The effect of grain boundary orientation on creep and rupture of IN-738 and nichrome

Creep rupture specimens taken from directionally solidified ingots of IN-738 and Nichrome in which the grain boundaries were oriented longitudinally, transversely, and 45 deg to the stress axis have been tested over a range of temperature and stress. For both alloys, the ductility was appreciably higher in the longitudinal orientation; but in IN-738, the creep strength was higher in the other two orientations. The net effect on rupture life was small for the superalloy. The nichrome showed much greater scatter which was due partly to inhomogeneous deformation and local recrystallization at the higher temperatures. Because of the recrystallization, even the longitudinal specimens showed intergranular failure for nichrome. The microstructural features of intergranular cracking both internally and on the surface are documented. It is suggested that surface cracking may be an important contributory factor in leading to reduced life with decreased section size which is commonly observed in conventionally cast superalloys.     

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.     

Surface intergranular cracking in large strain fatigue

The formation of surface intergranular cracks has been investigated with a coarse-grained polycrystal of nickel, deformed in low-cycle fatigue at 573 K. The evolution of the cracks was followed as a function of fatigue life fractions, and the factors favoring their formation were identified. It was found that in air, surface intergranular cracking occurs early in fatigue life and is induced by the impinging slip traces at the interface. Grain boundaries other than coherent twin boundaries and those with Σ < 5 are susceptible to such cracking. Depending on the boundary plane orientation and on the geometry of the operative slip vectors relative to the specimen surface, the grain boundary cracks may or may not grow to any appreciable extent. Crack growth is accelerated if the boundary plane makes a large angle with the stress axis and if the differential out-of-surface component of the operative slip vectors in the adjoining grains is large. In vacuum, slip is dispersed and surface rumplings become effective in grain boundary crack nucleation. The evolution of surface intergranular cracks, however, is delayed as opposed to tests conducted in air. The results are interpreted in terms of the interaction between crystal dislocations and grain boundaries and on the state of stress at the grain boundaries.     

Effect of Repair Welding on Electrochemical Corrosion and Stress Corrosion Cracking Behavior of TIG Welded AA2219 Aluminum Alloy in 3.5 Wt Pct NaCl Solution

The stress corrosion cracking (SCC) behavior of AA2219 aluminum alloy in the as-welded (AW) and repair-welded (RW) conditions was examined and compared with that of the base metal (BM) in 3.5 wt pct NaCl solution using the slow strain rate technique (SSRT). The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of both BM and welded joints. The results show that the ductility ratio (ε _NaCl/(ε _air)) of the BM was close to one (0.97) and reduced to 0.9 for the AW joint. This value further reduced to 0.77 after carrying out one repair welding operation. However, the RW specimen exhibited higher ductility than the single-weld specimens even in 3.5 wt pct NaCl solution. SSRT results obtained using pre-exposed samples followed by post-test metallographic observations clearly showed localized pitting corrosion along the partially melted zone (PMZ), signifying that the reduction in ductility ratio of both the AW and RW joints was more due to mechanical overload failure, caused by the localized corrosion and a consequent reduction in specimen thickness, than due to SCC. Also, the RW joint exhibited higher ductility than the AW joint both in air and the environment, although SCC index (SI) for the former is lower than that of the latter. Fractographic examination of the failed samples, in general, revealed a typical ductile cracking morphology for all the base and welded joints, indicating the good environmental cracking resistance of this alloy. Microstructural examination and polarization tests further demonstrate grain boundary melting along the PMZ, and that provided the necessary electrochemical condition for the preferential cracking on that zone of the weldment.     

含钒低合金钢铸坯高温延塑性研究

采用Gleeble-1500热模拟试验机测试了含钒低合金钢铸坯的高温延塑性,利用扫描电镜、金相显微镜对断口形貌及金相组织进行分析。低合金钢的第Ⅰ脆性温度区在Ts~1 370℃之间,第Ⅲ脆性温度区在915~710℃之间。第Ⅲ脆性区间由奥氏体低温域晶界滑移楔形裂纹造成的沿晶脆性断裂和奥氏体晶界先共析铁素体薄膜造成的沿晶韧性断裂两部分组成。钢中的V对钢的第Ⅲ脆性凹槽的影响比较大,脆化向低温区域延伸。     

Environmental effects on the creep behavior of a nickel-base superalloy

Environmental effects on the 760 °C creep behavior of a nickel-base superalloy are isolated by testing at varying stress levels, in laboratory air and vacuum, specimens of different gage diameters and grain sizes, and in a few cases, different grain boundary microstructures. For all specimens receiving a standard heat treatment that results in grain boundaries free of discrete carbides, the minimum creep rate is greater in air than in vacuum for a given specimen and grain size. In contrast, for specimens aged to precipitate carbides along the grain boundaries, the creep rate is lower in air than in vacuum. The minimum creep rate and the sensitivity of this rate to applied stress also are found to be functions of grain size, specimen size, and grain boundary microstructure to extents dependent on the test environments. Rationalizations of these environmental creep behaviors are suggested in terms of the apparently competing phenomena of the relative weakening of the alloy grain boundaries in the air environment, and the alloy creep strengthening in air due to the external surface oxide scale. Formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University     

Effect of Strain Rate on Hot Ductility Behavior of a High Nitrogen Cr-Mn Austenitic Steel

18Mn18Cr0.6N steel specimens were tensile tested between 1173 K and 1473 K (900 °C and 1200 °C) at 9 strain rates ranging from 0.001 to 10 s^?1. The tensile strained microstructures were analyzed through electron backscatter diffraction analysis. The strain rate was found to affect hot ductility by influencing the strain distribution, the extent of dynamic recrystallization and the resulting grain size, and dynamic recovery. The crack nucleation sites were primarily located at grain boundaries and were not influenced by the strain rate. At 1473 K (1200 °C), a higher strain rate was beneficial for grain refinement and preventing hot cracking; however, dynamic recovery appreciably occurred at 0.001 s^?1 and induced transgranular crack propagation. At 1373 K (1100 °C), a high extent of dynamic recrystallization and fine new grains at medium strain rates led to good hot ductility. The strain gradient from the interior of the grain to the grain boundary increased with decreasing strain rate at 1173 K and 1273 K (900 °C and 1000 °C), which promoted hot cracking. Grain boundary sliding accompanied grain rotation and did not contribute to hot cracking.     

ZTA15Cr铸造钛合金材料的组织及性能

研究了ZTA15Cr铸造钛合金材料在铸造状态和热等静压状态下的力学性能和微观组织。结果表明,ZTA15Cr铸造钛合金在铸造状态下组织以马氏体＋晶界α＋针状α为主,强度高而塑性较差,力学性能数据分布离散性强,不能满足使用要求;经热等静压处理后,组织中的板条状α＋β交叉呈网篮状,塑性有较大提高,可满足航空航天对钛合金铸件的使用要求。     

Hot ductility trough elimination through single cycle of intense cooling and reheating for microalloyed steel casting

Surface transverse cracking, especially corner cracking, is prone to generate in continuously cast slabs of microalloyed steels. The method of surface structure control (SSC) was supposed to the best way to avoid the detrimental defects. However, the mechanism of improving hot ductility by SSC and the specific parameters to control the process are still unclear for the reasonable adoption in production. In the present work, the impact of cooling rate, holding temperature and holding time on austenite decomposition, and the austenite grain size before and after intense cooling were investigated by thermal simulation method. With the increase of cooling rate, it is observed that the phase is transformed from austenite?→?grain boundary film-like alltromorph ferrite?→?Widmanstätten ferrite plates (or intragranular ferrite plates)?→?bainite+martensite. Mostly important, the film-like ferrite can be eliminated through intense cooling and the following reheating, but the austenite grain size is not observed to be refined through the single γ?→?α?→?γ cycle. Even though, the reduction of area (RA) is improved drastically to over 70% in the third ductility trough, whereas the RA value is just 相似文献   

