A. B. Lebedev

Softening of the elastic modulus in submicrocrystalline copper

Abstract Youngs modulus of copper polycrystals with an ultrafine-grained structure synthesized by two independent methods (compacting and heavy plastic deformation) has been measured within a temperature range from 20 to 200–300°C. It has been concluded that there is a component of the Youngs modulus softening which is peculiar to metals with nano- and submicrocrystalline structures, irrespective of the presence of porosity.

Amplitude-dependent elastic-modulus defect in the main dislocation-hysteresis models

The amplitude-dependent defect of the elastic modulus has been calculated for the three main dislocation-hysteresis models: (i) breakaway hysteresis of Granato-Lücke, (ii) Davidenkov hysteresis, and (iii) friction hysteresis without restoring force (WRF). The ratio r of the amplitude-dependent decrement to the modulus defect has been considered for all three types of loops, and it is shown that, in a general case, r depends on the vibration amplitude. In the particular case of power-law amplitude dependences of the decrement and the modulus defect, r does not depend on amplitude and depends only on the exponent n. Expressions have been obtained for the r(n) dependence for the three hysteresis-loop types, and it is demonstrated that r can serve to identify the loop shape. A comparison of calculated curves with experimental data accumulated to date shows that most of them lie closer to the Davidenkov and WRF hystereses. An analysis has been made of the applicability of the secant modulus-defect approximation used to derive the dislocation strain from internal-friction measurements.

Thermal stability of submicrocrystalline copper and Cu: ZrO2 composite

Ultrafine-grained (UFG) polycrystals are very attractive subjects for both theoretical and experimental research due to their unusual physical and mechanical properties. In recent years much attention has been paid to UFG polycrystals produced by heavy plastic deformation techniques, which allow one to obtain poreless materials with a grain size of about 0.1--0.2 {micro}m. A large number of investigations have been done in copper. Heavily deformed (HD) copper with UFG structure exhibits a high value of the yield stress and microhardness, and a softening of elastic moduli. However, annealing at temperatures of about 150--200 C for 20--60 min considerably increases the grain size, reduces the yield stress and microhardness, and increases the elastic constants. In the present communication the effect of annealing on the yield stress and Young`s modulus in HD Cu and precipitate hardened Cu:ZrO{sub 2} composite with UFG structure is reported. Good thermostability of the yield stress for HD Cu:ZrO{sub 2} within a temperature range 20--500 C is demonstrated.

Low-temperature deformation of nanocrystalline niobium

The strain characteristics of nanocrystalline niobium are measured in the temperature range 4.2–300 K. It is shown that the development of a strong local deformation with clearly delineated macroscopic slip bands occurs at 4.2 K and 10 K. The thermal effects at a stress jump observed upon transition of the sample (or a niobium strip placed close to the sample) from the superconducting state to the normal state are estimated. It is demonstrated that the temperature dependence of the yield point σs(T) can be divided into three portions: two portions (T<10 K and T>70 K) with a slight change in σs and the third portion with a strong dependence σs(T). The strain characteristics of polycrystals with nano-and larger-sized grains are compared with those of single crystals.

Influence of impurities on the low-temperature deformation of nanocrystalline copper

The influence of ZrO2 particles on the low-temperature deformation of nanocrystalline copper produced by strong plastic deformation is investigated using equichannel angular pressing. A comparison is made between the deformation characteristics in tension and compression in the temperature range 4.2–400 K, measured for copper and the composite Cu:0.3 vol. % ZrO2. It is shown that within 4.2–200 K the yield point σsm of the composite is higher than that for copper, attaining 680 MPa at 4.2 K, then the yield points are close in value up to room temperature, and diverge again as the temperature is raised. Possible causes of the dissimilar influence of an impurity on the strength and plasticity characteristics of nanocrystalline copper in various temperature intervals are discussed.