Diffusion controlled stress rupture of polycrystalline materials

The high temperature stress rupture process in polycrystalline solids is examined in terms of grain boundary parting by vacancy condensation at the tip of a crack within the boundary. The gradient for diffusion of these defects is considered in the model to arise from the stress gradient in advance of the flaw tip. A rupture stress function is derived which is shown to closely describe the stress rupture behavior of a number of metals and high temperature alloys.     

Effects of Dendritic Orientation on Stress Rupture Properties of DD6 Single Crystal Superalloy

 DD6 single crystal superalloy slabs were prepared with seed method in the directionally solidified furnace with high temperature gradient. The transverse stress rupture properties and fracture behaviour of the alloy at 760 ℃/758 MPa, 850 ℃/550 MPa and 980 ℃/250 MPa were investigated and compared with those of longitudinal specimens. The transverse stress rupture lives are corresponding with the longitudinal stress rupture lives at 760 ℃/758 MPa and 850 ℃/550 MPa. The transverse stress rupture lives are slightly less than the longitudinal stress rupture lives at 980 ℃/250 MPa. The fracture mechanism of the transverse stress rupture of the alloy at 760 ℃/758 MPa shows quasi-cleavage mode and the fracture mechanism at 980 ℃/250 MPa shows dimple mode, while the fracture mechanism at 850 ℃/550 MPa shows quasi-cleavage and dimple mixture mode. At higher temperature and lower stress, the microcracks are easier to initiate and interconnect in the transverse specimen than those in longitudinal specimen because there are interdendritic regions perpendicular to the axis of stress.     

超超临界火电机组用T23钢的高温蠕变断裂行为

 通过研究和分析超超临界火电机组用T23钢持久试样的断口形貌及其在600℃高温蠕变过程中的组织演变,探讨了不同应力水平下T23钢的蠕变断裂机制。研究结果表明,T23钢在高应力条件下的蠕变断裂机制类似于常温下典型的韧性断裂,蠕变空洞主要形核于晶内的夹杂物处;而在低应力条件下的蠕变断裂机制表现为脆性沿晶断裂,蠕变空洞则主要形核于晶界第二相处。     

Effect of stress and temperature on the creep and rupture behavior of a 1.25 Pct chromium—0.5 Pct molybdenum steel

The creep and stress rupture behavior of a normalized 1.25 pct chromium-0.5 pct molybdenum steel has been investigated over a temperature (T) range of 510 to 620°C and a stress(σ) range of 65 to 425 MN/m². The creep rate ( ) and time to rupture (t _r ) data have been analyzed in terms of the general expression ort _r -A σⁿ exp (Q/RT), whereA is a constant,n is the power exponent of stress,Q is an empirical activation energy for the rate controlling process andR is the universal gas constant. At each temperature, the logarithmic plots of creep rate and time to rupture as functions of stress consist of two linear segments, separating the data into low stress and high stress regimes. The stress exponent has approximate values of 4 and 10 in the low stress and high stress regimes respectively in the appropriate expressions for both creep rate and for time to rupture. The activation energy has values of 367 and 420 kJ/mole in the low stress regime for time to rupture and creep rate respectively. In the high stress regime, the respective values of activation energy are 581 and 670 kJ/mole. Fractographic observations show that the changes from low stress to high stress behavior in creep rate and time to rupture approximately coincide with the transition in fracture mode from intergranular to transgranular cracking as well as with the transition in the rupture ductility from a region of linear variation with stress to one of constant ductility. These observations suggest that the transition from low stress to high stress behavior may be associated with a change in deformation mode from predominantly grain boundary sliding at low stress to transgranular matrix deformation at high stress. Analysis of the creep rate data based on this premise enables calculation of the ratio of the contributions of the grain boundary sliding mode to the total deformation (ε _gb /ε _T ) at various values of stress and temperature. Results of this analysis are consistent with numerous experimental observations reported in the literature.     

