Effects of Alloying Elements on Creep Properties of Mg-Gd-Zr Alloys

The minimum creep rate and microstructures of aged samples of Mg-Gd-Zr alloys, with and without alloying additions of Zn and/or Y, have been investigated in the present work. The creep tests were performed at 523 K (250 °C) and under 80 to 120 MPa, and the microstructures before and after creep tests were characterized using scanning electron microscopy, transmission electron microscopy, and the high-angle annular dark-field imaging technique. It is found that dislocation creep predominates in the steady-state creep stage for all alloys. The Mg-2.5Gd-0.1Zr (at. pct) alloy, strengthened by the β′ precipitates, has minimum creep rates in the range 1.0 × 10^?8 to 3.8 × 10^?8 s^?1 under 80 to 120 MPa. The addition of 1.0 at. pct Zn to the Mg-2.5Gd-0.1Zr alloy reduces the 0.2 pct proof strength and increases the minimum creep rate, resulting from the formation of γ′ basal plates at the expense of β′ precipitates. The replacement of 1.0 at. pct Gd by Y in the Mg-2.5Gd-1.0Zn-0.1Zr alloy leads to a substantial reduction in the minimum creep rate, even though it does not cause much change to the 0.2 pct proof strength. The reduced minimum creep rate is attributed to a much lower diffusivity of Y atoms than Gd in the solid magnesium matrix. An increase in the Gd content from Mg-1.5Gd-1.0Y-1.0Zn-0.1Zr to Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr leads to a denser distribution of precipitates, a higher 0.2 pct proof strength, and a further reduction in the minimum creep rate.     

The creep behavior of W-5 re

Polycrystalline W-5 wt pct Re was creep-tested in tension from 1500° to 1900°C at stresses from 2500 to 10,000 psi in a vacuum of 10^?8 torr. The steady-state strain rate was directly proportional to stress to the 5.5 power, and the apparent activation energy for creep was 104 kcal per mole. Dislocation substructure that developed during high-temperature deformation was studied by transmission electron microscopy. The total dislocation density was dependent on stress to the 2.1 power and was insensitive to temperature and strain. No subgrains were found in creep tested specimens. The rate-controlling deformation mechanism was ascribed to dislocation climb where the governing diffusion process was dislocation core diffusion. Comparison of creep data for tungsten, W-5 wt pct Re, and W-25 wt pct Re showed that W-5 wt pct Re alloy has significantly better creep properties than the other two materials.     

Tensile properties and creep behavior of a new Ti-Al-Nb intermetallic alloy with an O+<Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript> microstructure

A new Ti-Al-Nb alloy with a composition of Ti-27.5Al-13Nb (at. pct) was proposed. The density of this alloy was 4.7 g/cm³, which is about 13 pct lower than that for O+B2 alloys. After hot processing, the alloy was heat treated under two conditions: directly aged at 850 °C (DA treatment), or cooled from above the β-transus temperature with a cooling rate of 3 °C/min and then aged at 850 °C (BCA treatment). Under the present heat-treatment conditions, the phase constitution was primarily O+α ₂. A very fine Widmanstätten microstructure was obtained after the DA treatment, while a microstructure with coarse O plates was obtained after the BCA treatment. The tensile properties were investigated at 20 °C to 950 °C, and the creep behavior was investigated at 650 °C to 750 °C/90 to 380 MPa. The elongation to fracture at room temperature for the DA-treated tensile specimen was as high as 2.6 pct, despite the high Al content in this alloy. In comparison with the O+B2 ternary alloys, this alloy showed higher specific proof stress at temperatures over 800 °C and higher creep strength. The stress exponent and the apparent activation energy for creep were calculated. The fracture mechanism was discussed.     

