Elevated temperature deformation behavior of a dispersion-strengthened Al-Fe,V,Si alloy

The deformation behavior of a rapidly solidified, dispersion-strengthened Al alloy containing 11.7 pct Fe, 1.2 pct V, and 2.4 pct Si was studied at test temperatures up to 450 °C using constantstress creep and constnt strain-rate tensile tests. Apparent stress exponents (n) up to ∼24 and an activation energy of 360 kJ/mol were obtained with the standard Arrhenius type power-law creep equation, which also suggested a change in behavior at ∼300 °C. Substructure-invariant and dislocation/dispersoid interaction models were found to be inadequate for explaining the behavior. When the data were replotted as vs σ, two regimes were found between 350 °C and 450 °C. A model with a pseudothreshold stress (σ _Th′ ) for the higher stress regime resulted inn ∼3, indicating solute drag in this regime. Transmission electron microscopy (TEM) showed departureside pinning of dislocations at higher stresses. In the lower stress regime, TEM showed dislocation subgrain structures. Here, the model resulted in a stress exponent of ∼4.5 indicating the dislocation climb mechanism. At temperatures below ∼300 °C, a single regime was found along with lower activation energies and a stress dependence of ∼3. Dislocation pipe diffusion is proposed to explain the lower activation energy. The origin ofσ _Th′ has been tied to dislocation generation at the grain boundaries.     

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.     

Elevated temperature fracture toughness of Al-Cu-Mg-Ag sheet: Characterization and modeling

The plane-strain initiation fracture toughness (K _JICi ) and plane-stress crack growth resistance of two Al-Cu-Mg-Ag alloy sheets are characterized as a function of temperature by a J-integral method. For AA2519 +Mg+Ag, K _JICi decreases from 32.5 MPa√m at 25 °C to 28.5 MPa√m at 175 °C, while K _JICi for a lower Cu variant increases from 34.2 MPa√m at 25 °C to 36.0 MPa√m at 150 °C. Crack-tip damage in AA2519+Mg+Ag evolves by nucleation and growth of voids from large undissolved Al₂Cu particles, but fracture resistance is controlled by void sheeting coalescence associated with dispersoids. Quantitative fractography, three-dimensional (3-D) reconstruction of fracture surfaces, and metallographic crack profiles indicate that void sheeting is retarded as temperature increases from 25 °C to 150°C, consistent with a rising fracture resistance. Primary microvoids nucleate from smaller constituent particles in the low Cu alloy, and fracture strain increases. A strain-controlled micromechanical model accurately predicts K _JICi as a function of temperature, but includes a critical distance parameter (l*) that is not definable a priori. Nearly constant initiation toughness for AA2519+Mg+Ag is due to rising fracture strain with temperature, which balances the effects of decreasing flow strength, work hardening, and elastic modulus on the crack-tip strain distribution. Ambient temperature toughnesses of the low Cu variant are comparable to those of AA2519+Mg+Ag, despite increased fracture strain, because of reduced constituent spacing and l*.     

Elevated temperature deformation behavior of an Al-8.4 wt pct Fe-3.6 wt pct Ce alloy

The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS) aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable particles, such as γ^′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in strength of at least some of the second phase particles at temperatures above 723 K. Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

Elevated temperature strength of fine-grained INCONEL alloy MA754

Elevated temperature tensile and creep-rupture tests were performed on INCONEL alloy MA754 in an as-rolled, fine-grained condition. Tensile tests were performed at 25 °C, 800 °C, 900 °C, and 1000 °C; creep-rupture tests were performed at 800 °C, 900 °C, and 1000 °C. the elevated temperature strength in the fine-grained condition was approximately 25 pct of that the coarse-grained, annealed condition. While good ductility was observed in tensile tests at a nominal strain rate of 1 × 10⁻³s⁻¹, ductility in creep-rupture tests was very low, with failure elongations less than 5 pct and no reduction in area. Creep deformation appeared to occur primarily by cavity formation and growth.     

