Interactions between creep,fatigue, and strain-aging in two refractory metal alloys

The application of low-amplitude, high-frequency fatigue vibrations during creep testing of two strain-aging refractory alloys (molybdenum-base TZC and tantalum-base T-lll) significantly reduced the creep strength of these materials. This strength reduction caused dramatic increases in both the first stage creep strain and the second stage creep rate. The magnitude of the creep rate acceleration varied directly with both frequency andA ratio (ratio of alternating to mean stress), and also varied with temperature, being greatest in the range where the strainaging phenomenon was most prominent. It was concluded that the creep rate acceleration resulted from a negative strain rate sensitivity which is associated with the strain-aging phenomenon in these materials. (A negative rate sensitivity causes flow stress to decrease with increasing strain rate, instead of increasing as in normal materials.) By combining two analytical expressions which are normally used to describe creep and strain aging behavior, an expression was developed which correctly described the influence of temperature, frequency, andA ratio on the TZC creep rate acceleration.     

Nanotechnology for copper and copper alloys

This article surveys and analyzes the literature data on nanotechnologies for copper and copper alloys. It describes the main methods used to obtain nanomaterials, including powder metallurgy, crystallization from the liquid state with a controlled rate of cooling, intensive plastic deformation, dispersion hardening, and dispersion strengthening with internal oxidation. Preference is given to nanophase materials in the discussion. __________ Translated from Metallurg, No. 8, pp. 40–46, August, 2007.     

Atmospheric corrosion of copper and some copper alloys

Abstract

Two-year atmospheric corrosion tests in an urban environment were conducted on specimens of copper, Muntz metal, and copper-zinc-aluminum alloys. X-ray diffraction and X-ray spectrographic studies were made of the films on the top surface and underside of the exposure coupons which were exposed facing south but inclined at a 45° angle to the vertical. Differences between the two surfaces are attributed to the leaching action of rain as well as the possible effect of direct sunlight. The corrosion products of copper were found to consist predominantly of Cu₂O, but also included a substance of undetermined composition, copper chloride and basic copper chloride. The zinc-containing alloys formed similar products, along with zinc sulphates, and showed considerable dezincification. A preoxidation treatment improved the tarnished appearance of the ternary alloy.

Résumé

Des essais de corrosion atmosphérique en milieu urbain ont été exécutés sur du cuivre, du métal Muntz et des alliages Cu-Zn-Al. Les échantillons étaient placés à un angle de 45°, face au sud et des mesures de diffraction aux rayons-X et de spectrographie X ont été faites sur chacune des faces. Les differences entre les deux faces d'un même échantillon peuvent étre attribuables à la pluie et peut-être aussi au solell. Les produits de corrosion du cuivre sont surtout formés de Cu₂O et également du chlorure de cuivre, du chlorure basique de cuivre et un composé non identifié sur les alliages contenant du zinc des produits semblables se sont formés avec aussi des sulfures de zinc; on y notait aussi une dézincification. Dne oxydation avant les essais améliorait l'apparence finale des alliages ternaires.     

Induced creep and creep/fatigue of a nickel-base superalloy at ambient temperatures

The stress controlled fatigue of Nimonic*115, a typical γ’-strengthened nickel-base superalloy, was studied at ambient temperature, using a trapezoidal wave form at 1 Hz, with stresses chosen to produce failure in the lO⁴ to lO⁴ cycle range. In tests with maximum stress greater than the proportional limit, most of the fatigue damage occurs within the first few test cycles. Much of this strain is accumulated under static load and is therefore identified as creep strain. Transmission electron microscopy shows that these creep strains occur in slip bands which disrupt the ordered γ’ precipitates. Strain is found to follow a logarithmic time dependence, which suggests a low activation energy mechanism.     

Microstructure and creep properties of dispersion-strengthened aluminum alloys

The mechanical properties of dispersion-strengthened aluminum alloys, with various dispersoid types, volume fractions, and grain structures, were investigated in conjunction with systematic microstructural examinations. New theoretical concepts, based on thermally activated dislocation detachment from dispersoid particles, were used to analyze the creep behavior. A particularly strong dispersoid-dislocation interaction was identified as reason for the excellent creep properties of carbide dispersion-strengthened aluminum. Oxide particles (Al₂O₃,MgO) seem to exert a weaker interaction force and are therefore less efficient strengtheners. Although fine crystalline in the as-extruded condition, all alloys are remarkably resistant against diffusional creep. It is demonstrated that this behavior can be consistently understood by extending the concept developed for the interaction between bulk dislocations and dispersoids to grain boundary dislocations. Formerly Project Group Leader, Max-Planck-Institut fur Metallforschung     

Direct creep curve assessment and optimization of creep equations for high temperature alloys