Elevated temperature creep and fracture properties of the 62Cu-35Au-3Ni braze alloy

The Cu-Au-Ni braze alloys are used for metal/ceramic brazes in electronic assemblies because of their good wetting characteristics and low vapor pressure. We have studied the tensile creep properties of annealed 62Cu-35Au-3Ni alloy over the temperature range 250 °C to 750 °C. Two power-law equations have been developed for the minimum creep rate as a function of true stress and temperature. At the highest temperatures studied (650 °C and 750 °C), the minimum creep rate is well described with a stress exponent of 3.0, which can be rationalized in the context of Class I solid solution strengthening. The inverted shape of the creep curves observed at these temperatures is also consistent with Class I alloy behavior. At lower temperatures, power-law creep is well described with a stress exponent of 7.5, and normal three-stage creep curves are observed. Intergranular creep damage, along with minimum values of strain to fracture, is most apparent at 450 °C and 550 °C. The lower stress exponent in the Class I alloy regime helps to increase the strain to fracture at higher temperatures (650 °C and 750 °C). The minimum creep rate behavior of the 62Cu-35Au-3Ni alloy is also compared with those of the 74.2Cu-25. 8Au alloy and pure Cu. This comparison indicates that the 62Cu-35Au-3Ni has considerably higher creep strength than pure Cu. This fact suggests that the 62Cu-35Au-3Ni braze alloy can be used in low mismatch metal-to-ceramic braze joints such as Mo to metallized alumina ceramic with few problems. However, careful joint design may be essential for the use of this alloy in high thermal mismatch metal-to-ceramic braze joints.     

The influence of matrix microstructure and particle reinforcement on the creep behavior of 2219 aluminum

The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 °C to 250 °C. At 150 °C, reinforcement led to an improvement in creep resistance, while at 250 °C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 °C and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 °C can be attributed to diffusional relaxation processes at the higher temperature.     

Creep and fracture of a Laves phase strengthened ferritic alloy

The microstructural changes which occurred during creep were examined in a carbon-free ferritic alloy containing 1 at. pct Ta and 7 at. pct Cr. The alloy derived its creep resistance from a uniform dispersion of almost spherical particles of the Laves phase Fe₂Ta in a bcc matrix. There was a lath-like substructure in this alloy, with dislocation tangles and subgrains within the laths. Partial recovery during creep led to the formation of a regular subgrain structure. The Laves phase particles prevented total recovery by pinning individual dislocations and subgrain boundaries. The apparent activation energy for creep, the estimated stress sensitivity of minimum creep rate, and the substructural features observed in crept specimens suggested that creep deformation of the alloy occurred by two or more independent processes. Fracture under creep conditions was initiated by void formation at precipitate-matrix interfaces. Intergranular void formation played an important role in the fracture of creep specimens tested at 1200°F (649°C), but not at lower temperatures.     

Low-temperature creep of a carburized steel

The low-temperature creep behavior of carburized 4320 steel with retained austenite contents of 35 and 14 pct and two uncarburized 4320 steels was investigated. The temperature range in the experiments was from 70 °C to 195 °C. The creep rate obeyed a logarithmic law when the stress level was below or near the proportional limit. A kinetic model is presented which de-scribes the low-temperature creep behavior of this steel under different stress and temperature conditions. The techniques for determining the constants in the model are given. Formerly Visiting Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois     

The compressive creep behavior of a Pu-1 wt pct Ga δ-stabilized alloy at elevated temperatures

Constant stress compression creep tests were performed in vacuum on a high-purity Pu-1 wt pct Ga ö-stabilized alloy over the temperature range from 252° to 382°C for stresses from 700 to 2500 psi. Although the primary creep behavior could not be correlated by established techniques, the creep rates developed after true creep strains of about 0.15 provided good agreement with the temperature and stress dependence of creep for pure metals and dilute alloys. A power stress law for steady-state creep of the alloy was found forδ/E values less than 5 x 10′4, with the stress exponent being 4.0, and it was concluded that the alloy exhibits Class I solid solution behavior. For higher stress, exponential stress dependence was observed. The true activation energy for creep was found to be 38,900 cal per mole which is in good agreement with the value for self-diffus ion of plutonium in the ô-stabilized alloy. The primary creep behavior could be divided into three types: 1) at low strain rates, the creep rate gradually increases to a nearly steady-state; 2) at intermediate strain rates, the creep rate first decreases and then increases to steady-state; and 3) at high strain rates, the creep rate decreases gradually to steady-state. It was concluded that the failure of established creep correlations for primary creep of Pu-1 wt pct Ga was the result of some temperature-dependent component of creep structure, possibly resulting from radiation damage byα particles.     