Effects of heat treatment and testing temperature on fracture mechanics behavior of low-Si CA-15 stainless steel

This research studied the effects of heat treatment and testing temperature on fracture mechanics behavior of Si-modified CA-15 martensitic stainless steel (MSS), which is similar to AISI 403 grade stainless steel, which has been widely used in wall and blanket structures and in the pipe of nuclear power plant reactors, turbine blades, and nozzles. The results indicated that fracture toughness of low-Si CA-15 MSS is better than that of AISI 403. The specimens of the low-Si CA-15 MSS after austenitization at 1010 °C and then tempering at 300 °C have higher plane-strain fracture toughness (K _IC ) values for both 25 °C and −150 °C testing temperatures. However, the specimens tested at 150 °C cannot satisfy the plane-strain fracture toughness criteria. The fatigue crack growth rate is the slowest after austenitization at 1010 °C for 2 hours and tempering at 400 °C. Observing the crack propagation paths using a metallographic test, it was found that the cracking paths preferred orientation and branched along ferrite phase, owing to martensite-phase strengthening and grain-boundary-carbide retarding after 300 °C to 400 °C tempering. Also, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.     

Fatigue and fracture behavior of an aluminum-lithium alloy 8090-T6 at ambient and cryogenic temperature

An investigation has been made of the fatigue and fracture behavior of an Al-Li-Cu-Mg-Zr 8090-T6 alloy at room (300 K) and liquid nitrogen (77 K) temperatures. The fatigue and fracture strengths, as well as ductility of the alloy, have been found to increase with decreasing temperature. The observations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the changes in fatigue and fracture behavior with temperature are considered to be associated with the change in the deformation and fracture modes. It has been found that the occurrence of the localized shear deformation bands in which the hard precipitates are sheared by moving dislocations is responsible for the reduction of fatigue and fracture strengths as well as ductility of the alloy at room temperature. However, the improvement of both strength and ductility of the alloy at liquid nitrogen temperature might be attributed to the deeper and larger delamination that occurred on the fracture surface.     

铝合金材料焊接接头断裂行为研究

近年来社会经济步入了高速发展阶段,也促进了汽车行业、造船行业、航天航空领域方面的发展,在高新技术领域当中,涉及到许多精密的零件,更是保障技术及产品质量的关键部分,其中铝合金的可塑性较高、且密度很小,高强度的特性被广泛应用于汽车、航空等制造业当中,在工业生产过程中,焊接是重要的连接方式之一,其中由于多方面因素的影响,焊接...     

Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.     

Investigation on the fracture behavior of shape memory alloy NiTi

This work investigates the fracture behavior of shape memory alloy NiTi (50.7 at. pct Ni) at room temperature. Macroscopic mechanical tests, microscopic in situ observations of tensile fracture processes by scanning electron microscopy (SEM), and detailed analyses of fracture surfaces were carried out. The results reveal that specimens with different thicknesses show various shape memory effects and superelasticities. The main crack with a quasi-cleavage mode that combines cleavage with ductile tearing is initiated at the notch tip and is stress-control-propagated in line with the direction of the maximum normal stress. The microstructure has little effect on the direction of crack propagation, but coarser substructures show lower resistance to the crack propagation. In specimens with various types of notches, various notch acuities present different effects on the crack initiation and propagation and result in different fracture behaviors.     

含铜镍基合金的高温氧化行为研究

采用SEM分析了不同试验条件下含铜镍基合金表层的高温氧化产物,分析其产生的机理,并讨论了其在工业生产中对产品质量的影响及改善措施。结果表明,含铜镍基合金在长时间高温氧化后,表层会形成镍及铜的嵌套氧化产物,且随时间长短的不同,表现不同程度的结构比例。该氧化结构对钢板表面质量产生直接影响,可通过调整加热温度及时间等因素控制其产生的严重程度。     

Surface composition of ternary cu-ag-au alloys: part i. experimental results

The enthalpies and entropies of segregation for silver and gold, along with equilibrium surface composition measurements on three Cu-Ag-Au ternary specimens, are reported and discussed. The experimentally studied compositions were Cu-0.3 at. pct Ag-2.1 at. pct Au, Cu-0.55 at. pct Ag-6.5 at. pct Au, and Cu-1.09 at. pct Ag-6.9 at. pct Au. The experimental enthalpies of segregation, entropies of segregation, and surface compositions are compared with those of binary alloys having nearly identical bulk solute contents in order to isolate ternary effects on surface segregation. The results indicate that the equilibrium surface composition of the ternary alloys is dominated by site competition with respect to the more weakly segregating species (gold). The surface composition of the more strongly surface-active species (silver) is largely unaffected by ternary bulk gold additions of up to 7 at. pct.     

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.     

