Propagation of cracks in structural alloys at a temperature of 4.2 K

We study distinctive features of the process of crack propagation in structural materials under conditions of discontinuous yield and under the action of pulses of electric current at a temperature of 4.2 K. The kinetics of fracture processes in steel under the indicated conditions is studied by the method of stereofractographic analysis. We investigate the applicability of approaches to the evaluation of the characteristics of the crack-growth resistance of materials that take into account the effect of discontinuous yield and pulses of electric current on the process of formation of a plastic zone at a crack tip. Translated from Problemy Prochnosti, No. 6, pp. 36–40, November–December, 1997.     

Structural state of Ti-45 wt %Nb alloys strained at 4.2 K

The structure of Ti-45 wt %Nb alloy treated under different conditions and strained at 4.2 K has been studied by transmission electron microscopy (TEM). A considerable difference in defect structures after low-temperature straining was found between recrystallized samples and the samples drawn and drawn with subsequent annealing at 375° C. Twinning on the [112] and apparently on [110] planes was observed in the recrystallized samples strained at 4.2 K. Partial detwinning was found after rewarming to room temperature.     

Propagation of small surface cracks in stainless steel at high temperature

Growth behavior of small surface cracks in stainless steel is experimentally examined at 500°C and room temperature. The crack is confirmed to maintain a semi-elliptic shape during its growth. Cyclic deformation behavior of the material is investigated to evaluate elasto-plastic fracture mechanics parameters. It is found that crack growth rates have a good correlation with the strain intensity factor range ΔK and J-integral range ΔJ.     

On a possible mechanism of instability of plastic deformation of metals and alloys at 4.2 K

The fine structure of spontaneous deformation jumps and jumps initiated by both the shock action on a deformation device and a strong fast-changing magnetic field (up to 2.7 MA/m) has been investigated in metals and alloys at liquid-helium temperatures. Oscillography of the signals of changes in the load on the sample and of acoustic emission has made it possible to reveal the relationship between types of jumps and postulate on the mechanism of their appearance.     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.     

Effect of the scale factor on special features of deformation of structural alloys in static loading at low temperatures (4.2 K)

Investigations were carried out into the effect of the scale factor on special features of intermittent yielding of structural alloys under rigid static loading at the boiling point of liquid helium (4.2 K). The results show that an increase of the deformed volume of the metal is accompanied by reduction of the specific surface,which impairs the conditions of heat removal from the gauge part of the specimen. This in turn changes the nature of intermittent yielding of the material. It is important to take into account the effect of the form and dimensions of specimens as well as the structural elements on special features of their deformation at low temperatures (4.2 K).Translated from Problemy Prochnosti, No. 5, pp. 61–66, May, 1990.     

Properties of CuSn bronze at 4.2 K

Mechanical properties of bronze at 4.2 K were determined as a function of tin content. The 0.02% offset yield strength varied from 76 MPa for pure copper to 241 MPa for an 8.2 w/o Sn bronze. The work-hardening modulus varied correspondingly from 1.9 GPa to 9.6 GPa. Electrical resistivity at 4.2 K was also measured as a function of tin content and found to vary from 1.27 × 10^?8 ohm-cm (pure copper) to 1.3 × 10^?5 ohm-cm (10.7 w/o Sn bronze).     

Temperature dependence of the linear expansion coefficients of some Fe-Cr-Ni alloys in the temperature range 4.2 to 300 K

The temperature dependence of the linear expansion coefficients of Cr18Ni10, Cr18Ni15 and Cr18Ni25 alloys has been studied in the temperature range 4.2 to 300 K. Using dilatometric curves the temperature range (195-60 K) of the martensitic transformation on cooling was determined for metastable Cr18Ni10 alloy single crystals.The linear expansion coefficients of the stable structure Cr18Ni25 and Cr18Ni15 and metastable Cr18Ni10 alloys were found to become negative around 25 K; this effect is assumed to be associated with the antiferromagnetic rearrangement in austenite     

Electron transport properties of palladium-ruthenium alloys from 50 mK to 4.2 K

The thermopowerS, resistivity , thermal conductivity , and thermoelectric ratioG of alloys nominally of 0.1, 0.5, 1.0, and 5 at % Ru in Pd have been measured from 50 mK to 4 K. The thermopowerS varies essentially linearly withT, and is only a slowly varying function of concentration up to 1 at % Ru. The characteristic thermopower of Ru in Pd for the 0.1 at % alloy is (0.23±0.01) T µ V/K. At temperatures below 0.15 K there are departures from linearity, presumably caused by magnetic impurities. In the same temperature rangeG shows large departures from the expected constant value. The Lorenz ratioL obtained from two independent sets of data,S/GT and /T, are in good agreement. Departures ofL from the Sommerfeld valueL ₀ are discussed.Work supported by the National Science Foundation under Grant number DMR-75-01584.     

Anti-Stokes Luminescence in ZnSe Single Crystals at 4.2 K

Photoluminescence (PL) and Raman scattering in ZnSe single crystals were studied at 4.2 K. Under excitation by He–Ne and Ar laser lines within the spectral window of ZnSe, both Stokes and anti-Stokes PL signals were observed in a backscattering geometry. The anti-Stokes PL is due to excitonic and impurity recombination processes. The results open up possibilities of qualitative and quantitative analysis of impurities and defects in the bulk of the semiconductor by analyzing its low-temperature excitation spectrum.     

Effect of low temperature thermocyling on the structure and stress-strain curve parameters of a TiAl alloy at 4.2 K

Transmission electron microscopy was used to study the features of the defect structure of the Ti-(3.5-5)%Al alloy induced by repeated thermocycling between 300 and 77 K and by low temperature plastic deformation at 4.2 K. The low temperature thermocycling combined with an applied stress brings about significant changes in the structural state of the alloy (dislocation density increase, redistribution of internal stresses, twinning difficulty), resulting in increasing strength and plasticity of the alloy at the liquid helium temperature.     

Effect of strain rate on features of discontinuous flow for titanium alloys under static tension conditions at 4.2 K

The effect of strain rate on features of discontinuous flow for titanium alloys at 4.2 K is studied. It is shown that an increase in strain rate with static tension in a medium of liquid helium leads to marked qualitative and quantitative changes in the nature of material deformation. The reason for this is an increase in the temperature of the specimen gauge length determined by the amount of thermal flow which forms as a result of plastic deformation. The effect of this factor should be considered in evaluating the strength of structures under extreme cooling conditions.Translated from Problemy Prochnosti, No. 9, pp. 26–30, September, 1990.     

Operation of the TVO temperature sensors in the range from 4.2 K up to 425 K

The report continues our investigations on cryogenic thermometers. To estimate the influence of warming the well known TVO temperature sensors [Proceedings of the ICEC17 (1998) 699, Cryogenics 41 (2001) 213, Advances in Cryogenic Engineering 45B (2000) 1817, Proceedings of the ICEC18 (2000) 627] up to 425 K on their calibration curve, a series of experiments have been carried out. The number of thermal cycles in the range from 425 K down to 77.3 K was 105. Comparison of readings of the sensors at 293, 77.3 and 4.2 K was performed with initial calibration curves for 100 cycles. Then several sensors were re-calibrated, and a new comparison was done during five additional thermal cycles at the same temperatures. The obtained results are discussed and they seem to be optimistic.