初始晶粒尺寸对2024铝合金热变形行为和组织的影响

利用永磁搅拌近液相线铸造和普通铸造方法制备不同晶粒尺寸的2024铝合金铸锭,利用Gleeble-1500热模拟试验机研究初始晶粒尺寸对不同压缩变形条件下2024铝合金的热变形行为和变形后显微组织的影响。研究表明:2024铝合金的热变形行为依赖于变形条件和初始组织。初始晶粒尺寸对流变应力的影响是:当应变速率小于0.1 s~(-1)时,流变应力随晶粒尺寸减小而减少;当应变速率为10 s~(-1)时,流变应力随晶粒尺寸减小而增大。降低变形温度会弱化晶粒尺寸对流变应力的影响。热压缩流变应力随应变速率增大而增大,随变形温度升高而减小。应变速率为10 s~(-1)时,热压缩应力应变曲线呈现周期性波动;只在粗晶2024铝合金中发现变形剪切带。     

The low-cycle fatigue and fatigue-crack-growth behavior of HAYNES® HR-120 alloy

The low-cycle fatigue and fatigue-crack-growth behavior of the HAYNES HR-120 alloy was investigated over the temperature range of 24°C to 980°C in laboratory air. The result showed that increasing the temperature usually led to a substantial decrease in the low-cycle fatigue life. The reduction of fatigue life could be attributed to oxidation and dynamic strain-aging (DSA) processes. The strain vs fatigue-life data obtained at different temperatures were analyzed. It was also found that the fatigue-crack-growth rate per cycle generally increased with increasing temperature and R ratio (R=σ _min/σ _max, where σ _min and σ _max are the applied minimum and maximum stresses, respectively). The relationship between the stress-intensity-factor range and fatigue-crack-growth rate was determined. Scanning-electron-microscopy (SEM) examinations of the fracture surfaces revealed that the fatigue cracks initiated and propagated predominantly in a transgranular mode.     

High-temperature low-cycle fatigue and lifetime prediction of Ti-24Al-11Nb alloy

The influence of hold time on low-cycle fatigue (LCF) of Ti-24Al-11Nb was studied at 650 °C. At 0.167 Hz, the alloy exhibits cyclic hardening at all strain levels studied and obeys the well-known Manson-Coffin behavior. A 100-second hold at peak tensile or compressive strain at ±0.6 pct strain has no observable effect on cycles to failure. For hold times at ±0.5 pct strain, however, the fatigue lives are nearly halved and specimens show secondary cracking normal to the stress axis. The increase in inelastic strain as a result of hold time appears to be primarily responsible for the observed loss in fatigue life. A linear life fraction model, which considers both fatigue and creep damage, is found to provide good correlation of measured lives with predictions. For the range of test conditions employed, the total and the tensile hysteretic energy per unit volume, absorbed until fracture, remain nearly constant. The tensile hysteretic energy appears to be a more useful measure of fatigue damage for life prediction. On leave from Defence Metallurgical Research Laboratory, Hyderabad, India 500-258     

Temperature and strain-rate effects on low-cycle fatigue behavior of alloy 800H

The effects of strain rate (4 × 10^-6 to 4 × 10^-3 s^-1) and temperature on the low-cycle fatigue (LCF) behavior of alloy 800H have been evaluated in the range 750 ° to 950 °. Total axial strain controlled LCF tests were conducted in air at a strain amplitude of ± 0.30 pct. Low-cycle fatigue life decreased with decreasing strain rate and increasing temperature. The cyclic stress response behavior showed a marked variation with temperature and strain rate. The time- and temperature-dependent processes which influence the cyclic stress response and life have been identified and their relative importance assessed. Dynamic strain aging, time-dependent deformation, precipitation of parallel platelets of M₂₃C₆on grain boundaries and incoherent ledges of twins, and oxidation were found to operate depending on the test conditions. The largest effect on life was shown by oxidation processes. On leave from the Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India.     

Influence of grain size on the low-cycle fatigue lives of austenitic stainless steels at high temperatures

The influence of grain size on the fatigue lives was investigated for eight kinds of austenitic stainless steels with the grain size numbers from 9 to 1. Fatigue tests were carried out at 600 and 700 °C under triangular wave shapes at strain rates of 6.7 × 10^-3/s and 6.7 × 10^-5/s, respectively, and under truncated wave shape with 30 m;n hold-time at tension side. When a strain rate was 6.7 × 10^-3/s at both 600 and 700 °C, the fracture modes were always transgranular, and the fatigue lives scarcely depended on the type of steels or the grain size. When a strain rate was 6.7 × 10^-5/s at 600 °C, the fracture modes changed from a dominantly transgranular mode to a completely intergranular one and the fatigue lives decreased with decreasing the grain size number. When a strain rate was 6.7 X 10^-5/sVs at 700 °C, grain size dependence of the fatigue lives was divided into two groups of the steels depending on the type of steel. The fracture modes of some types of the steel were completely intergranular, and others mixed. In hold-time tests, the grain size dependence of the fatigue lives was similar to that in the tests of triangular wave shape at a strain rate of 6.7 × 10^-5/s.     

The effect of lamellar morphology on tensile and high-cycle fatigue behavior of orthorhombic Ti-22Al-27Nb alloy

The room-temperature tensile and high-cycle fatigue (HCF) behavior of orthorhombic Ti-22Al-27Nb alloy with varying lamellar morphology was investigated. Varying lamellar morphology was produced by changing the cooling rate after annealing in the single B2 phase region. A slower cooling rate of 0.003 K/s, for example, resulted in several large packets or colonies of similarly aligned O-phase lamellae and a nearly continuous massive α ₂ phase at the prior B2 grain boundaries, while a faster cooling rate of 0.1 K/s led to the refinement of colony sizes and the O-phase lamellae. The interface of O-phase lamellae and B2 phases was semicoherent. Water quenching produced a very fine tweed-like microstructure with a thin continuous O phase at the prior B2 grain boundaries. The 0.2 pct yield stress, tensile strength, and HCF strength increased with increasing cooling rate. For example, the tensile strength and HCF strength at 10⁷ cycles of 0.003 and 0.1 K/s-cooled were 774 and 450 MPa, and 945 and 620 MPa, respectively. Since the fatigue ratio, which is the ratio of HCF strength at 10⁷ cycles to tensile strength, did not show a constant value, but instead increased with increasing cooling rate, part of the fatigue improvement was the result of improved resistance to fatigue associated with the microstructural refinement of the lamellar morphology. Fatigue failure occurred by the subsurface initiation, and every initiation site was found to contain a flat facet. Concurrent observation of the fatigue initiation facet and the underlying microstructure revealed that the fatigue crack initiated in a shear mode across the colony, irrespective of colony size, indicating that the size of the initiation facet corresponded to that of the colony. Therefore, the colony size is likely a major controlling factor in determining the degree of fatigue improvement due to the microstructural refinement of lamellar morphology. For the water-quenched specimens, fatigue crack initiation appeared to be associated with shear cracking along the boundary between the continuous grain boundary O phase and the adjacent prior B2 grain.     

High-temperature low-cycle fatigue of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr

The low-cycle fatigue (LCF) behavior of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr in fully lamellar (FL) and nearly lamellar (NL) microstructural conditions is studied at 650 °C and 800 °C, with and without hold times. At 650 °C and 800 °C, the alloy in either condition exhibits cyclic stability at all strain levels studied, excepting the NL structure which shows slight cyclic hardening at higher strain levels at 650 °C. Fracture in the FL condition occurs by a mixed mode comprising delamination, translamellar fracture, and stepwise fracture. On the other hand, fracture occurs mostly by translamellar mode in the NL condition. At both test temperatures, the alloy in the FL condition obeys the well-known Manson-Coffin behavior. The fatigue resistance of the alloy at 650 °C in the FL condition is very much comparable to, while in the NL condition it is superior to, that of Ti-24Al-llNb alloy. At 650 °C, a 100-second peak tensile strain hold doubles the fatigue life of the alloy in the FL condition, while a 100-second hold at compressive peak strain or at both tensile and compressive peak strain degrades fatigue life. The observed hold time effects can primarily be attributed to mean stress. Irrespective of the nature of the test, the hysteretic energy (total as well as tensile) per cycle remains nearly constant during the majority of its life. The total and tensile hysteretic energy to fracture, at both test temperatures, increase with cycles to failure, and the variation follows a power-law relationship. Formerly NRC Senior Resident Associate, Wright Laboratory.     

The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy

The high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 °C to 515 °C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of M _g =1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the grain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.     

The effect of grain size on the ambient temperature creep deformation behavior of a beta Ti-14.8 V alloy

The effect of grain size in the range of 18 to 350 μm on the ambient-temperature creep behavior of a metastable β Ti-14.8 pct V alloy was investigated at a stress level of 95 pct yield stress. The main modes of deformation were found to be stress-induced plate (SIP) formation and slip. In the 350-μm grain-size material, the plates were found to nucleate and grow with time, thereby contributing to the ambient-temperature creep strain. The extent of SIP formation and growth were found to decrease with decreasing grain size, i.e., creep strain was found to decrease with decreasing grain size. The SIPs observed during creep were found to be similar to those observed in an earlier investigation during tensile testing, which were reported to be {332}〈113〉 twins.     

The effect of aging and cold working on the high-temperature low-cycle fatigue behavior of alloy 800h: part ii: continuous cyclic loading

The individual and combined effects of cold working (5 and 10 pct) and aging (4000 and 8000 h in the temperature range 538 to 760 °C) on the high-temperature low-cycle fatigue behavior of alloy 800H have been investigated. The specimens were tested at the aging temperatures. Both the saturation stress range and the fatigue life were found to be history dependent. A history-independent hardening mechanism, dynamic strain aging, was found to operate over the temperature range ~450 to 650°C and to be maximized at ~55O °C. It is speculated that carbon is responsible for this dynamic strain aging. Finally, at temperatures above 538 °C the Coffin-Manson plots show a history-independent deviation from linearity. Formerly a Staff Scientist at General Atomic Co.     

The effect of heat treatment on the fatigue and corrosion fatigue behavior of a CuNiCr alloy

Fatigue experiments were conducted on a CuNiCr alloy (IN838) in air and in 0.5 N NaCl solutions under conditions of free corrosion and of applied anodic currents. The alloy was heat treated to produce a solutionized structure and also to produce a precipitation hardened structure. The fatigue behavior of the solutionized alloy was unaffected by free corrosion although increased corrosion rates resulted in a decrease in fatigue resistance for small applied anodic currents. The age hardened alloy showed a decrease in fatigue resistance under free corrosion conditions and a further decrease in resistance with small applied anodic currents. In both heat treatments fatigue in air resulted in mixed transgranular-intergranular crack initiation and propagation while corrosion increased the relative amount of intergranular cracking. These results can be explained by a consideration of previously developed fatigue and corrosion fatigue models of pure copper and copper aluminum alloys. H. N. Hahn, formerly with Rensselaer Polytechnic Institute     

The effect of hold time and waveform on the high-temperature, low-cycle fatigue properties of a Nb-A286 alloy