Effects of crack tip stress states and hydride-matrix interaction stresses on delayed hydride cracking

A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature. For test temperatures approached from below, factors such as the hydridematrix accommodation energies, the stress state at the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward and inside the plastic zone boundary with increasing temperature. At lowK _I values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition forK _IH . For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above parameters become less evident.     

Stress Rupture Fracture Model and Microstructure Evolution for Waspaloy

Stress rupture behavior and microstructure evolution of nickel-based superalloy Waspaloy specimens from tenon teeth of an as-received 60,000-hour service-exposed gas turbine disk were studied between 923 K and 1088 K (650 °C and 815 °C) under initial applied stresses varying from 150 to 840 MPa. Good microstructure stability and performance were verified for this turbine disk prior to stress rupture testing. Microstructure instability, such as the coarsening and dissolution of γ′ precipitates at the varying test conditions, was observed to be increased with temperature and reduced stress. Little microstructure variation was observed at 923 K (650 °C). Only secondary γ′ instability occurred at 973 K (700 °C). Four fracture mechanisms were obtained. Transgranular creep fracture was exhibited up to 923 K (650 °C) and at high stress. A mixed mode of transgranular and intergranular creep fracture occurred with reduced stress as a transition to intergranular creep fracture (ICF) at low stress. ICF was dominated by grain boundary sliding at low temperature and by the nucleation and growth of grain boundary cavities due to microstructure instability at high temperature. The fracture mechanism map and microstructure-related fracture model were constructed. Residual lifetime was also evaluated by the Larson–Miller parameter method.     

9CrMoWV耐热钢的高温持久蠕变性能及组织行为研究分析

为了表征9CrMoWV耐热钢在实际应用中的力学行为,对9CrMoWV钢在不同温度和应力条件下进行持久试验,并对持久试样进行OM、SEM、TEM观察,分析不同温度、应力和持久试验时间对9CrMoWV钢显微组织和宏观、微观断口的影响,同时统计Laves相的尺寸变化及其他第二相分布情况。结果表明,随着试验时间和温度的增加,断口韧窝尺寸增大,板条状马氏体回复程度增大,Laves相的尺寸变化明显,分布于晶界的M₂₃C₆与Laves相均发生不同程度聚集粗化,这是高温低应力区试样断裂的原因之一。     

Strengths and failure mechanisms of a Co-15Cr-13TaC directionally solidified eutectic alloy

The tensile and stress rupture properties of a Co(Cr)-TaC directionally solidified eutectic alloy have been investigated and compared to those of a single phase, directionally solidified Co(Cr) alloy corresponding in composition to that of the eutectic matrix. The temperature for 100 h stress rupture life at 20,000 psi (138 MN/m²) is about 200°F (111°C) better than that of any cast nickel-base superalloy now used in aircraft or land gas turbines. The degree of superiority becomes progressively less at higher stresses, and at 50,000 psi (345 MN/m²), the temperature for 100 h stress rupture life in the eutectic is about 150°F (83°C) less than for several high strength superalloys. This behavior is related to a bimodal stress rupture mechanism. A model predicts that at low stresses, failure is controlled by the stress rupture behavior of the matrix; and at high stresses failure occurs by a stress relaxation mechanism which causes early fiber failure. Fractographic observations are in agreement with the existence of two stress rupture mechanisms. It was also observed that both stress rupture mechanisms can occur at the same temperature, with specimens failing by the fiber-related mode having lives 1 to 2 orders of magnitude in time less than those which fail by the matrix-related mode at the same stress level. Both authors were associated with the General Electric Corporate Research and Development Center, Schenectady, N. Y. at the time this study was performed     

Effect of Hf on Stress Rupture Properties of DD6 Single Crystal Superalloy After Long Term Aging

 The specimens of the second generation single crystal superalloy DD6 with different Hf contents were prepared in the directionally solidified furnace with a high temperature gradient. The long term aging of the specimens after full heat treatment was performed at 1040 ℃ for 800 h. The effect of Hf on the microstructure and stress rupture properties under 980 ℃/250 MPa of the alloy after long term aging was investigated. The results show that the γ′ coarsening and rafting and no topologically close packed phase (TCP) are observed in the microstructures of DD6 alloy with different Hf contents after aged at 1040 ℃ for 800 h. It indicates that DD6 alloy with different Hf contents all possesses good microstructure stability. With increasing Hf content the rupture life after long term aging turns shorter and the elongation represents the increasing first and decreasing afterwards. The fracture mechanism of the alloy with different Hf contents at 980 ℃/250 MPa all shows dimple model. The influence of the microstructures on the stress rupture properties of the alloy is also discussed.     

