account. Firstly, the films were applied on specimens before drawing. The observation that the effect characteristic of each type of coating is retained in multiple-draft (!) drawing is unexpected and very important in practice. Secondly, the films after multiple-draft drawing substantially add to the plasticity at the ultimate extent of hardening (reduction of area). Note that the tests did not involve anneals, which is important. Thirdly, not only the deposited films were strained together with the tube, but they were (at least, the film on the outer tube surface was) in direct contact with the extrusion device (die). However, the films each retained their distinctive effects on the mechanical properties of tubes in multiple-draft drawing. This provides grounds to suggest that not only are films conserved on the tube surface, but they even cause a certain influence on the mechanical properties of the substrate metal. Unlike the ordinary conditions of mechanical tests, there is a direct action of the extrusion device in this case. Therefore, the film may be regarded as a solid lubricant. Lastly, the fact itself that the strength and plasticity increase in tandem in drawing is evidence that films are capable of substantially changing the conditions of contact micro plastic strain of the substrate metal. The most significant consequence of this is the lowering of the critical level of the deformation defect density in the substrate (the level that causes fracture of the material). The material after being drawn has a more perfect structure regarding its mechanical properties. Based on experimental evidence (especially, from the sets of specimens on which an oxide film was overlaid each time they were drawn), we infer that the ability of the oxide coating to inhibit premature surface breakdown of the substrate is the dominant. In other words, the film creates a situation such that the surface layer remains more plastic than the metal bulk even under ultimate strain hardening. In this regard, the film
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