The conventional use of a time temperature parameter method for the basic assessment of a multi-heat creep data set facilitates the establishment of a creep equation for a material type. However, the parameter can introduce some distortion to the creep data and hence the creep equation. To avoid this disadvantage, a new method for the direct creep curve assessment of multi-heat data was developed. Systematic deviations in creep behaviour of individual test materials in respect to the mean behaviour of the multi-heat data set are reduced and the creep curves for individual stresses are transformed to mid creep curves of a limited number of stress classes. With this method large multi-heat creep data sets of Alloy 100 and Alloy 738 LC could be reduced to mean creep curves. On this basis, existing parameter based creep equations were examined and optimized, if necessary. Further, the confidence limits of these equations were determined. With a stress modification the creep behaviour of a material similar in structure can be described. In finite element analyses some verification experiments which simulate typical loading conditions of components could be successfully recalculated with the optimized creep equations.     

Mechanisms of creep deformation in Mg-Sc-based alloys

Binary Mg-Sc alloys show only a very weak age-hardening response due to the low diffusivity of Sc in Mg and exhibit inferior creep resistance compared to WE alloys. The addition of a small amount of Mn (<1.5 wt pct) improves their creep behavior markedly, decreasing the minimum creep rates by up to about two orders of magnitude at temperatures above 300 °C compared to WE alloys. This is due to the precipitation of fine Mn₂Sc phase basal discs, which are very effective obstacles in controlling creep at temperatures at which cross-slip of basal dislocations and nonbasal slip are the rate controlling mechanisms. The addition of Ce improves the creep resistance even more due to the effect of the grain boundary eutectic. The effect of Mn₂Sc discs can still be seen in alloys with a low Sc content (∼1 wt pct) and with the addition of rare earth (RE) elements (Gd, Y, Ce ∼4 wt pct). Very thin hexagonal plates containing RE and Mn, which lie parallel to the basal plane of the Mg matrix, augment the effect of the Mn₂Sc precipitates at elevated temperatures (∼250 °C). The triangular arrangement of prismatic plates of metastable or stable phases of Mg-RE systems controls effectively the motion of basal dislocations during the creep of these alloys at elevated or high temperatures. The combined control of basal slip, cross-slip of basal dislocations, and of nonbasal slip in low Sc content alloys ensures minimum creep rates of about one order of magnitude lower than those observed in WE alloys, both at elevated and high temperatures. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee.     

Solid solution creep behavior of Sn-xBi alloys

This study details the steady-state creep properties of Sn-1 wt pct Bi, Sn-2 wt pct Bi, and Sn-5 wt pct Bi as a function of stress and temperature. All data, including previous work on pure Sn, are described by the following empirical equation:

Corrosion and creep resistance of some dental amalgam alloys

Corrosion behaviour, creep values and phase transformation of conventional, admixed, unicompositional and two palladium-enriched amalgam alloys were investigated. Weight-loss method and solution analysis for dissolved metal ions were used. Creep values were determined after different immersion periods. X-ray diffraction was carried out to follow up phase transformation after different immersion periods and/or aging in dry air at 37 degrees up to one year. The results have shown that addition of palladium has improved corrosion and creep resistance and reduced gamma 1 to beta 1 phase transformation, and that gamma 2 did not appear again in high copper or palladium-enriched amalgam up to one year.     

Hardness and creep of Bi-Sb alloys under spherical indentation

The deformation behavior of Bi, 10 pct Sb-Bi, 25 pct Sb-Bi, 50 pct Sb-Bi and Sb has been studied under spherical indentation for loads from 50 to 150 kg applied up to 10,000 s from 0.5 to 0.7 of the melting temperature. The softening parameterB and the apparent “activation energy”B’ were calculated from the short-time hardness data, and the true activation energyQc and the stress coefficient for creep α from the long-time indentation data. The α values were found to be greater than five indicating a breakdown of the power law in the range of loads and temperatures utilized. The values ofQ _c increased slightly with temperature whereas those of α increased with load and decreased with temperature, the observations indicating the effect of stress in the breakdown region.     

Comment on harper-dorn creep in Al alloys

Metallurgical and Materials Transactions A -     

Steady state creep in two Ni-Al alloys

Creep of two Ni-AI alloys containing 4.8 and 7.0 wt pct Al was studied in the temperature range 873 to 1073 K and stress range 30 to 400 MPa. The former alloy represents the solid solution of aluminum in nickel, the latter a solid solution strengthened by NI3AI particles. As to its creep behavior the solid solution alloy belongs to the Class n of solid solu-tions,i.e. the creep controlling mechanism is the same as in pure nickel. From the analy-sis of an effective stress dependence of steady state creep rate it follows that the mo-tion of jogged screw dislocations can be considered as the most probable creep control-ling mechanism. The apparent activation energy of creep in the two phase alloy increases with tempera-ture. This effect is caused by changes in the volume fraction of second phase particles and by the onset of climb around particles at high temperatures. At lower temperatures particles are cut by dislocation pairs.     