A theoretical model for creep crack growth

A theory of creep crack growth has been developed with the presumption that the crack growth occurs by the diffusion of vacancies along the grain boundaries. This is consistent with many experimental results that show that creep fracture is generally of intergranular type and the activation energies for crack growth rates fall within the range of grain boundary diffusion energies. The theory is based on the concept that creep crack growth results from a balance of two competing processes-the diffusion of point defects that contributes to the growth and the creep deformation process that retards the growth and causes even its arrest. The present analysis shows that crack growth via grain boundary diffusion occurs within some temperature range. The upper limiting temperature is determined by the bulk diffusion process which disperses the vacancies, that are diffusing to the crack tip, to the plastic zone ahead of the crack front. The lower temperature limit is set by the fact that the grain boundary diffusion rates decrease with the decrease in temperature and thus large stress intensities approaching the fracture toughness value are required to accomplish crack growth by the grain boundary diffusion. Outside these limits creep crack growth occurs via deformation which is significantly slower than growth by the grain boundary diffusion process. The importance of the present analysis rests on the fact that service conditions for many high temperature structural materials fall within the regime wherein creep crack growth occurs via grain boundary diffusion.     

Effect of hydride precipitation on the elastoplastic stress field near a crack tip

Dissolved hydrogen in zirconium diffuses up the hydrostatic stress gradient and forms hydride platelets in the region of high stress. The hydride formation is accompanied by an expansion. The effect of hydride expansion on the elastoplastic stress, strain and displacement fields, is investigated in this paper by a finite-element discretization technique. It is shown that hydride formation causes an elastic unloading in the crack tip (reduction of the peak stress) and appearance of a peak stress at the front end of the hydride. Further unloading occurs along the hydride platelet, and depending on the hydride expansion, reversed plastic deformation may take place. Effects of the hydride length, its location with respect to the crack tip, and geometry of the front end extremity of the hydride, are also investigated.     

退火对GaAs窗口晶体断裂模数的影响

LEC GaAs晶片经高温退火后，残余应力得以部分释放；从而减小残余应力诱生断裂的可能性，提高了GaAs晶体的断裂模数。原生GaAs晶片加工的样品的断裂模数平均值约为135MPa，经退火的GaAs晶片加工样品的断裂模数平均值更高，约为150MPa，断裂模数最高值达163MPa。     

Stress-corrosion cracking of sensitized type 304 stainless steel in thiosulfate solutions

The stress corrosion cracking of a sensitized Type 304 stainless steel has been studied at room temperature using controlled potentials and two concentrations of sodium thiosulfate. In both constant extension rate and constant load tests, the crack velocities attain extremely high values, up to 8 μm s^-1. Scratching electrode experiments conducted at various pH values on simulated grain boundary material show that both the crack initiation frequency and crack velocity are closely related to the repassivation rate of the grain boundary material as expected on a dissolution-controlled mechanism; however, the maximum crack velocity at any potential is consistently about two orders of magnitude higher than that predicted from the electrochemical data. Frequent grain boundary separation ahead of the crack tip is thought to occur, but retarded repassivation of the grain boundary material is a necessary feature of the cracking. Effects of strain-generated martensite are discussed.     