Creep and fracture of a laves phase strengthened ferritic alloy

The microstructural changes which occurred during creep were examined in a carbon-free ferritic alloy containing 1 at. pct Ta and 7 at. pct Cr. The alloy derived its creep resistance from a uniform dispersion of almost spherical particles of the Laves phase Fe₂Ta in a bee matrix. There was a lath-like substructure in this alloy, with dislocation tangles and subgrains within the laths. Partial recovery during creep led to the formation of a regular subgrain structure. The Laves phase particles prevented total recovery by pinning individual dislocations and subgrain boundaries. The apparent activation energy for creep, the estimated stress sensitivity of minimum creep rate, and the sub-structural features observed in crept specimens suggested that creep deformation of the alloy occurred by two or more independent processes. Fracture under creep conditions was initiated by void formation at precipitate-matrix interfaces. Intergranular void formation played an important role in the fracture of creep specimens tested at 1200°F (649°C), but not at lower temperatures. M. Dilip Bhandarkar was formerly associated.     

The effect of low gold concentrations on the creep of eutectic tin-lead joints

The effects of low Au concentrations on the creep properties of a eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90 °C, 0, 0.2, and 1.0 wt pct Au at 65°C, and 0.2 wt pct Au at 25 °C. In the absence of Au, the creep of finegrained eutectic Sn/Pb is dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.     

Unidirectionally solidified Ti−TiB and Ti−Ti5Si3 eutectic composites

Induction melting and electron beam melting techniques were employed in the production of unidirectionally solidified eutectic composites of Ti-1.7 wt pct B and Ti-8.5 wt pct Si. The grown eutectics were reinforced by 7.7 volume pct of TiB fibers and 31 volume pct of Ti₅Si₃ fibers respectively. Controlled dendritic solidification of a hypereutectic composition of Ti-12 wt pct Si was also accomplished. Tensile, compressive, creep, and stress rupture specimens were cut from the eutectic composites and tested with reinforcing fibers parallel to the load axis. Ti?TiB eutectic was found to have less than the critical volume fraction of fibers necessary for reinforcement, while Ti?Ti₅Si₃ composite attained a compressive yield strength of 275,000 psi and a compressive Young's modulus of 30×10⁸ psi after heat treatment. The 500 and 4000 hr stress rupture properties of Ti?Si eutectic were superior to commercial titanium alloys at 1000° and 1200°F. The minimum creep rate of Ti?Ti₅Si₃ eutectic composite was lower than all other titanium alloys at 1000°F. Tensile, compressive, and creep properties of the Ti-8.5 wt pct Si eutectic are discussed in terms of the current theories of composite behavior.     

High-Temperature Creep Degradation of the AM1/NiAlPt/EBPVD YSZ System

The failure mechanisms of a NiAlPt/electron beam physical vapor deposition yttria-stabilized-zirconia thermal barrier coating system deposited on the AM1 single crystalline substrate have been investigated under pure creep conditions in the temperature range from 1273 K to 1373 K (1000 °C to 1100 °C) and for durations up to 1000 hours. Doubly tapered specimens were used allowing for the analysis of different stress states and different accumulated viscoplastic strains for a given creep condition. Under such experiments, two kinds of damage mechanisms were observed. Under low applied stress conditions (i.e., long creep tests), microcracking is localized in the vicinity of the thermally grown oxide (TGO). Under high applied stress conditions, an unconventional failure mechanism at the substrate/bond coat interface is observed because of large creep strains and fast creep deformation, hence leading to a limited TGO growth. This unconventional failure mechanism is observed although the interfacial bond coat/top coat TGO thickening is accelerated by the mechanical applied stress beyond a given stress threshold.     