Low-Cycle fatigue of dispersion-strengthened copper

The cyclic deformation behavior of a dispersion-strengthened copper alloy, GlidCop Al-15, has been studied at plastic strain amplitudes in the range 0.1 pct ≤Δε _p/2 ≤ 0.8 pct. Compared to pure polycrystalline copper, the dispersion-strengthened material exhibits a relatively stable cyclic response as a consequence of the dislocation substructures inherited from prior processing and stabilized by the A1₂O₃ particles. These dislocation structures remain largely unaltered during the course of deformation; hence, they do not reveal any of the features classically associated with copper tested in fatigue. At low amplitudes, the fatigue lifetimes of the dispersion-strengthened copper and the base alloy are similar; however, the former is more susceptible to cracking at stress concentrations because of its substantially greater strength. This similarity in fatigue lifetimes is a consequence of the dispersal of both deformation and damage accumulation by the fine grain size and dislocation/particle interactions in the GlidCop alloy. The operation of these mechanisms is reflected in the fine surface slip markings and rough fracture surface features for this material. Formerly Graduate Research Assistant, University of California, Davis, CA     

Observations of room-temperature creep recovery in titanium alloys

Conventional α(hcp) and α(hcp)/β(bcc) titanium alloys exhibit significant primary creep strains at room temperature and at stresses well below their macroscopic yield strength. It has been previously reported in various materials systems that repeated unloading during primary creep testing may either accelerate or retard the accumulation of creep strains. These effects have been demonstrated to depend on both microstructure and the applied stress. This article demonstrates that significant room-temperature recovery occurs in technologically relevant titanium alloys. These recovery mechanisms are manifested as a dramatic increase in creep rates (by several orders of magnitude) upon the introduction of individual unloading events, ranging from 1 minute to 365 days, during primary creep tests. Significant increases in both creep rate and the total accumulated creep strain were observed in polycrystalline single α-phase Ti-6Al, polycrystalline α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and individual α/β colonies of Ti-6242. Based on transmission electron microscopy (TEM) studies of the active deformation mechanisms, it is proposed that the presence of significant stress concentrations within the α phase of these materials, in the form of dislocation pileups, is a prerequisite for significant room-temperature recovery. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Observations of room-temperature creep recovery in titanium alloys

Conventional α(hcp) and α(hcp)/β(bcc) titanium alloys exhibit significant primary creep strains at room temperature and at stresses well below their macroscopic yield strength. It has been previously reported in various materials systems that repeated unloading during primary creep testing may either accelerate or retard the accumulation of creep strains. These effects have been demonstrated to depend on both microstructure and the applied stress. This article demonstrates that significant room-temperature recovery occurs in technologically relevant titanium alloys. These recovery mechanisms are manifested as a dramatic increase in creep rates (by several orders of magnitude) upon the introduction of individual unloading events, ranging from 1 minute to 365 days, during primary creep tests. Significant increases in both creep rate and the total accumulated creep strain were observed in polycrystalline single α-phase Ti-6Al, polycrystalline α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and individual α/β colonies of Ti-6242. Based on transmission electron microscopy (TEM) studies of the active deformation mechanisms, it is proposed that the presence of significant stress concentrations within the α phase of these materials, in the form of dislocation pileups, is a prerequisite for significant room-temperature recovery. M.F. SAVAGE, formerly with the Department of Materials Science and Engineering, The Ohio State University Columbus, OH. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Diffusional creep and creep-degradation in dispersion-strengthened Ni-Cr base alloys

Dispersoid-free regions were observed in the dispersion-strengthened alloy TD-NiCr (Ni-20 Cr-2 ThO₂) after, slow strain rate testing (stress rupture, creep, and fatigue) in air from 1145 to 1590 K. Formation of the dispersoid-free regions appears to be the result of diffusional creep. The net effect of creep in TD-NiCr is the degradation of the alloy to a duplex microstructure. Creep degradation of TD-NiCr is further enhanced by the formation of voids and intergranular oxidation in the dispersoid-free bands. Void formation was observed after as litte as 0.13 pct creep deformation at 1255 K. The dispersoid-free regions apparently provide sites for void formation and oxide growth since the strength and oxidation resistance of Ni-20 Cr is much less than Ni-20 Cr-2 ThO₂. This localized oxidation does not appear to reduce the static load bearing capacity of TD-NiCr since long stress-rupture lives were observed even with heavily oxidized microstructures, but this oxidation does significantly reduce the ductility and impact resistance of the material. Dispersoid-free bands and voids also were observed in two other dispersion-strengthened alloys, TD-NiCrAl (Ni-16Cr-4 Al-2 ThO₂) and IN-853 (Ni-20 Cr-2.5 Ti-1.5 Al-1.3 Y₂O₃). Thus, it appears that diffusional creep is characteristic of dispersion-strengthened alloys and can play a major role in the creep degradation of these materials.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190 °C and 260 °C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases.