Plastic deformation behaviour and deformation substructure in Al-rich TiAl single crystals deformed at high temperatures

Plastic deformation behaviour in Ti–54.7 at.%Al and Ti–58.0 at.%Al single crystals was examined around and above the anomalous strengthening peak temperature (T_p) focusing on the effect of Al₅Ti₃ superstructure. The Al₅Ti₃ superstructure developed in the L1₀ matrix of Ti–58.0at.%Al, and the size of the Al₅Ti₃ phase once increased during annealing at 8008C and then decreased withincreasing temperature, while no significant evidence of the Al₅Ti₃ particles was obtained in Ti–54.7 at.%Al from TEM observation although diffuse scattering corresponding to the spots for the Al₅Ti₃ superstructure was observed. The transition of slip plane for ½<110]; ordinary dislocations from {111} to {110) and/or (001) occurred at and above T_p due to anisotropy of anti-phase boundary energies on {111}, {110) and(001) in the Al₅Ti₃ superstructure. Anomalous strengthening is related to the development of this superstructure which may assist the cross-slip of some parts of ½<110] ordinary dislocations onto {110) and/or (001) resulting in the formation of dragging points to the motion of the dislocations.     

The plastic deformation of potassium single crystals at low temperatures

The yield stress of potassium single crystals was measured in the temperature range of 1.5 to 30 K for a wide range of orientations. The effects of nobs itutional solutes (sodium and rubidium) on the yield strength of potassium single crystals were also investigated. The Schmid law of critical resolved shear stress for slip is invalkid in the temperature-dependent region of yield stress, and the orientation dependence of yield stress is in agreement with the Peierls mechanism for screw dislocation motion. The validity of the Schmid law may be questionable even in the athermal region, as suggested by the observations at 30K. Alloy softening is observed in the temperature-dependent region of yield stress by adding substitutional solutes to potassium.     

Plastic deformation of Al2O3 single crystals by indentation at temperatures up to 750° C

Vickers hardness measurements have been made on three differently oriented sapphire single crystals in the temperature range from room temperature to 750° C. The hardness generally decreased with increasing temperature but varied with specimen orientation. In the regions surrounding the indents, twinning lamellae and slip lines have been studied systematically and could be associated with dislocation systems. Rhombohedral {01¯1 2} and basal (0 0 0 1) twinning as well as prismatic {1 1¯ 2 0} and basal (0001) slip have been observed. Plastic deformation occurred during placement of Vickers microhardness indentations. The hardness was markedly influenced by the number and kind of activated systems. A change in the activated systems caused variation and inversion in hardness ratios. Investigating high-index planes gave much more information on anisotropy effects than would have been obtained had only basally and/or prismatically oriented specimens been used. The latter is commonly the case in the literature. It was shown that in brittle materials, such as ceramics, plastic deformation occurs at temperatures below 0.5T/T _m if a stress field with a large hydrostatic component is applied. It is suggested that this is also the case in abrasion.     

Anelastic relaxation in high purity molybdenum single crystals at medium temperatures

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.     

Fracture and deformation behaviour of melt growth composites at very high temperatures

Unidirectionally solidified Al₂O₃/Y₃Al₅O₁₂ (YAG) or Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10^–4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al₂O₃/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase.     

镁合金AZ31高温形变机制的织构分析

利用X射线衍射和背散射电子衍射方法测定了镁合金AZ3l高温动态再结晶和超塑形变时的宏观和微观织构,分析了晶粒内部的形变机制.结果表明,在动态再结晶和超塑形变过程中,晶粒内部的滑移机制仍起重要作用,表现为再结晶晶粒出现择优取向以及一些晶粒可充分均匀形变成长条状.宏观织构的测定表明,具有不同初始织构的两类样品高温动态再结晶时,新晶粒有不同的取向择优过程,形成相似的织构;长条形变晶粒内部开动的滑移系也有一定的差异.分析了不同温度下相同的织构对应的不同塑变机理取向成像分析表明,基面织构取向的晶粒间总伴随着较高比例的小角晶界和30°(0001)的取向关系,这是六方结构的六次对称性限制了动态再结晶时(亚)晶粒间取向差的有效增大的缘故.     

Plastic deformation in diacetylene (PTS) monomer single crystals

The slip systems of diacetylene (PTS) monomer crystals which were deformed with a shear deformation and a micro indentation were determined. The (1 0 0) [0 0 1], (1 0 0) [0 1 0] and (1 0 2) [0 1 0] slip systems eventually were identified as the main slip systems from the stress-strain curves, slip lines and etch-pit arrangements.     

The yield strength and dynamic behaviour of dislocations in MgO crystals at high temperatures

The mechanical behaviour and mobility of dislocations in MgO crystals in the temperature range 20 to 800° C are shown to be dependent on the concentration and the valence state of impurities. Impurities of trivalent state are found to interrupt the motion of screw dislocations and lead to an increase of the yield strength throughout the temperature range investigated. Impurities of divalent state, of which Ca²⁺ ions are the most prevalent, are shown to retard the motion of edge dislocations by forming an atmosphere at elevated temperatures and are responsible for a peak of the yield stress appearing around 700° C.     