5A01铝合金高温压缩流变行为

采用Gleeble-1500热模拟机高温压缩试验,研究5A01铝合金在应变速牢为0.01～1s-1、变形温度为350～450℃条件下的流变行为,并利用光学显微镜分析合金在不同压缩条件下的组织形貌特征.结果表明:应变速率和变形温度的变化强烈影响合金流变应力的大小,流变应力随变形温度升高而降低,随应变速率提高而增大.采用双曲正弦形式ARRHENIUS的关系来描述5A01铝合金高温压缩变形时的流变应力行为,获得的材料常数A、α、n和Q分别为0.068 31 s-1、0.009 4 MPa-1、2.708 9和161.14 kJ/mol:在应变速率为0.01 s-1及变形温度低于400℃条件下变形时,5A01铝合金组织为纤维组织,而当变形温度升高到450℃时,再结晶程度很高,出现大量等轴晶.     

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.     

Phase stability investigations of the palladium-cadmium system: part i. thermodynamic studies

The vapor pressure of cadmium in palladium-cadmium alloys was determined between 830 and 1350 K and between 33 and 65 at. pct Cd by two different isopiestic methods. An analysis of the resulting partial molar quantities of Cd in the β₁-phase in terms of Chang's theoretical model for the Ll₀-structure yielded a disorder parameter α= 7 × 10^-4 and a second-nearest neighbor parameter η = 0.75 at 1073 K. Available literature data for the thermodynamic activities of Cd in the α-phase (solid solution of Cd in Pd) were evaluated using a subregular solution model. By combining these values with the present experimental data, all relevant partial and integral thermodynamic properties for the α- and β₁-phase could be derived. At the stoichiometric composition of the β₁-phase, values of ΔG = -38.0 kJ(g-atom)^-1 and ΔH = -53.7 kJ(g-atom)^-1, referred to Pd(s) and Cd(l), were obtained for a temperature of 1073 K. Y. MAA, formerly Graduate Student, Materials Department, University of Wisconsin-Milwaukee Y. A. CHANG, formerly Professor of Materials Engineering and Associate Dean for Research, Graduate School, University of Wisconsin-Milwaukee W. SCHUSTER, formerly a Postdoctoral Research Associate, Materials Department, University of Wisconsin-Milwaukee.     

Fatigue and fracture toughness of a Nb-Ti-Cr-Al-X single-phase alloy at ambient temperature

The fatigue and fracture resistance of a commercially made, single-phase Nb-base alloy with 35 at. pct Ti, 5 at. pct Cr, 6 at. pct Al, and several elements to increase solid solution strengthening have been investigated. The threshold for fatigue crack growth was determined to be ≈7 MPa√m and fracture toughness ≈35 MPa√m. Crack growth was intermittent and sporadic; the fracture path was tortuous, crystallographic, and appeared to favor the {100} and {112} planes. Fatigue crack closure was measured directly at the crack tip. The fatigue and fracture properties of the commercial alloy are compared against those of Nb-Cr-Ti and Nb-Cr-Ti-Al alloys. The comparison indicated that Ti addition is beneficial for, but Al addition is detrimental to, both fracture toughness and fatigue crack resistance.     

Nb-Ti-Al合金及其硅化物涂层的高温氧化行为

采用电子束和真空自耗电弧熔炼法制备Nb-40Ti-7Al(质量分数,%)合金,利用料浆熔烧法在合金表面制备Si-Cr-Ti涂层,研究在1 400℃下合金与涂层的氧化行为。通过X射线衍射(XRD)、扫描电镜(SEM)、能谱分析(EDS)及电子探针微区分析(EPMA)研究基体与涂层氧化前后的组织形貌变化及成分分布。结果表明:Nb-40Ti-7Al合金在1 400℃氧化1～11 h后,氧化产物均主要为TiNb2O7、TiO2、Al2O3;氧化前,涂层主要由(Nb,Ti,Cr,Al)Si2主体层与(Ti,Nb,Al)5Si3过渡层组成,高温氧化后涂层表面形成含有Al2O3、TiO2的SiO2阻挡层;合金与涂层的氧化行为均遵循抛物线规律,合金在1 400℃氧化11 h的单位面积质量增量为161.98 mg/cm2,而涂覆涂层后单位面积质量增量降至9.56 mg/cm2,表明Si-Cr-Ti涂层具备良好的高温抗氧化性能。     

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.