Creep and tensile properties of several oxide dispersion strengthened nickel base alloys

The room temperature and 1365 K tensile properties and 1365 K tensile creep properties at low strain rates were measured for several oxide dispersion strengthened (ODS) alloys. The alloys examined included ODS Ni, ODS Ni-20Cr and ODS Ni-16Cr-4J5Al. Metallography of creep tested, large grain size ODS alloys indicated that creep of these alloys is an inhomogeneous process. All alloys appear to possess a threshold stress for creep. It is believed that the threshold stress is associated with diffusional creep in the large grain size ODS alloys and normal dislocation motion in perfect single crystalline ODS alloys. Threshold stresses for large grain size ODS Ni-20Cr and Ni-16Cr-4J5A1 type alloys are dependent on the grain aspect ratio. Because of the deleterious effect of prior creep on room temperature mechanical properties of large grain size ODS alloys, it is speculated that the threshold stress may be the design-limiting creep strength property.     

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.     

Abnormal grain growth behavior of an

To obtain a superior high temperature creep strength, the transformation of fine-grained structure to large elongated grains by abnormal grain growth is an important process for oxide dispersion strengthened superalloys. The present study investigated the abnormal grain growth behavior of TMO-2, an experimental alloy possessing higher creep strength than existing ODS alloys. It was found that abnormal grain growth was triggered by oxide dispersoid coarsening rather than γ^′ dissolution. Preanneal increased the grain growth temperature and decreased the grain growth rate in the same manner as low-γ^′ or γ^′-free ODS alloys. However, with regard to stored energy, far fewer dislocations were found, and grain growth was considered to be driven predominantly by surface energy. The allowance of normal grain growth during preanneal or hot work to preserve abnormal grain growth ability was investigated and correlated with the amount of softening.     

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     

New aspects on the superplasticity of fine-grained 7475 aluminum alloys

Thermomechanical processes were developed which give fine grain sizes of 6 and 8 μm in the 7475 Al alloy. Superplastic properties of this material were evaluated in the temperature range of 400 °C to 545 °C over the strain-rate range of 2.8 x 10^-4 to 2.8 X 10^-2 s^-1. The maximum ductility exhibited by the alloy was approximately 2000 pct, and optimum superplasticity was achieved at a strain rate of 2.8 X 10^-3 s^-1 which is higher by an order of magnitude than other 7475 Al alloys. This result is attributed to the presence of fine dispersoids which maintain the fine grain size at high homologous temperatures. The flow stress and strain-rate sensitivity strongly depend on the grain size. The superplastic 7475 Al alloy has strain-rate sensitivities of 0.67 (6 μm) and 0.5 (13 μm) and an activation energy which is similar to the one for grain boundary diffusion of aluminum. Microstructural investigation after superplastic tests revealed zones free of dispersoid particles at grain boundaries primarily normal to the tensile direction. These dispersoidfree zones (DFZs) appear even after 100 pct elongation and are occasionally as large as 5 μm across. This result demonstrates the importance of diffusional flow in superplastic deformation of the fine-grained 7475 Al alloy especially at low elongations.     

Air embrittlement of a cobalt-base superalloy

The stress-rupture and tensile properties of a cast cobalt base superalloy, MM509, are compared after high temperature exposure in air and vacuum. A loss in tensile ductility after air exposure is observed over the entire temperature range from room temperature to 1000 °C with the effect being most severe at 400 ° to 600 °C. This appears to be related to grain boundary oxygen penetration and is compared and contrasted with similar observations in nickel base alloys. The cobalt base alloy has high intrinsic ductility at high temperatures and, even after air exposure, there is no loss in creep life associated with the embrittlement. It is argued that in this alloy the critical strain for fracture of embrittled grain boundaries is much higher than that for high strength nickel base superalloys.     

Creep and rupture of an ods alloy with high stress rupture ductility

The creep and stress rupture properties of an oxide (Y₂O₃) dispersion strengthened nickel-base alloy, which also is strengthened by γ′ precipitates, was studied at 760 °C and 1093 °C. At both temperatures the alloy YDNiCrAl exhibits unusually high stress rupture ductility as measured by both elongation and reduction in area. Failure was transgranular, and different modes of failure were observed including crystallographic fracture at intermediate temperatures and tearing or necking almost to a chisel point at higher temperatures. While the rupture ductility was high, the creep strength of the alloy was low relative to conventional γ′ strengthened superalloys in the intermediate temperature range and to ODS alloys in the higher temperature range. These findings are discussed with respect to the alloy composition; the strengthening oxide phases, which are inhomogeneously dispersed; the grain morphology, which is coarse and elongated and exhibits many included grains; and the second phase inclusion particles occurring at grain boundaries and in the matrix. The creep properties, in particular the high stress dependencies and high creep activation energies measured, are discussed with respect to the resisting stress model of creep in particle strengthened alloys. RICHARD M. ARONS, formerly a Graduate Student at Columbia University     

