High-purity Molybdenum Single Crystals Research Articles

Anelastic relaxation in high purity molybdenum single crystals at medium temperatures

Abstract The internal friction of deformed molybdenum single crystals with two different orientations has been measured in the 300–1300 K temperature range. After annealing to 950 K, a relaxation peak is seen in the 880–840 K range, with a hysteresis between the warming and cooling runs. For higher annealing temperatures, the peak position change to 970 K for the 〈1 1 0〉 and 1040 K for the 〈1 4 9〉 sample. The influence of a bias stress on the sample relaxation was studied. Possible mechanisms for this relaxation have been considered, and an interaction of dislocations with vacancy type point defects has been proposed.

Microplastic Relaxations of Single and Polycrystalline Molybdenum

The microplasticity of high-purity molybdenum single crystals and of Mo polycrystals of technical purity has been investigated by relaxation step tests in uniaxial compression. A new model for the evaluation of relaxation tests in the microplastic range of b.c.c metals is presented which takes into account the decrease of the mobile dislocation density due to exhaustion of non-screw dislocations. The model allows an independent determination of the activation volume and of the microstructure parameters controlling dislocation exhaustion. The results indicate that in the high-purity single crystals the deformation rate is controlled by interactions of non-screw dislocations with the grown-in network. In the polycrystals additional interactions with impurity atoms seem to occur. In the single crystals the activity and subsequent exhaustion of two different glide systems was observed, followed by a gradual onset of screw dislocation motion. Die Mikroplastizität von hochreinen Molybdän-Einkristallen und von Mo Polykristallen technischer Reinheit wurde durch Stufen-Relaxationsversuche in uniaxialer Kompression untersucht. Es wird ein neues Modell für die Auswertung von Relaxationsversuchen im mikroplastischen Bereich kubisch-raumzentrierter Metalle präsentiert, welches die Abnahme der mobilen Versetzungsdichte infolge von Erschöpfung der Nicht-Schraubenversetzungen berücksichtigt. Das Modell ermöglicht die getrennte Bestimmung des Aktivierungsvolumens und der Mikrostrukturparameter. Die Ergebnisse deuten darauf hin, daß die Verformungsrate in den hochreinen Einkristallen durch Wechselwirkungen mit dem eingewachsenen Versetzungsnetzwerk bestimmt ist. In den Polykristallen dürften zusätzliche Wechselwirkungen mit Verunreinigungen eine Rolle spielen. In den Einkristallen wurde die Betätigung und anschließende Erschöpfung zweier verschiedener Gleitsysteme beobachtet. Danach erfolgte ein allmähliches Einsetzen der Schraubenversetzungsbewegung.

Effects of 14 MeV Neutron Irradiation on Mechanical Properties of High-Purity Molybdenum Single Crystals

High-purity molybdenum single crystals with the antitwinning axial orientation were irradiated with 14 MeV neutrons at room temperature, 473 K and 673 K under low neutron fluences of (2.1-2.7)×10 21 n/m 2 using the Rotating Target Neutron Sources (RTNS-II), and then were deformed in tension at 290 K. The irradiations led to the inhomogeneous yielding accompanied by large yield drops. The active slip plane, which of the unirradiated specimen was the plane (101), was shifted more and more towards the maximum shear stress plane (211) with decreasing irradiation temperature. By the isochronal annealing, the pronounced yield drop disappeared above 973 K

On the dislocation internal friction in high-purity molybdenum single crystals

On the dislocation internal friction in high-purity molybdenum single crystals

Effect of Point Defects on the Character of Distortions in High-Purity Molybdenum Single Crystals

The effect of point defects on the distortion character of the crystal lattice in high-purity Mo single crystals is studied. The diffuse scattering intensity is measured at T = 150 K near to the reflection 330 in the directions [110], [110], and [001]. The elements of the double force tensor and their tetragonality of (A - B)/(A + 2B) = 0.26 are calculated. The distortions are weak because they are due to point defects. The concentration of the impurity atoms determined by X-ray measurement is compared with those estimated from the residual resistivity ratio (RRR) and from the spectroscopic analysis. The good agreement suggests that the impurities in the Mo crystals investigated give rise to dilatation-type distortions. [Russian Text Ignored].

Variation of low temperature internal friction at microplastic deformation of high purity molybdenum single crystals

AbstractAmplitude and temperature spectra of ultrasound absorption in weakly deformed high purity molybdenum single crystals of different orientations were measured. The results were discussed in terms of parameter changes related to quasiparticle‐ or dislocation oscillations, respectively, dislocation‐point defect interactions as well as defect generation at microplastic deformation.

Slip systems and orientation dependence of yield stress in high purity molybdenum single crystals at 4.2 K and 77 K

Slip systems and orientation dependence of yield stress in high purity molybdenum single crystals at 4.2 K and 77 K

DISLOCATION RELAXATION PEAKS IN MOLYBDENUM

In high purity molybdenum single crystals measurements of the damping and modulus are reported in the temperature range of the α and γ-relaxation. Activation volumes of the α-subpeaks were determined from the amplitude dependence of the Internal Friction. The results clearly support the explanation of the α-relaxation in terms of kink pair fromation in 71°-dislocations and diffusion of different types of geometrical kinks in screw dislocations. Deformation induced point defects interact with these dislocation segments giving rise to the β-relaxation. Some characteristics of the γ-peak (kink pair formation on screw dislocations) are discussed.

Interaction of dislocations with localized pinning points in high-purity molybdenum single crystals

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Low-temperature mechanical properties of high-purity molybdenum single crystals doped with carbon

High-purity molybdenum single crystals of mid-orientation, doped with carbon, are hardness and tensile tested after various heat-treatments. Two-stage age-hardening is observed on isochronal aging. The two stages correspond to the two stages in resistivity coange and are considered to be due to the formation of carbon clusters. The age-hardened crystals containing more than 10 at.ppm carbon are brittle and cleaved along approximately {100} in a tensile test at 77 K. Above 179 K, an aged specimen containing 300 at. ppm carbon is ductile. Work-softening is observed in as-quenched specimens tested at 77 K and in aged crystals with more than 300 at. ppm carbon tested at the ductile temperature range. The yield stresses of brittle crystals and those showing work-softening are larger than 1100 MN M −2, which is approximately the maximum flow stress of a pure molybdenum crystal. Aged specimens with less than 10 at. ppm carbon show lower yield and flow stresses than the pure crystal.

The early stages of flow in molybdenum single crystals

The early stages of flow of high purity molybdenum single crystals with central axial orientations have been studied by thermal activation strain rate analysis over the temperature range 80 K to 398 K. The results clearly demonstrate that it is necessary to independently measure the true plastic strain rate when analyzing deformation behavior in the early stages of flow. This is because a significant amount of plastic strain is required before the plastic and total strain rates approach the value dictated by the cross-head speed. Thermal activation analysis indicates that more than one dislocation mechanism operates at strains less than about 3 pct.

Anomalous {110} slip in high-purity molybdenum single crystals and its comparison with that in V(a) metals

UHV, zone-refined, high-purity molybdenum single crystals, with the residual resistivity ratio between 293 and 4.2 K larger than 8000, oriented near the middle of the unit stereo triangle were deformed in tension in liquid nitrogen. The crystals show pronounced anomalous {110} slip with coarse and straight slip lines. Transmission electron microscopy (TEM) revealed that the anomalous {110} slip consists of lamellar network structures of irregular shaped screw dislocations aligned in the {110} plane. These observations are compared with those in V(a) metals, and it is concluded that the anomalous {110} slip observed in molybdenum is of the same nature with that in V(a) metals. The appearance of the anomalous {110} slip is successfully explained by the mechanisms already proposed by the present authors. The difference in persistence of the anomalous slip between that in V(a) metals and in molybdenum is discussed in terms of the stability of the anomalous slip band.

Constrained deformation of molybdenum single crystals

High-purity molybdenum single crystals were deformed at room temperature in both free compression and plane strain compression. In free compression, crystals were oriented with [100], [110] and [111] parallel to the compression axis. The orientations for plane strain compression were (110)[001], (110)[110], (100)[011] and (100)[010], with (hkl) being the plane of compression and [uvw] the elongation direction. Change in width of the specimen is suppressed by a channel. The resulting stress-strain (σ-ε) curves were converted to resolved shear stress: resolved shear strain (τ-γ) curves using the Taylor-Bishop-Hill analysis. It was found that the τ-γ curves do not form a narrow band if {110}〈111〉 slip is assumed. A narrow band is formed, however, if asymmetric {112}〈111〉 slip is assumed along with {110}〈111〉 slip, with τ 112 H = τ 110 = 1.3 τ 112 E , where τ 110, τ 112 H and τ 112 E are the CRSS for {110} slip, {112} slip in the antitwinning sense and {112} slip in the twinning sense respectively. Thus it may be concluded that the Schmid Law is obeyed in high-purity Mo deformed at room temperature by assuming a combination of asymmetric {112}〈111〉 and {110}〈111〉 slip systems.