The brittle-to-ductile transition in doped silicon as a model substance

In order to investigate the importance of the dislocation velocity for the brittle-to-ductile transition temperature, fracture experiments were performed on precleaved, dislocation-free silicon single crystals at elevated temperatures. The well known doping dependence of the dislocation velocity in silicon is used to obtain detailed information about the conditions in the plastic zone. At high temperatures, dislocations are generated at an applied stress intensity distinctly lower than the critical stress intensity for inducing cleavage in the dislocation-free samples at room temperature. As a consequence of this, the plastic zone which develops around the crack tip is saturated before the tensile stress at the crack tip reaches the cohesive stress. Because of saturation of the plastic zone, the brittle-to-ductile transition temperature is determined by the velocity at which dislocations move outward from the crack tip at low shear stresses existing in the vicinity of the shielded crack.     

Stress Corrosion Cracking Behavior at Inconel and Low Alloy Steel Weld Interfaces

Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed.     

Modeling on Dendrite Growth of Medium Carbon Steel during Continuous Casting

Dendrite growth is an important phenomenon during steel solidification. In the current paper, a numerical method was used to analyse and calculate the dendrite tip radius, dendrite growth velocity, liquid concentration, temperature gradient, cooling rate, secondary dendrite arm spacing, and the dendrite tip temperature in front of the solid/liquid (S/L) interface for the solidification process of medium carbon steels during continuous casting. The current model was well validated by published models and measurement data. The results show that in the continuous casting process, the dendrite growth rate is dominated by the casting speed. Dendrite growth rate, liquid concentration at the S/L interface, temperature gradient and cooling rate decrease with proceeding solidification and solid shell thickness growth, while other parameters such as dendrite tip radius, secondary dendrite arm spacing, and dendrite tip temperature in front of the S/L interface become larger with solidification progress and solid shell thickness growth. Parametric investigations were carried out. The effects of the stability coefficient, temperature gradient and casting speed on the micro‐structural parameters were discussed. Under the same conditions, higher casting speed promotes coarser secondary dendrite arm spacing and enlarges the dendrite tip radius, while decreasing temperature gradient, reducing the dendrite growth rate and making the solute distribute more uniform.     

高压强脉冲电流对金属裂纹的止裂效果

如何产生高强度脉冲电流是应用电磁热效应遏制金属裂纹扩展的一个关键问题.介绍了高压脉冲放电装置的工作原理,从理论、数值模拟、实验三个方面,通过对不同尺寸的裂纹试件导入强脉冲电流而产生的电磁热效应的止裂效果进行了分析和研究,研究结果表明:强脉冲电流产生的电磁热效应,能够使裂纹尖端处熔化形成微小的焊口,遏制了裂纹的扩展.试件的通流尺寸、电阻率、导入电流密度是影响裂纹尖端温度场和温度梯度场的主要因素.     

The high temperature creep behavior of doped tungsten wire

The creep behavior of 0.018 cm diam doped tungsten wire has been studied over a range of stress from 30 to 90 MPa and temperature from 2400 to 2800 K. Grain aspect ratio (gar) had a strong influence on creep and rupture of the recrystallized wires, and separated the creep behavior into two regimes with a transitional gar of about 11 between the two. The low gar regime showed lower strength and characteristics typical of grain boundary sliding. In the high gar regime, properties were independent of gar, and evi-dence is presented to show that creep is governed by dislocation-bubble dispersion strengthening. Formerly with the Lamp Business Division of General Electric Company, E. Cleveland, Ohio     

High temperature deformation and fracture mechanisms in a dendritic Ni3Al alloy

The mechanisms that control high temperature deformation and rupture were studied in a Ni₃Al alloy that was thermo-mechanically treated to produce a non-porous dendritic grain structure. Comparisons of data corresponding to the dendritic grain morphology with that for the equiaxed grain structures indicate that the dendritic morphology results in significantly lower creep rates as well as substantially greater times to rupture. Comparison of the data with numerical calculations suggests that this difference in creep strength is due to an inherent resistance to grain boundary sliding by the dendritic grain structure. A constrained cavity growth model was adapted based on microstructural observations to account for cavitation within the dendritic microstructure. The success of the model indicates that rupture time is primarily determined by constrained cavity growth on isolated dendrite boundary segments.