The effect of hold time and waveform on the high-temperature, low-cycle fatigue (HTLCF) of a Nb-modified A286 alloy was investigated at 650 °C. It was found that the fatigue life with strain hold was lower than that of continuous low-cycle fatigue, and this was due to the additive creep deformation during hold time. The apparent activation energy for the creep deformation during the tensile and compressive hold times was determined to be about 260 to 270 kJ/mole from stress-relaxation curves. This value is in good agreement with the apparent activation energy required for the formation of η phase at the grain boundary in the Nb-A286 alloy. Therefore, the grain-boundary precipitates and the corresponding cavitation along the grain boundary were estimated to be the main damage of the creep-fatigue interaction of the Nb-A286 alloy and to cause the intergranular failure. Also, the driving force for grain-boundary cavitation leading to intergranular failure under a compressive strain hold is reduced compared to the case of a tensile strain hold. This fact is known to be due to the healing effect of the grain-boundary cavitation. Therefore, the strain hold in compression leads to less reduction in fatigue life than that in tension.     

The effect of grain size on the high-strain,high-strain-rate behavior of copper

Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-μm grain-size specimens and virtually absent for the 9.5-μm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges. Formerly with the Department of Applied Mechanics and Engineering Sciences, University of California. Formerly with the Department of Applied Mechanics and Engineering Sciences.     

The effect of aging and cold working on the high-temperature low-cycle fatigue behavior of alloy 800H: Part I. The effect of hardening processes on the initial stress-strain curve

The individual and combined effects of cold working (5 and 10 pct) and aging (4000 and 8000 h at 538 to 760°) on the microstructure and high-temperature yield strength of al-loy 800H have been investigated. The specimens were tested at the aging temperatures. During testing some of the specimens showed the phenomenon of serrated yielding. In order of importance, the principal hardening agents observed in this work were: cold work, the precipitation of Cr₂₃3C₆ at the grain boundaries, and, in some cases, the precipitation of a Perovskite-type γ’ phase in the grain interiors.     

Strain distribution effects on the low-cycle fatigue behavior of Fe-C-Mo steels

The strain distributions obtained from monotonic finite element method (FEM) calculations have been employed to model the low-cycle fatigue (LCF) behavior of Fe-C-Mo dual-phase steels. The microstructures considered have a continuous ferrite matrix (with Mo₂C precipitates) surrounding martensite packets. Two microstructural parameters have been controlled: (1) the volume fraction of martensite and (2) the strength of the ferrite matrix. The FEM approximations show that highly strained regions dominate LCF lifetimes. The experimentally observed reductions in plastic strain life for increasing martensite volume fractions are described usingM _ε , the strain magnification factor, which is obtained from the FEM analyses. Strengthening the ferrite matrix or reducing the volume fraction of martensite reducesM _ε . The cyclic softening observed is qualitatively correlated with FEM predictions of increasing plastic strain in the martensite as the ferrite strength increases. The overall cyclic hardening-softening behavior results from the combination of ferritic hardening combined with martensitic softening. Formerly Graduate Student, Department of Materials Science, University of Virginia.     

The effect of microstructure on the fatigue crack propagation behavior of an Al-Zn-Mg-Cu alloy

The effect of slip distribution on the fatigue crack propagation behavior in vacuum of a high purity Al-5.9Zn-2.6Mg-l.7Cu alloy in various age-hardened conditions has been investigated. The crack propagation resistance was observed to be significantly higher for underaged microstructures containing shearable precipitates in comparison to overaged conditions with nonshearable precipitates. The improved crack propagation resistance is attributed in part to an increased amount of reversed slip in the plastic zone at the crack tip due to a higher degree of planar slip for conditions with shearable precipitates. The observed increase in fatigue crack propagation resistance with decreasing precipitate size for microstructures containing a constant volume fraction of shearable precipitates cannot be explained on the basis of such slip reversibility alone. The variation in ductility for the different microstructures has also to be taken into account. It was found that the enhanced crack propagation resistance can be correlated to the increased ductility with decreasing precipitate size. This explanation was supported by the experimental observation that microstructures containing different volume fractions and sizes of shearable precipitates but exhibiting the same ductility showed approximately the same resistance against fatigue crack propagation. formerly with German Aerospace Research Establishment (DFVLR), Cologne, Germany. formerly with Ruhr-University, Bochum, Germany.