Plastic deformation of Zr-Sn polycrystals at intermediate temperatures

The yield stress and the activation volume for Zr-Sn alloys with 0.74, 2.85, 4.27 and 6.19 wt% Sn have been measured at temperatures between 400 and 750 K. The temperature dependence of the yield stress exhibits a plateau except for the alloy with the highest content of tin. The yield stress increases with increasing content of tin atoms. A non-monotonic variation of the activation volume with temperature has been observed for pure zirconium and for Zr-Sn alloys with 0.74 and 2.85 wt% Sn. The maximum value of the activation volume (at about 600 K) decreases with increasing content of tin. Dynamic strain ageing is considered to be responsible for the maximum in the temperature dependence of the activation volume. The dislocation structure has been observed. The experimental results are interpreted in terms of a simple model which considers that the flow stress is determined by thermally activated glide of dislocations through obstacles, dynamic strain ageing and a strengthening effect of tin atoms.     

Strain-dependence of deformation microstructures in ultra-low-C IF steel deformed to high strains by torsion at elevated temperatures

Single-phase bcc-ferrite in an interstitial free(IF)steel was deformed to different strains in a wide range from low to high strains(ε=1–7)by torsion under different Zener-Hollomon(Z)conditions.The specimens were rapidly quenched after the torsion to preserve microstructures formed under different deformation conditions.The results showed that during high-Z(low-temperature)deformation,grains were subdivided by geometrically necessary boundaries(GNBs)via the grain subdivision mechanism.Deformation to high strains(ε>5)led to the ultrafine lamellar structures(with grain sizes<1μm)mainly composed of GNBs having high misorientation angles.Decreasing Z with increasing temperature and/or decreasing strain rate accelerated thermally activated processes,such as dynamic recovery and boundary migration.Unlike the ultrafine lamella formed under the high-Z condition,a variety of microstructures having equiaxed morphologies with fine to coarse grain sizes(>1μm)were realized with decreasing Z.The significance of the grain subdivision and the thermally activated phenomena on the formation of various microstructures under different deformation conditions is discussed.     

Tensile impact behavior and deformation mechanism of duplex TiAl intermetallics at elevated temperatures

Investigations are made on the effects of strain rates on the tensile behavior and deformation modes of Duplex Ti–46.5Al–2Nb–2Cr (DP TiAl) at temperatures ranging from room temperature to 840 °C and under strain rates of 0.001, 320, 800, and 1350 s⁻¹. The dynamic strength is higher than quasi-static strength but does not change much over the high strain rate range. Yield stress anomaly is not found. Brittle-to-ductile transition temperature (BDTT) increases with the increased strain rates. A Zerilli–Armstrong constitutive model with appropriate coefficients is chosen to describe the high strain rate flowing behavior. TEM analysis indicates that both ordinary dislocations and superdislocations are found and dislocation pile-up only appears in samples deformed under quasi-static loadings at elevated temperatures. The deformation twins are common in equiaxed grains and the proportion of twinned grains increases with the increased strain rate from 46–72% under quasi-static loadings to 69–95% under high strain rate loadings. No deformation twins are found in lamellar colonies.     

Recovery in deformed copper and nickel single crystals

The static recovery behaviour of copper and nickel single crystals deformed in multiple slip was investigated in mechanical tests and in HVEM in situ annealing experiments. Recovery of Type I, as previously identified in aluminium was observed in both its microscopic and macroscopic features: a sharpening of the cell-wall structure without a change in substructure scale, correlated with a short transient in the strain-hardening behaviour during retesting. The substructure development with increasing recovery time and temperature is similar to that during dynamic recovery, i.e. with increasing strain in a continuous test. After longer recovery times or at higher annealing temperatures, the specimens recrystallize; after larger strains, they recrystallize dynamically. An intermediate stage akin to the Type II recovery found in aluminium was never observed, either in its macroscopic manifestation of a long reloading transient, or as a general coarsening of the subgrain structure. Examples of local sub-boundary mobility and dissolution were, however, seen in situations close to static or dynamic recrystallization. It is concluded that the fluctuations occurring during subgrain coarsening are stable in aluminium, leading to Type II recovery and extended steady-state deformation, but unstable in copper and nickel, leading to static or dynamic recrystallization.     

Plastic deformation and fatigue properties of ternary CoTi based alloys containing Al at room and high temperatures

Mechanical and fatigue properties of ternary CoTi based single crystals containing 2% and 5% Al was investigated. It was shown that in these two alloys the addition of Al improve the strength of CoTi at all tested temperatures, but by increasing Al content the peak temperature shifted to lower temperature and the effect of anomalous strengthening was weakened. The number of cycles to failure in fatigue test for CoTi (2 at.% Al) single crystals showed a minimum, at high temperatures. This phenomenon seems to be due to the activation of new slip systems. Also by increasing Al, the fatigue limit at constant temperature can be changed. It was shown that Al addition is not effective in improving fatigue life at low temperatures but it can improve it at high temperatures.