铸坯取样位置对经济型双相不锈钢2101热塑性的影响

为了解大型铸锭在轧制过程中产生边裂的原因,通过对比铸坯中部和边部的成分、不同温度下相比例、两相硬度差等的变化规律,利用光学显微镜,扫描电子显微镜和电子背散射衍射观察分析试验钢的微观组织和断口形貌,分析了边部容易开裂的原因.结果表明,和中部相比,边部晶粒细小,且铁素体含量较多,但边部开裂更严重.这说明晶粒尺寸和相比例并不是影响使边部开裂严重的主要原因.而和中部比,铸锭边部试样两相硬度差较大,使两相在热变形过程中应变分配不均匀,容易在相界处产生应力集中,导致开裂.同时边部析出物较中部多,相界析出物的产生破坏了基体的连续性,容易在相界处产生显微裂纹,导致开裂.      

Environmental damage of a cast nickel base superalloy

Exposure in air in the temperature range 900 to 1100 °C produces a major loss in stress rupture life and ductility of IN738. The sensitivity to this environmental damage increases with decreasing test temperature in the range 1000 to 700 °C. Oxygen is identified as the source of the damage for air exposure and indirect evidence supports grain boundary penetration of the gas to considerable depth. It is argued that oxygen segregation can lead to grain boundary immobilization and unstable intergranular fracture at intermediate temperatures. It is shown that compositional modifications, particularly boron and hafnium additions, may reduce the oxygen damage susceptibility, and that a cobalt base coating effectively eliminates the susceptibility. The relevance of these observations in understanding the effect of test environment on creep-rupture and fatigue crack propagation is considered.     

Recovery of creep properties of a vanadium-strengthened steel (1Cr1Mo0.75VTiB) by reheat treatment

A reheat treatment cycle for Durehete 1055 has been developed, the first stage of which removes grain boundary creep cavitation. Subsequent stages control microstructural parameters such as prior austenite grain size and allow in-service degradation of the vanadium carbide precipitate distribution to be reversed. Samples have been examined using a range of optical and electron optical techniques, and the compositions of extracted carbide precipitates have been determined using STEM-EDS X-ray microanalysis. It is shown that by reheat treatment the uniaxial creep life of creep-ruptured, ex-service material can be restored, and the creep ductility of previously coarse-grained material improved. The application of the results to life regeneration of service components is briefly discussed.     

Plastic deformation of CVD textured tungsten—I. Constitutive response

The constitutive response of CVD textured tungsten in 〈001〉, 〈011〉 and random orientations under uniaxial compression subjected to a range of strain rates is investigated. Both 〈001〉 and 〈011〉 textured specimens revealed a strong strain rate sensitivity. The 〈001〉 specimens showed large ductility at all strain rates and significant strain softening during high strain rate deformation. The 〈011〉 texture undergoes homogenous deformation under quasi-static loading but failed prematurely due to extensive grain boundary cracking during high strain rate loading. The random orientation specimens exhibited brittle failure. The temperature rise during high strain rate deformation was measured using high speed infrared detectors. A physically based crystal plasticity model motivated by dislocation reactions in b.c.c. metals is outlined. The model predicts the dependence of constitutive response on the texture in tungsten polycrystals and suggests that the transverse tensile stresses which develop in 〈011〉 orientation due to the crystallographic asymmetry could be responsible for observed grain boundary cracking.     

Serrated grain boundary formation potential of Ni-based superalloys and its implications

The serrated grain boundary formation potential of a large number of conventionally forged, powder processed, and investment cast Ni-based superalloys is reviewed. A mechanism of serrated grain boundary formation by which grain boundaryγ′ particles move and displace the local grain boundary segment is discussed and the prerequisite conditions for its occurrence are highlighted. The practical implications of the serrated grain boundary formation are also discussed. It is suggested that modifying the existing heat-treatment cycles in some investment cast and powder processed Ni-based superalloys would improve their properties. The possibility of minimizing weld cracking in superalloys by creating serrated grain boundaries in the base metal and the heat affected zone is also discussed.     

