The high-temperature elastic properties of palladium single crystals

A thin-line acoustic method has been utilized to measure the single-crystal elastic moduli of palladium in the temperature range 30–1200 K. The single-crystal constants behave, at high temperatures, in a classical manner in contrast to a large modulus defect observed in polycrystalline palladium. The latter effect has been interpreted as a result of grain boundary relaxation.     

On the primary creep of CMSX-4 superalloy single crystals

The effect of orientation on the primary-creep mechanism of the Ni-based single-crystal superalloy CMSX-4 is examined. Four specimens with orientations within 20 deg of the [001] axis are deformed at 750 °C and 750 MPa and show a decreasing amount of primary creep as the tensile axis approaches the [001]–[011] symmetry boundary. Of these, specimen N lies within 1 deg of [013] and shows a negligible primary creep and a very low secondary creep rate. For this specimen, direct observation in the transmission electron microscope (TEM) and analysis of the shape change show that 〈112〉{111}, stacking-fault shear, is absent. Analysis of the dislocations present in the gamma precipitates and at the γ/γ′ interface shows that the only dislocations present have the Burgers vectors a/2[101] and a/2[10-1], which cannot combine to nucleate the dislocations necessary for stacking-fault shear. Thus, it is argued that the reason for the low degree of primary creep for orientations close to the [001]–[011] symmetry boundary is not the interaction between two equally stressed 〈112〉{111} systems, but the lack of either. As the orientation moves away from the [001]–[011] boundary, the range of dislocation Burgers vectors increases and stacking-fault shear is nucleated; the amount of primary creep increases as a consequence.     

Continuous bending and straightening technology of Q345c slab based on high-temperature creep deformation

A new continuous bending and straightening casting curve with the aim of full using of high-temperature creep deformation was proposed.The curvature of bending and straightening segment varies as sine law with arc length.The basic arc segment is shortened significantly so that the length of bending and straightening area can be extended and the time of creep behavior can be increased.The distance from solidifying front in the slab was calculated at 1 200°C by finite element method.The maximum strain rate of new casting curve at different locations inside the slab is 6.39×10~(-5) s~(-1) during the bending segment and it tends to be 3.70×10-5 s~(-1) in the straightening segment.The minimum creep strain rate is 7.45×10~(-5) s~(-1)when the stress is 14 MPa at 1 200°C.The strain rate of new casting machine can be less than the minimum creep strain rate.Thus,there is only creep deformation and no plastic deformation in the bending and straightening process of steel continuous casting.Deformation of slabs depending on creep behavior only comes true.It is helpful for the design of the new casting machine and improvement of old casting machine depending on high temperature creep property.     

Diffusion creep by dislocation climb in beryllium and Be-Cu single crystals

High temperature creep of beryllium and Be-Cu alloy (10 wt pct Cu) single crystals com-pressed along the -c axis was shown to proceed by pure climb of -c dislocations (Burgers vector [0001]). Experimental results can be explained if we assume that -c dislocation loops grow in the basal planes. The activation energy for the climb of the -c dislocations is found to exceed the activation energy for self diffusion by 10 ±5 kcal/mole for both beryllium and Be-Cu. This leads us to think that thec dislocation climb rate is controlled by the processes of vacancy emission or absorption at jogs rather than by lattice diffusion.     

Influence of molybdenum on the creep properties of nickel-base superalloy single crystals

The influence of Mo on the creep properties of single crystals of a model nickel-base superalloy has been investigated. The Mo content was systematically varied from 9.8 to 14.6 wt pet in an alloy series based on Ni-6 wt pet Al-6 wt pet Ta. The optimum initial γ-γ′ microstructure for raft development and creep strength was produced in each alloy prior to testing. The creep lives at 982 °C and 234 MPa exhibited a steep peak as a function of Mo content, with the maximum in life occurring at about 14.0 wt pet Mo. Deviations of less than 1 wt pet Mo from the optimum composition resulted in an order of magnitude drop in properties. As the Mo content was increased from 9.8 to 14.0 wt pct, the magnitude of lattice mismatch significantly increased, which was believed to be beneficial because of stronger γ-γ′ interfaces. As the Mo content was increased further from 14.0 to 14.6 w/o, the mechanical properties degraded because of the precipitation of a deleterious third phase. The results suggest that small variations in refractory metal content and initial gg′ size can have profound effects on mechanical properties. Hence, composition ranges and microstructures for the attainment of optimum mechanical properties may be somewhat limited and require close process control.     