The effect of predeformation on the creep and stress rupture of an oxide dispersion strengthened mechanical alloy

The effect of higher strain rate predeformation on creep behavior and stress rupture life of the oxide dispersion strengthened nickel-base alloy MA 754 was studied. Both the predeformation and creep testing were conducted at 760 °C. It was found that the minimum creep rate decreased as the amount of prestrain increased and was a factor of two lower at 1.2 pct prestrain. Predeformation also shortened the duration of primary creep. Transmission electron microscopy revealed dislocations being emitted from particle-matrix interfaces after prestraining and an increase in dislocation density with increasing prestrain. These observations are discussed with respect to the mechanical results. Formerly a Graduate Student at Columbia University     

Creep deformation in near-γ TiAl: II. influence of carbon on creep deformation in Ti-48Al-1V-0.3C

The influence of interstitial strengthening and microstructure on creep deformation has been examined in the near-γ TiAl alloy Ti-48Al-lV-0.3C. Creep studies were conducted under constant load in air at 815 °C in the stress range of 50 to 200 MPa. Significant improvement in creep resistance was observed in this alloy compared with a similar alloy (Ti-49Al-lV) containing low levels of carbon (0.07 at. pct). The degree of strengthening resulting from the addition of carbon was found to be dependent on microstructure. At 815 °C and 150 MPa, the addition of carbon reduced the minimum creep rate by a factor of approximately 20 in the equiaxedy and duplex microstructures and by a factor of 3 in the fully lamellar microstructures. Carbide precipitation occurred in this alloy when aged in the temperature range of 700 °C to 950 °C. The addition of carbon leads to a decrease in the stress exponent from 4 to 3 in the duplex and equiaxedy microstructures and the inhibition of sub-boundary formation in the duplex microstructure. This suggests that solute/dislocation interaction mechanisms, rather than a direct effect of carbide precipitates, are responsible for the significant increase in creep resistance observed in this alloy. Brian D. Worth, formerly with the Department of Materials Science and Engineering, The University of Michigan.     

Phase transformations in Ti-6Al-4V-xH alloys

Microstructures, phases, and phase transformations in Ti-6Al-4V alloy specimens containing 0, 10, 20, and 30 at. pct hydrogen were investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and microhardness testing. Alloying with hydrogen was achieved by holding the specimens in a pure hydrogen atmosphere of different pressures at 780 °C for 24 hours. The phases present in the temperature range of 20 °C to 1000 °C were determined by microstructural characterization of the specimens quenched from different temperatures. Increasing the hydrogen addition from 0 to 30 at. pct lowered the beta-transus temperature of the alloy from 1005 °C to 815 °C, significantly slowed down the kinetics of the beta-to-alpha transformation, and led to formation of an orthorhombic martensite instead of the hexagonal martensite found in quenched specimens containing 0 pct H. A hydride phase was detected in specimens containing 20 and 30 at. pct hydrogen. The time-temperature-transformation (TTT) diagrams for beta-phase decomposition were determined at different hydrogen concentrations. The nose temperature for the beginning of the transformation decreased from 725 °C to 580 °C, and the nose time increased from 12 seconds to 42 minutes when the hydrogen concentration was increased from 0 to 30 at. pct.     

Creep of fine wires of W-ThO2 alloys

The increasing interest in the application of fine wires as high strength structural components,e.g., in high temperature composite materials, makes an understanding of the differences between the creep behavior of large specimens and that of fine wires desirable. In this investigation, the creep properties of fine wires of recrystallized W-1 wt pct ThO₂ were studied over the temperature range between 1800° to 2750°C. In tungsten wires in which the dispersion of fine thoria particles stabilized a fine grained structure, the stress dependence of the creep rate varies with test temperature and stress. For test temperatures below 2500°C, a stress dependence ofn ~ 5 was found, indicative of creep deformation due to dislocation climb processes, while for temperatures above 2500°C and low stresses, values ofn < 2 were obtained, indicative of deformation by grain boundary sliding and diffusional creep processes. In wires which recrystallized to a large-grained structure, having a large aspect ratio, a high stress dependence of 15 was found when tested at 1800°C.     