On the Role of Grain Boundary Serration in Simulated Weld Heat-Affected Zone Liquation of a Wrought Nickel-Based Superalloy

The effects of grain boundary serration on boron segregation and liquation cracking behavior in a simulated weld heat-affected zone (HAZ) of a wrought nickel-based superalloy 263 have been investigated. The serrated grain boundaries formed by the developed heat treatment were highly resistant to boron segregation; the serrated sample contained 41.6 pct grain boundaries resistant to boron enrichment as compared with 14.6 pct in the unserrated sample. During weld thermal cycle simulation, liquated grain boundaries enriched with boron were observed at the peak temperature higher than 1333 K (1060 °C) in both unserrated and serrated samples; however, serrated grain boundaries exhibited a higher resistance to liquation. The primary cause of liquation in this alloy was associated with the segregation of the melting point depressing element boron at grain boundaries. The hot ductility testing result indicated that the serrated grain boundaries showed a lower susceptibility to liquation cracking; the grain boundary serration led to an approximate 15 K decrease in the brittle temperature range. These results reflect closely a significant decrease in interfacial energy as well as a grain boundary configuration change by the serration.     

Effect of the Charging Temperature on the Hot Ductility of Nb‐Containing Steel in the Simulated Hot Charge Process

In order to develop a comprehensive understanding of the effect of hot charging temperature on the hot ductility of a Nb‐containing steel, direct hot charging process was simulated by using a Gleeble thermo stress/strain machine. Three kinds of thermal histories were introduced to assess the hot ductility of the steel during continuously cast, hot charging, and cold charging process by means of hot tensile test in relation to surface cracking of hot charging processed steel slabs. The ductility of the specimens charged at the temperature within the range of ferrite/austenite two‐phase region and charged at the temperature just below the A_r1 of the steel is largely reduced. These results can be ascribed to the retained ferrite films at the boundaries of austenite encouraging voiding at the boundaries and these voids gradually link up to give failure around 750°C, and the combination of inhogeneous austenite grain size and precipitations aggravating the ductility trough by encouraging grain boundary sliding at 950°C. The steel via the conventional cold charge process experienced a complete phase transformation from austenite to ferrite and pearlite structure during the cooling to the ambient temperature. This steel can be charged into a reheating furnace and rolled without experiencing hot embrittlement due to the recrystallization and the precipitates are trapped inside a newly formed grain of austenite. In comparison with the hot ductility results, the hot tensile strength is only slight influenced by the charging temperature.     

Microstructural damage and residual mechanical properties in helium-bearing gas metal Arc weldments

The influence of entrapped helium on microstructural damage and residual mechanical properties subsequent to applying low-penetration gas metal arc (GMA) weld overlays was examined for an AISI Type 304 stainless steel. Two helium levels were examined: 22.5 and 85.0 atomic parts per million (appm) He. Detailed scanning electron microscopy (SEM) revealed the presence of intergranular cracks in the weld heat-affected zone (HAZ). The crack surfaces exhibited a dimple structure that was characteristic of a gas bubble embrittled material. Transmission electron microscopy (TEM) revealed that the size and spacing of the grain boundary helium gas bubbles remained virtually unchanged (relative to that established by the charging and aging procedure) at distances greater than 1 mm from the fusion line. Within this first millimeter, the diameter of the bubbles increased rapidly, and the bubble spacing increased to the characteristic spacing of the dimples that decorated weld-induced cracks. Mechanical testing revealed a loss in strain-to-fracture and ultimate tensile strength (UTS) at the higher helium level. While the majority of the fracture occurred in a transgranular, ductile manner, some deformation-induced intergranular cracking was observed. This cracking occurred over a very narrow region localized to the HAZ of the weldment. At the lower helium level, ductility and strength were unaffected compared to helium-free specimens.     

Effects of grain boundary chemistry on the intergranular cracking behavior of Ni-16Cr-9Fe in high-temperature water

An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr-9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creepinduced grain boundary void formation and fracture.     

Effects of grain boundary chemistry on the Intergranular Cracking Behavior of Ni-16Cr-9Fe in High-Temperature Water

An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr- 9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creep- induced grain boundary void formation and fracture.