The assessment of creep damage in certain aged alloys based on Al-Cu-Mg system

The mechanism of creep failure was investigated for three different test alloys namely Al-4.0%Cu-0.3%Mg, Al-4.0%Cu-0.3%Mg-0.4%Cd and Al-4.0%Cu-0.3%Mg-0.4%Ag at 125 and 150°C in the fully hardened condition. The room temperature tensile properties of these alloys increased in the order of ternary alloy, Cd-containing alloy and Ag-containing alloy. The creep performance of these alloys also improved in the similar order. The present studies revealed the dominance of intercrystalline creep failures in all the alloys at both the test temperatures. The grain boundary microstructures contained precipitates with narrow Precipitate Free Zones (PFZ’s) with large difference in particle spacings. Ag-containing alloy recorded minimum grain boundary particle spacing as compared to that of ternary and Cd-containing alloys. The creep damage assessment in terms of damage distribution in the gauge portion showed maximum damage in the Ag-containing alloy as compared to other two alloys. In all these alloys, failures occurred by the coalescence of several cracks and the negotiations of few boundary junctions rather than the propagation of single major crack.     

Effect of carbon addition on the strength and creep resistance of FeAl alloys

We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100°C and 1300°C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacanies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300°C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300°C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700°C, and yield strength up to 800°C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys. The work was carried out when the authors were with Chrysalis Technologies Inc., Richmond, VA. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Role of grain boundary segregation in diffusional creep

The high temperature deformation of polycrystalline materials by the stress directed flow of vacancies is now a well established creep mechanism which operates in two temperature regimes: high temperature, or Nabarro-Herring creep, in which lattice diffusion is rate determining, and low temperature, or Coble creep, in which grain boundary diffusion predominates. Basic studies have been conducted mostly with pure metals for which there exists in general a good correspondence between predicted and observed behavior. Multicomponent engineering alloys will normally experience, as part of their processing history or service lives, the segregation enrichment of interfaces such as grain boundaries by species present in solid solution. The aim of this paper is to evaluate the experimental information and to explore the manner in which this segregation affects the principal forms of diffusional creep. Cases of retarded Herring-Nabarro creep are analyzed in terms of the efficacy of grain boundaries as sources and sinks for vacancies: strongly bound segregant atoms at grain boundaries affect the mobility of defects and hence control the operation of vacancy sources. Recently, observations have been made on the effect of strongly segregating solutes on grain boundary diffusivity. Such behavior influences Coble creep rates, producing in general a retardation. Here we assess the magnitude of the effect induced by various surface active species on grain boundary diffusivity and consequently on Coble creep; predictions show that in general, small amounts of highly surface active impurities induce a remarkable inhibition of this form of creep.     

Deformation mechanisms of a rapidly solidified Al-8.8Fe-3.7Ce alloy

The mechanisms of deformation of a rapidly solidified and compacted Al-8.8Fe-3.7Ce (wt pct) alloy were investigated in the stress range 20 to 115 MPa and temperature range 523 to 623 K. The stress dependence of the steady state strain rates indicated a transition from diffusional creep to power law creep, the transition stress decreasing with increasing temperature from 70 MPa (σ/G = 3.1 × 10^-3) at 523 K to 40 MPa (σ/G = 1.9 × 10^-3) at 623 K. The activation energy in the power law creep regime was close to that of bulk self-diffusion in aluminum, while the activation energy in the diffusional creep regime was close to that of grain boundary self-diffusion in Al. The creep strain rates in the power law creep regime were found to be predicted much better by the substructure-invariant creep law (Sherby, 1981) than by the semi-empirical Dorn equation for Al, with the inclusion of a “threshold” stress. In the Coble creep regime, it was found that the cell/subgrain boundaries are inefficient vacancy sources/sinks and that their contribution to Coble creep is totally suppressed in this alloy. The Coble creep rates could be explained by using the average diameter of the powder particles as the effective grain size in the Coble creep equation.     

Effects of RE on Microstructures and Mechanical Properties of Hot-Extruded AZ31 Magnesium Alloy

Effects of rare earth (RE) additions on microstructure and mechanical properties of the wrought AZ31 magnesium alloy were investigated. The results show that, by adding 0.3%, 0.6% and 1.0% RE elements, the as-cast microstructure can be refined, and the as-cast alloys‘ elongation and tensile strength can be improved. After extrusion, the alloy with 0.3 % and 0.6% RE additions obtain a finer microstructure and the best mechanical properties, but the alloy with 1.0% RE addition has the coarse A1-RE compound particles in grain boundaries which decreased elongation and tensile properties. Usually, Rare earth (RE) elements were used to improve the creep properties of aluminium-containing magnesium pressure die cast alloys at elevated temperatures. In this paper, it is also found that the high temperature strength of extruded materials can be increased by RE elements additions.     

