Lower Creep Rate Research Articles

The role of precipitation parameters upon the creep rupture properties of inconel alloy X-750

The effects of grain size in the range of 9–200 μm and of precipitation parameters on the creep rupture properties of Inconel alloy X-750 at 700°C have been investigated. Superior creep properties of 113 μm grain size material are associated with very low minimum creep rates. This in turn is a consequence of an optimum combination of microstructural parameters—viz., a wide precipitate free zone (PFZ), discontinuous precipitates along grain boundaries, together with a fine distribution of intragranular γ' (Ni 3 Al, Ti), precipitates. At low temperature, the material was relatively ductile and exhibited cup and cone type transgranular fracture, while at intermediate temperatures the fracture mode was intergranular. At intermediate temperatures, fracture occurred by cavitation, failure resulting late in the tertiary stage of creep when a crack of critical size to propagate spontaneously formed. At high temperatures, large areas of fracture surface were created by plastic tearing. Many cracks were present in the small (9 μm) grain size material, owing to the large number of triple junctions which acted as barriers to crack propagation while in coarse grained material no cavitation damage was observed in failed specimens. Water quenching during heat treatment in general produced continuous grain boundary precipitates which are detrimental to the creep rupture properties. In cases where continuity of grain boundary precipitates and the precipitate free zone (PFZ) were such as to have opposing effects on creep rupture properties, the former played the dominant role in limiting rupture life.

Effect of low-pressure oxygen on the creep properties of W-25 pct Re

The creep properties of W-25 pct Re were measured in low-pressure oxygen to simulate its interaction with oxygen derived from the decomposition of oxides at high temperatures. The results are compared with the creep properties of the alloy in a vacuum of 10−7 torr. Between 1650° and 2000°C and stresses of 1000 and 2000 psi, the creep rates were lower in 10−5 torr O2 than in vacuum by factors as high as 250; however, rupture strains were only 2 to 7 pct in the O2 environment. Specimens exposed to oxygen before creep testing in vacuum were also stronger than specimens tested in vacuum. The oxygen content of specimens tested in oxygen was generally lower than in the as-received material. The lower creep rates of W-25 pct Re in oxygen are attributed to the presence of a surface layer of σ phase. This reaction layer results from the formation of volatile tungsten oxides at a higher rate than rhenium oxides.

The Influence of γ Precipitation upon the Creep of γ′ (Ni3(Al, Ti)) Single Crystals

Earlier work by R. K. Ham, R. H. Cook, and G. R. Purdy (Metal Sci. J, 1972, 6, 73) has demonstrated that it is possible to produce a precipitate of the disordered nickel-rich terminal solid solution, γ, in a matrix of the ordered γ′ phase Ni3 (Al,Ti). The present paper shows that this precipitate increases the steady-state creep-resistance of the matrix and induces a prolonged initial stage of very low creep rate (‘nilcreep stage’). The end of the nil-creep stage was associated with loss of coherency of the γ precipitate and an increase in matrix dislocation activity. A model is proposed in which the particles are treated as attractive trapping sites for superlattice dislocations, so that measurable deformation can occur only when Ostwald ripening has increased the particle spacing to a level sufficient for Orowan looping of dislocations between the particles. This model accounts for many of the observed features of the nil-creep stage.

Influence of Vacuum Environment on the Composition, Structure, and Mechanical Behavior of the Tantalum Tungsten Hafnium Alloy T-111