Dislocation-depth distribution in high-temperature creep

The dislocation density as a function of depth from the surface was determined by X-ray diffraction line-broadening techniques on a specimen of 1100 Al allowed to creep to various strains at a temperature aboveT _m/2. The dislocation density was found to be higher in the surface layer, ρ_s, than in the interior, ρ_i. With increasing strain, ρ_s increased by ρ_i remained constant. During the annealing of the strained specimens, the recovery occurred in two stages: a fast stage (U_I) and a slow stage (U_II). The activation energy (U_I) equaled 14.3 kJ/mole, whereas (U_II) equaled 127.9 kJ/mole. The rate of decrease of dislocations in the interior was impeded by the presence of the surface layer.     

High temperature creep of Ni-20Cr-2ThO2 single crystals

The high temperature creep properties of single crystals of Ni-20Cr-2ThO₂ have been investigated in the temperature range 650–1300°C. Single crystal tensile specimens were cut from plates having a large elongated grain structure and constant stress vacuum creep tests were conducted at various stresses and temperatures.By including the temperature dependence of the elastic modulus in the analysis of the data, the creep activation energy was found to be very nearly equal to that for self diffusion of Ni-20Cr. The stress dependence of the creep rate was observed to be large and variable, with power law exponents ranging from 9 to 75 over the temperature range studied. The ductility of these crystals as measured by percentage reduction of area was very high, typically ranging from 40 to 60%. The percentage elongation varied inversely with the stress exponent and ranged from 1 to 28%.The creep properties of these dispersion strengthened crystals can be described accurately by considering the creep strength to be given by a simple sum of the creep strength of unthoriated polycrystalline Ni-20Cr and the Orowan strength, as calculated from the measured ThO₂ particle distribution. This simple model describes the measured creep rates to within a factor of three at all stresses and temperatures.     

X-ray diffraction study of subgrain misorientation during high temperature creep of tin single crystals

Creep tests in the high temperature range (0.68 T_m-0.94 T_m, where T_m is the melting temperature) have been performed on high purity tin single crystals with the [100] direction as the tensile axis. Above the transition temperature, about 0.84 T_m (150 °C), the creep activation energy was found to be almost equal to the self diffusion energy. Below the transition temperature the creep activation energy is about half of the self diffusion energy and has a value of the order expected for dislocation pipe diffusion. A method based on X-ray rocking curves was developed to measure subgrain misorientation angles of crept specimens. The average subgrain misorientation angles for specimens crept at 70 °C and 100 °C increased continuously with creep strain at rates a factor of 20 to 40 smaller than that predicted by a cross-slip controlled process. The average subgrain misorientation angles at a given strain decrease slightly with increasing temperature. However, no abrupt change of the angle was observed at the transition temperature. It was concluded that a cross-slip controlled mechanism cannot control below the transition temperature. Instead, our results lend support to Sherby and Weertman’s argument that dislocation climb controls creep over the entire high temperature regime.     

Substructure formation and nonuniformity in strain during high temperature creep of copper single crystals

The change in substructure during high temperature creep of copper single crystals was examined to make clear the relation between the substructure and the creep rate. The structural observations by means of etch-pit technique were made in the same area of the specimen surface after successive straining by creep. Different types of substructures were formed in different localities at an early stage of transient creep. Small subgrains elongating in the direction of the deformation band were found in some regions (region A). Large subgrains containing cells within them were observed in other regions (region B). With the progress of creep, the substructures in both regions underwent gradual changes to become alike in their appearance with each other. The creep strains in regions A and B were examined at various creep stages. The ratio of creep strain to the average creep strain throughout the specimen,ε/ε _av, was a few times larger in region B than in region A at an early stage of transient creep. With the structural changes mentioned above, the difference between the ratios in the two regions came to disappear, both becoming almost unity at the later stage of creep. These results suggest the close relation between the substructure and the creep rate.     

Effect of plastic anisotropy on the creep strength of single crystals of a nickel-based superalloy

The effect of plastic anisotropy, which is caused by the arrangement of slip systems, on the creep strength of notched specimens of Ni-based superalloy single crystals, is described. It was revealed that the creep strength of the notched specimens was affected by the crystallographic orientation, not only in the tensile direction, but also in the thickness direction. The creep strength was superior in the notched specimen with the [011] tensile and [100] plate-normal orientation, whereas the notched specimen with [011] tensile and [100] plate-normal orientation exhibited extremely poor creep strength. In the case of specimens whose tensile orientation was [011], results of the creep-rupture tests at 973 K were in agreement with the assumption of the operation of {111}〈112〉 slip systems. The creep strength in the notched specimen with [001] tensile and [100] plate-normal orientations was superior to that in the notched specimen with [001] tensile and [011] plate-normal orientations. When the tensile orientation was [001], the results of the creep-rupture tests were in agreement with the assumption of the operation of {111}〈112〉 slip systems during primary creep region and {111}〈101〉 slip systems during secondary creep region.     

Application of fracture mechanics to high-temperature creep rupture

Abstract

This paper reviews the crack opening displacement approach to fracture testing of materials at ambient temperature and considers extension of the work to higher temperatures, as one phase of an approach aimed at bringing various cracking behaviours within the same over-all fracture mechanics treatment.

Testing procedure uses BS 15 mild steel as l-inch plate at 350°C, with loading times up to 1000 hours. A rectangular specimen about 18 × 6 inches is slit for three quarters of its length along the centre of width, and the notched region is immersed at high temperature. The split ends protruding from the furnace are strained apart, and a transducer is projected into the furnace to provide a continuous measurement of crack opening displacement. CaDs produced are in reasonable agreement with values predicted by the general yielding theory. Intergranular crack extensions occurred at CODs of 14 to 17 × 10^?3 inches, were of the ‘pop-in’ type.

One specimen was cooled to 0°C after test so that the crack could be extended by cleavage. The COD at initiation was 5 × 10^?3 inches, indicating that the material had been substantially embrittled by hot straining.

Résumé

Après avoir revu l'ouverture et le déplacement des fissures pour des essais de rupture à température ambiante, les auteurs considèrent l'élargissement de ces conclusions a haute température comme approche pour examiner divers comportements des fissures dans un mécanisme global de la rupture.

Pour les essais, une tôle d'un pouce d'épaisseur d'acier doux BS?15 a été utilisée a 350°C, le temps maximal de mise en charge étant de 1000 heures. L'éprouvette rectangulaire déenviron 18 po. × 6 po. est entaillée en son centre sur les trois quarts de sa longueur; la région entaillée est maintenue à haute température. Les extrémités fendues de la tôle sont écartées l'une de l'autre hors du four et un palpeur dans le four donne en continu la propagation de la fissure. La propagation des fissures est en bon accord avec les valeurs données par la théorie de l'écoulement. L'extension de fissure intergranulaire se produit pour des valeurs de la propagation des fissures de 14 à 17 × 10^?3 pouces et sont réentrantes. Une éprouvette fut refroidie à 0°C aprés l'essai de fa?on à pouvoir faire avancer la fissure par clivage. La propagation de la fis sure était de 5 × 10^?3 pouces indiquant une fragilisation du métal par deformation à chaud.     

High-temperature creep and the defect structure of nickel-based superalloy single crystals after hot isostatic pressing

The influence of hot isostatic pressing on the high-temperature creep of single crystals of a nickel-based superalloy containing rhenium and ruthenium is studied. The microstructural damages caused by the development of creep have been studied. Hot isostatic pressing is found to weakly influence the life and creep of the superalloy at 1150°C. The distribution of deformation pores over the length and cross section of failed samples has been studied. A high pore concentration is shown to exist at the sites of severe plastic deformation.