Elevated temperature mechanical properties of the iron base oxide dispersion strengthened alloy ma 956 bar

1144 to 1477 K elevated temperature tensile, stress rupture, and creep tests and residual room temperature tensile tests following creep exposures were conducted on the iron-base oxide dispersion strengthened alloy MA 956, nominally Fe-20Cr-4.5Al-0.5Ti-0.5Y₂0₃. While the majority of the testing was in the longitudinal bar direction, a few tests in the long transverse bar direction were also conducted. Under slow strain rate conditions in the longitudinal direction, MA 956 deforms via a crack nucleation and growth mechanism eventually leading to sudden fracture. The longitudinal direction is stronger than the long transverse direction. Small amounts (∼0.1 pct) of prior creep strain do not degrade subsequent room temperature tensile properties.     

扩散蠕变理论的基础问题研究

Nabarro-Herring蠕变模型一直被认为是解释合金高温扩散蠕变的经典理论模型。然而,在20世纪末,Ruano等在分析当时几乎所有已知合金高温低应力条件下的蠕变试验数据后发现,按Nabarro-Herring扩散蠕变模型计算的蠕变速率数据与实验结果吻合度都很不理想,有的甚至相差1×103倍。研究认为,由于Nabarro-Herring扩散蠕变模型仅仅考虑了几乎不可能存在的空位扩散流,忽略了不均匀力化学势场导致的原子扩散流,因此在定量处理多晶材料的高温低应力蠕变数据时有很大偏差。在高温低应力条件下,由于位错、晶界等非平衡缺陷的存在,实际晶体材料中原子的力化学势并非处处相等,因此导致原子扩散流的产生,材料发生扩散蠕变。多晶材料高温低应力条件下的扩散蠕变可以认为是静水应力作用下的体积蠕变和非静水应力作用下的形状蠕变的叠加。本研究将重新审视Nabarro-Herring扩散蠕变模型的理论基础,初步建立能够合理解释多晶材料高温蠕变时晶界形成无沉淀区的定性模型,为有关问题的解决提供新的思路。     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Dispersion strengthened aluminum-4 pct magnesium alloy made by mechanical alloying

Mechanical alloying is a unique high energy milling process for producing metal powders with a controlled microstructure. When applied to aluminum based alloys, a uniform, equiaxed fine dispersion of oxygen- and carbon-based particles is obtained. In addition, a very fine grain structure, pinned by the dispersoid, is generated. Relatively low volume loadings of dispersoid may be employed to achieve attractive combinations of properties including ambient temperature tensile strength, corrosion resistance and stress corrosion cracking resistance. The development of a dispersion strengthened aluminum-4 pct magnesium alloy is discussed.     

Development of a High-Strength Ultrafine-Grained Ferritic Steel Nanocomposite

This article describes the microstructural and mechanical properties of 12YWT oxide-dispersion-strengthened (ODS)-ferritic steel nanocomposite. According to the annealing results obtained from X-ray diffraction line profile analysis on mechanically alloyed powders milled for 80 hours, the hot extrusion at 1123 K (850 °C) resulted in a nearly equiaxed ultrafine structure with an ultimate tensile strength of 1470 MPa, yield strength of 1390 MPa, and total elongation of 13 pct at room temperature comparable with high-strength 14YWT ODS steel. Maximum total elongation was found at 973 K (600 °C) where fractography of the tensile specimen showed a fully ductile dimple feature compared with the splitting cracks and very fine dimpled structure observed at room temperature. The presence of very small particles on the wall of dimples at 1073 K (800 °C) with nearly chemical composition of the matrix alloy was attributed to the activation of the boundaries decohesion mechanism as a result of diffusion of solute atoms. The results of Charpy impact test also indicated significant improvement of transition temperature with respect to predecessor 12YWT because of the decreased grain size and more homogeneity of grain size distribution. Hence, this alloy represented a good compromise between the strength and Charpy impact properties.     

Texture and room temperature mechanical properties of dispersion strengthened Ni−Cr alloys

The effects of preferred crystallographic orientation on the elastic and plastic properties of dispersion-strengthened Ni-Cr alloys have been observed at room temperature. One alloy possessed a heavy cube-texture with a twin component while another alloy did not show evidence of a preferred crystallographic orientation. The elastic modulus was found to vary within the plane of the sheet of the textured alloy, depending on the direction of the axis of the tensile specimen, from 22.9×10⁶ psi (1.58×10¹² d per sq cm) to 33.1×10⁶ psi (2.28×10¹²) d per sq cm). The ultimate strength varied between 108×10³ psi (7.45×10⁹ d per sq cm) and 127×10³ psi (8.75×10⁹ d per sq cm) again depending upon the orientation of the tensile axis. These variations are related to theories of plastic and elastic behavior in materials. It is concluded that the presence of the preferred orientation acts to texture strengthen the alloy at room temperature. The tensile properties of the two alloys at 2000°F (1093°C) indicate that texture is possibly partially responsible for the excellent high temperature strengths and creep resistance of the dispersion strengthened alloys.     

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni₃Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.