Various heats of a tantalum base alloy containing 8% tungsten and 2% hafnium together with 30 to 130 ppm oxygen as a residual impurity were creep tested in a vacuum environment of less than 1 × 10−8 Torr. Analyses of the residual gas composition indicated that the major constituents of the vacuum environment were carbon monoxide, hydrogen, argon, and water. Loads and temperatures were selected to provide approximately 1% creep strain in times ranging between 1000 and 40 000 h. Chemical analysis of the test materials before and after creep testing indicated that the exposure to the 10−8 Torr environment caused a significant decrease in the amount of oxygen present in the tantalum alloy. This decrease was accompanied by the disappearance of a grain boundary network of hafnium oxide inclusions which was shown by electron microprobe studies to be present in the material prior to testing. This loss of oxygen during testing had a pronounced influence on creep behavior of the T-111 alloy in the 1600–2200 °F temperature range where an oxygen-associated dynamic strain aging effect has been noted. The strain aging behavior appeared to cause an anomalously low creep rate in the early stages of T-111 creep testing; for example, creep rates below 10−8/h were measured at 1600 °F and 35 ksi. This period of very slow creep was followed by a transition to a much higher creep rate and this transition was thought to occur as a result of the loss of the oxygen from the test material. Analysis of the creep rates measured after the rate transition provided an activation energy value which was the same order as the self-diffusion coefficient for pure tantalum, indicating after the oxygen was lost that creep proceeded by a conventional diffusion controlled process.

High temperature creep in a Cu-11 at. % Al solid solution

Abstract The creep in a Cu-11 at. % solid solution was investigated by the isothermal tests technique. In the region of lower temperatures and lower creep rates, respectively, the apparent activation energy of creep is stress independent and its value is close to that expected for the activation enthalpy of self-diffusion of aluminium in the solid solution. The stress exponent n = η In δέ/δ In a increases with stress from 4 to 5·5. Viscous motion of dislocations connected with the interaction of aluminium atoms with dislocations was suggested to be the rate-controlling mechanism. In the region of higher temperatures and higher creep rates, respectively, two mechanisms seem to operate. The apparent activation energies are strongly stress dependent and are considerably higher than the activation enthalpy of self-diffusion of copper in the solid solution. The stress exponents n generally depend on stress and temperature. The results for the higher stress interval 6·0–11·0 Kg mm−2 can be correlated with the m...

Cavity Growth by Grain-Boundary Sliding during Creep of Copper

AbstractThe recent results of Davies and Williams apparently confirm the suggestion made earlier that at high creep rates cavity nuclei grow by grain-boundary sliding. At lower creep rates, however, growth may occur by a combination of grain-boundary sliding and vacancy condensation.

The Role of Internal Stress in the High-Temperature Deformation of Copper

AbstractPolycrystalline copper of 99·99% purity has been deformed in tensile creep at a variety of strain rates and temperatures. The internal stress developed during creep deformation was measured by a modified stress-relaxation technique. The magnitude of the internal stress remains constant during steady-state deformation but the ratio of internal stress to applied stress depends critically on the creep rate and test temperature. At low creep rates the internal stress closely approaches the value of the applied stress but as the creep rate increases the magnitude of the effective stress becomes greater. The magnitude of the internal stress is related to the scale and nature of the dislocation substructure.

Creep mechanism in a Cu-5·5 at. yo A1 alloy

Abstract The high temperature creep of a Cu-5·5 at. % Al alloy was investigated by the isothernial tests technique. Two dislocation mechanisms operating in parallel were detected. In the region of lower creep rates ( ) the apparent activation energy depends linearly on stress; to the stress σ = 0 corresponds a value close to that expected for the activation enthalpy of lattice self-diffusion of copper in the alloy. The stress dependence of minimum creep rate can be described by a power function of the σn type, where n depends both on stress and temperature. The results are in good qualitative agreement with the model of non-conservative motion of jogs on screw dislocations. In the region of higher creep rates the apparent activation energy decreases with the increasing stress and is higher than that in the lower rates region.

Creep of a Cu-2·5 at. pct. Al alloy at high temperatures

The creep of a Cu-2·5 at.pct. Al alloy has been investigated by the isothermal tests technique. Two dislocation mechanisms operate in parallel in the investigated temperature region. In the region of lower minimum creep rates (\(\dot \varepsilon \)<3×10−5 sec−1) — region 1 — the activation energy of creep is close to the expected value of the activation enthalpy of the lattice self-diffusion of copper in the alloy. The stress dependence of the minimum creep rate can be described by a power function of theσ n type, where n=n1=5·8. The results for region 1 can be satisfactorily correlated with the model of nonconservative motion of jogs on screw dislocations dependent on lattice self-diffusion. If a power function of theγ m type is used to describe the minimum creep rate dependence on the stacking fault energyγ, thenm is equal to −0.66.

Cross-Linking–Effect on Physical Properties of Polymers

Many of the polymers used in composite systems and in other applications are cross-linked or thermoset polymers. How do such cross-linked polymers differ in properties from the better-understood linear or thermoplastic polymers? This review paper attempts to answer this question. The paper is written from the practical viewpoint of the experimental scientist who is using cross-linked polymers but who is not an expert on the theory of cross-linking. In spite of their intractable nature once they have been formed, and the difficulty of fabricating highly cross-linked polymers, such materials have some outstanding properties that make them ideal for many applications. The properties include (1) excellent dimensional stability and low creep rates, (2) resistance to solvents, and (3) in many cases, high heat-distortion or softening temperatures.

Dislocation Density and Dislocation Arrangement during Creep of 20% Cr–35% Ni Stainless Steels

Abstract Measurements of dislocation density and observations of dislocation structure have been carried out on two 20% Cr–35% Ni steels, one (A) solid solution and the other (B) precipitation-hardened by Ni3 (Ti, Al), both creep-tested at 700° C. The dislocations were arranged in a three-dimensional network. Subgrains were formed only in the solid solution. A linear relationship between creep stress (τ) and (dislocation density)½ (ρ½) was established for both materials. This may be considered as supporting the view that the recovery-creep model applies to materials hardened by a second phase. The τ vs. ρ½ relationship had a positive stress intercept for steel B and may be regarded as a measure of the decrease in the driving force for recovery due to the precipitates. The lower creep rate of alloy B could be entirely accounted for by this decrease in driving force. The solid-solution alloy exhibited a negative stress intercept in the τ vs ρ½ relation and this is a consequence of the increase in dislocatio...

Eventualite du glissement visqueux aux joints des grains, au cours du fluage d'eprouvettes d'uranium de purete nucleaire courante

In view of the important role played by viscous displacement at grain boundaries during creep in many metals, we have sought to investigate this phenomenon in uranium of normal nuclear purity. In the first place we showed that at a temperature near 1 2 T t deformation by viscous displacement at grain boundaries seems to be of negligible importance in comparison with intergranular slip. For this reason we went on to establish the creep conditions which are known to be favourable, in other metals, for grain boundary displacement, i.e. a temperature above 1 2 T t , low load, low creep rate, and small grain size. We showed that the phenomenon for uranium in qualitatively identical with that observed in other metals, but that in a quantitative sense its role seems distinctly less important than in other metals. This is no doubt due to the poor purity of the uranium used in these experiments.

MAR-M Alloy 509

Abstract MAR-M Alloy 509 is a vacuum melted, cobalt-base alloy having high strength, low creep rates, and resistance to oxidation and thermal shock. It is recommended for aircraft and industrial gas turbine applications. This datasheet provides information on composition, physical properties, hardness, and tensile properties as well as creep. It also includes information on high temperature performance and corrosion resistance as well as casting, heat treating, machining, and joining. Filing Code: Co-54. Producer or source: Martin Metals Division, Martin Marietta Corporation.

The effect of creep rate on the mechanism of cavity growth

Abstract The effect of creep rate on the angular distribution of cavitated grain boundaries has been investigated. It was found that at high creep rates cavities form mainly on boundaries at 45° to the stress axis while at lower creep rates cavitation occurs preferentially on boundaries making greater angles with the stress axis. It is suggested that at high creep rates cavities grow by grain boundary sliding and at low creep rates by absorbing vacancies from the neighbouring grain boundary.

The effect of nitriding upon the creep properties of some molybdenum alloys

100-h creep tests at temperatures between 950°C and 1250°C have been carried out on strip specimens of Mo-0.49% Ti, Mo-0.98% Ti and Mo-1.5% Zr in both the dispersion-hardened and recrystallised conditions. The nitrided specimens had much longer creep lives and lower creep rates for the same stress and temperature compared with the un-nitrided Mo-Ti specimens, although the Mo-Zr alloy exhibited no steady-state creep. The activation energy and stress dependence of the creep rate was evaluated for the nitrided Mo-0.49% Ti alloy, and a metallographic and fractographic study was made of the tested specimens. The results are discussed in terms of current theories of creep in dispersion-hardened alloys.

COOPER ALLOY 22W

Abstract Cooper Alloy 22W is a high strength, heat resistant casting alloy with a low creep rate. It is recommended for heat applications where stress and hot gas corrosion rate are very high. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on corrosion resistance as well as casting, heat treating, machining, joining, and surface treatment. Filing Code: SS-146. Producer or source: Cooper Alloy Corporation.

REFRACTALOY 70

Abstract REFRACTALOY 70 is a super heat and oxidation resistant alloy containing high percentages of nickel, chromium, cobalt and molybdenum. It has an exceedingly low creep rate at elevated temperatures and shows marked high temperature stability. This datasheet provides information on composition, physical properties, hardness, and tensile properties as well as creep. It also includes information on high temperature performance as well as forming, heat treating, machining, and joining. Filing Code: Co-2. Producer or source: Westinghouse Electric Corporation. Originally published July 1953, revised August 1961.

REFRACTALOY 80

Abstract REFRACTALOY 80 is a super heat and oxidation resistant alloy containing high percentages of cobalt, chromium nickel and molybdenum. It has an exceedingly low creep rate at elevated temperatures and shows marked high temperature stability. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as creep. It also includes information on high temperature performance as well as forming, heat treating, machining, and joining. Filing Code: Co-7. Producer or source: Westinghouse Electric Corporation.

Rupture and Creep Characteristics of Titanium-Stabilized Stainless Steel at 1100 to 1300 F

Abstract Rupture and creep strengths were established at 1100, 1200, and 1300 F for 18-8 + Ti steel heat-treated at 1900 and 2050 F. Several Ti/C ratios were included as a variable. Creep and rupture strengths were higher for the higher-temperature heat-treatment. The lower Ti/C ratio steel had the higher strengths for a given heat-treatment, although this effect was somewhat less than that of temperature of heat-treatment. Limited tests indicated that water-quenching prior to testing slightly improves strength at 1100 F and reduces it at 1300 F. The steels varied in susceptibility to sigma-phase development during testing. The lower Ti/C ratio steels were less susceptible than the higher. In the commercial steels tested, those steels which developed sigma phase had abnormal creep characteristics, with first-stage creep virtually absent and increasing creep rates at short-time periods and low creep rates. This indication of premature third-degree creep was false in that prolonged testing established a true secondary creep rate as predicted by the rupture tests. Use of the minimum creep rates of short duration for establishing the usual creep strengths led to values estimated to be 1000 to 3000 psi high. The creep and rupture strengths of 18-8 + Ti and 18-8 + Cb steels probably have about the same range in strengths at high temperatures. Under present specifications it is entirely possible to have very low or very high strengths for both steels.

The Creep of Wires at High Temperature

This paper describes apparatus developed to make creep tests on wires at high temperatures in a vacuum, and the results of a series of creep tests on wires of an alloy known as Konal at temperatures of from 800 to 1000°C. It is shown that Ludwik's logarithmic law connecting rate of deformation and stress holds for higher creep rates and stresses, although it certainly does not hold for the lower creep rates and stresses. In the range of temperatures tested, it is found that a linear relation best expresses the variation of the logarithm of the minimum creep rate with temperature. The method used is suggested as being readily adaptable to the making of short time creep tests, for obtaining comparative creep data in a relatively short time on materials used at high temperatures.