Small Punch Creep Studies for Optimization of Nitrogen Content in 316LN SS for Enhanced Creep Resistance

Small punch creep (SPC) studies have been carried out to evaluate the creep properties of 316LN stainless steel (SS) at 923 K (650 °C) at various stress levels. The results have been compared with uniaxial creep rupture data obtained from conventional creep tests. The minimum deflection rate was found to obey Norton power law. SPC rupture life was correlated with uniaxial creep rupture life. The influence of nitrogen content on the creep rupture properties of 316LN SS was investigated in the range of 0.07 to 0.14 wt pct. SPC rupture life increased and the minimum deflection rate decreased with the increase in nitrogen content. The trends were found to be in agreement with the results obtained from uniaxial creep rupture tests. These studies have established that SPC is a fast and reliable technique to screen creep properties of different experimental heats of materials for optimizing the chemical composition for developing creep-resistant materials.     

High temperature plastic deformation of two V-Ga alloys with A15 structure

A study of high temperature plastic deformation has been undertaken on alloys of V-18 at. pct Ga and V-23 at. pct Ga. The materials were prepared by arc melting, homogenizing, and transformation annealing, resulting in polycrystalline A15 structure. Through compression testing and load-relaxation testing, plastic deformation has been studied over a strain rate range from 10^-6 to 10^-2/s and a temperature range from 1000 to 1300 °C. Flow stress decreases with increased temperature and decreased strain rate. Stress-strain rate relations may be fitted with a power law creep expression. The flow stress is influenced by the length of the 1150 °C transformation anneal; longer anneals result in a decrease in flow stresses projected from load relaxation testing. Analysis of compressive yield strength data places the activation energy for A15 V-Ga creep roughly in the 400 kJ/mol range.     

Influence of long-term aging at 520 °C and 560 °C and the superimposed creep stress on the microstructure of 1.25Cr-0.5Mo steel

In order to understand the influence of high-temperature aging effects and those of the superimposed creep stress on the microstructural variations in a 1.25Cr-0.5Mo steel, the shoulder as well as gage portions of specimens subjected to stress-rupture tests at 520 °C and 560 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the microstructure of the steel consists of 90 pct ferrite and 10 pct bainite, and M₃C is the only carbide present in bainite and at a few ferrite grain boundaries. On aging at 520 °C for 5442 hours, Cr₂N precipitates in a fibrous form at ferrite-bainite interfaces, and the creep stress has enhanced this mode of precipitation. On holding for 13,928 hours at 520 °C, fibrous carbide is still present but its composition has changed to Mo₂C, while the superimposed creep stress has promoted the precipitation of Mo₂C needles with fine globular precipitates of M₂₃C₆. Aging at 560 °C for 1854 or 10,338 hours has resulted in the precipitation of longer Mo₂C needles and ellipsoidal M₂₃C₆ carbide precipitation; the superimposed creep stress has resulted in a more dense precipitation of shorter needles in both cases. There is some recovery of bainitic ferrite at 560 °C, though the cementite coarsening is negligible.     

The structure of tempered martensite and its susceptibility to hydrogen stress cracking

A series of 4130 steels modified with 0.50 pct Mo and 0.75 pct Mo were tempered at temperatures between 300 and 700 °C for one hour. The changes in the carbide dispersion and matrix substructure produced by tempering were measured by transmission electron microscopy. These measurements were correlated with resistance to hydrogen stress cracking produced by cathodic charging of specimens in three-point bending. Scanning electron microscopy showed that specimens tempered between 300 and 500 °C failed by intergranular cracking while those tempered at higher temperatures failed by a transgranular fracture mode. Auger electron spectroscopy showed that the intergranular fracture was associated with hydrogen interaction with P segregation and carbide formation at prior austenite grain boundaries. Transgranular cracking was initiated at inclusion particles from which cracks propagated to produce flat fracture zones extending over several prior austenite grains. The 4130 steels modified with higher Mo content resisted tempering and showed better hydrogen stress cracking resistance than did the unmodified 4130 steel. The transition in fracture mode is attributed to a decohesion mechanism in the low temperature tempered samples and a pressure mechanism in the highly tempered samples.