Distribution of impurities in high-purity molybdenum single crystals: G P Kovtun and V I Makarov,Growth of Single Cryst of Refract and Rare Metals, Coll, Nauka Moscow 1973, 85–86 ( in Russian).

Distribution of impurities in high-purity molybdenum single crystals: G P Kovtun and V I Makarov,Growth of Single Cryst of Refract and Rare Metals, Coll, Nauka Moscow 1973, 85–86 ( in Russian).

The preparation and characterisation of high-purity molybdenum single crystals

An investigation has been made of the preparation of high-purity molybdenum single crystals by a combination of electron-beam zone-refining and thermal anneals. The changes in impurity levels have been followed by means of resistivity ratio measurements and radiochemical analysis. The results show that the majority of the impurities are removed during zone-refining as a result of volatilisation or outgassing. Two exceptions are: tungsten, which is not reduced to any significant extent and carbon, which is progressively transported to one end of the crystal during zone-refining. Carbon can be reduced to a low level (< 4 at.ppm) subsequent to zone-refining by annealing at ~ 1600°C in an oxygen partial pressure of ~ 6 × 10 −5 Torr, and the oxygen can then be reduced to a similar level by annealing under high vacuum (< 10 −5 Torr) at ~ 1800°C. An important feature of the results is that they indicate that oxygen occupies substitutional sites when in solution in molybdenum.

Electron Microscopic Investigation of the Dislocation Structure in Molybdenum Single Crystals Deformed in Tension at 293 and 493 °K

AbstractThe dislocation structure of high purity molybdenum single crystals after weak tensile deformation at 293 and 493 °K is studied by transmission electron microscopy. The transition from parabolic stress—strain behaviour to three‐stage hardening which occurs between the considered temperatures is associated with a basic change in the deformation substructure. Crystals deformed at 293 °K contain a uniform distribution of long primary and secondary screw dislocations. On the contrary, the dislocation structure in stage I at 493 °K consists mainly of isolated clusters of primary edge dipoles. It is concluded that the effect of temperature on the mobility of screw dislocations is responsible for the change observed both in work‐hardening characteristics and related dislocation structures.

Dislocation velocity measurements in high purity molybdenum

Dislocation velocities have been measured directly as a function of stress at 77°K and 300°K in high purity molybdenum single crystals by the etch-pitting technique. The results are compared to data obtained from strain-rate change experiments at 77°K, 180°K, 300°K and 575°K in the macrostrain region ( ε > 10 −3) and at 300°K in the microstrain region ( ϵ ≤ 10 −3). All the experimental observations and the calculated deformation parameters (activation volume v *, activation energy ΔH, kink energy H k and stress exponent m) are shown to be self-consistent on the basis that dislocation mobility is controlled by the overcoming of Peierls-Nabarro hills by the nucleation of double kinks as proposed by Seeger. Thus v ∗ is found to be inversely proportional to τ ∗ with typical values of v * = 18 b 3 (at τ ∗ = 7 kg mm −2 ) and v *≈ 2000 b 3 (at τ * = 0.055 kg mm −2) in the macro- and microstrain regions respectively at room temperature. Similarly ΔH = 1.57 eV (at τ ∗ = 1 kg mm −2 ) and H k ≈ 0.63 eV. The experimentally measured dislocation velocity v is found to obey the equation v = ( τ ∗ τ 0 ) m with m ≈ 6.4 and m ≈ 23.0 at 300°K and 77°K respectively. It is shown that this relationship can be derived from the theory and that m is given by m = H k 4kT In accord with this relationship the experimental values of m, as determined from dislocation velocity measurements and strain rate change experiments, are found to be inversely proportional to the absolute temperature T. The results also indicate that the same rate controlling mechanism is operative in both the micro- and macrostrain regions and that the usual assumption that the mobile dislocation density does not change during a rapid strain rate change is valid.