High temperature creep cavitation mechanisms in a continuously cast high purity copper

Continuously cast high purity copper was used to study intergranular high temperature creep fracture mechanisms. With the help of an internal marker system due to impurity segregation, grain boundary sliding, GBS, was found to have occurred to a similar extent on cavitated and uncavitated boundaries. To explain this phenomenon a void nucleation model involving small nonwetting shearable particles is suggested. Metallographic observations and the apparent activation energy derived from fracture time data indicate the operation of the vacancy condensation mechanism at the lower temperatures and higher stresses. At the higher temperatures and lower stresses void growth is enhanced by GBS. This cavitation mechanism obtains strong support from measurements of the distribution of voids on grain boundaries as a function of the boundary angle with respect to the tensile direction. Computer analysis of these distributions, in terms of a model which properly accounts for the distribution of potential nuclei, yields bimodal curves exhibiting peaks at grain boundaries oriented for high shear stress (peak I), and for high normal stress (peak II). A phenomenological equation is proposed for the dependence of peak I on test conditions. Peak II is thought to be caused by nucleation by local GBS and growth by vacancy condensation under locally enhanced normal stress. A. RUKWIED, formerly Physicist, Engineering Metallurgy Section, Metallurgy Division, National Bureau of Standards, U. S. Department of Commerce, Washington, D. C.     

A numerical study of cavity growth controlled by coupled surface and grain boundary diffusion

The diffusive growth of both two dimensional and axisymmetric cavities initially having equilibrium shapes and located on grain boundaries loaded in tension is studied using finite difference techniques. The shape evolution and growth kinetics of individual cavities as well as the time required for adjacent cavities to grow together is studied as a function of applied stress and the ratio of grain boundary to surface diffusivity. A key feature of this treatment is that the diffusional processes in the grain boundary and on the cavity surface are coupled by boundary conditions at the tip of the cavity. When surface diffusion is much slower than grain boundary diffusion, the cavities become crack-like during growth, and the fracture time varies reciprocally with the third power of the applied stress. When grain boundary diffusion is the slower process, the cavities remain rounded during growth, and the fracture time varies reciprocally with the first power of the stress. The transition between these limiting kinds of behavior is described and the results are compared with previous treatments of these problems.     

Tempaloy AA-1钢时效组织结构及力学性能分析

 通过进行650 ℃高温时效试验,研究了Tempaloy AA-1钢在时效过程中M23C6合金相的析出与室温力学性能和冲击韧性的关系,并利用扫描电镜和透射电镜对材料在不同时间时效后Tempaloy AA-1的组织结构和室温冲击断口进行观察。结果表明：随着Tempaloy AA-1钢时效时间的增加,逐渐析出M23C6合金相,并稳定地分布在晶界处。Tempaloy AA-1时效500 h后,由于晶界处M23C6碳化物强化作用,屈服强度与抗拉强度达到最大值;时效8 000 h后,晶界处M23C6碳化物颗粒聚集增大,晶界强化作用弱化, Tempaloy AA-1钢的强度略有降低。长期时效后M23C6 型碳化物没有发现其过度团聚,因此,Tempaloy AA-1钢仍具有良好的时效冲击韧性。Tempaloy AA-1时效后具有良好的力学性能和冲击韧性是由于材料在长期高温时效后具有良好的组织稳定性。     

高氮不锈轴承钢碳化物分布与高温断裂机制

 通过高温拉伸试验研究高氮不锈轴承钢高温断裂行为,探究了170 ℃和470 ℃回火态钢中碳化物分布特征,分析了高温拉伸断裂及组织演变和碳化物分布规律。研究发现,回火温度从170 ℃升高至470 ℃,高氮钢中大于0.8 μm的碳化物明显增加,高氮钢中M₂₃C₆强化增量提高了2.59 MPa,固溶强化增量下降了118.82 MPa,470 ℃回火态钢的室温抗拉强度降低、拉伸断口表现为准解理和少量撕裂韧窝;拉伸温度升高至300 ℃,试样断口表现为等轴型韧窝特征,170 ℃和470 ℃回火态试样起裂源断裂碳化物尺寸分别为2.8~3.6 μm和5.5~6.7 μm;450 ℃拉伸断口表现为塑孔韧窝特征,170 ℃和470 ℃回火态试样起裂源断裂碳化物尺寸分别为2.7~3.4 μm和5.8~6.4 μm。拉伸温度从300 ℃提高至450 ℃,钢的固溶强化和位错强化作用减弱,金属原子间结合能下降,碳化物与基体不连续应力分布加剧变形不协调性,碳化物承担较高应力而发生断裂。单纯热作用下钢中0.5~0.8 μm尺寸碳化物数量比例增加;在热力耦合作用下,钢中应力所导致的位错增殖为碳元素扩散提供通道,钢中碳化物在晶界和位错线上形核析出0.2~0.8 μm碳化物。裂纹沿着与拉伸方向45°角的最大剪力方向快速扩展而断裂,最终形成锯齿状的断口,小尺寸碳化物增多阻碍位错滑移导致塑性降低;钢中大尺寸碳化物不均匀分布在碳化物间形成大变形塑孔而增加钢的塑性。     

The effect of external stress on the discontinuous precipitation in an AlZn alloy at high and low temperatures

The effect of external tensile stress on continuous precipitation (DP) has been investigated in an Al21.8 at.% Zn alloy at high (215°C) and low (75 and 50°C) temperatures. The ratio of the macroscopic lattice diffusivity, D, to the DP boundary velocity, v, (D/v) is estimated to be larger than the interatomic spacing, λ, at the high temperature, and smaller than λ at the low temperatures. Under tensile stresses, the DP rates are enhanced at the grain boundary segments oriented transverse to the stress direction and suppressed at those oriented parallel to it at both high and low temperatures. Furthermore, Yi and Park show the DP rate changing continuously with temperature over the range where D/v increased from values much smaller than λ to those much larger. These results show that the diffusional coherency strain is the major driving force for DP even at low temperatures where, with D/v < λ, no solute diffusion is usually assumed to occur in front of the moving DP boundaries.     

Stress calculation in roasted iron-ore pellets on cooling

A method is proposed for calculating the stress in roasted zonal fluxed pellets. The method is based on the appearance of a stress state at the boundary zones with different solidification temperatures of the slag inclusions on cooling. The stress state is calculated for Kachkanar pellets with different positions of the hematite and magnetite zones. It is shown that, on cooling annealed zonal pellets in air, tensile normal stress appears in the radial direction at the boundary of the magnetite core and the hematite shell. The presence of a three-zone structure in the pellets (magnetite core + hematite shell + surface magnetite film) facilitates the redistribution of normal stress over the cross section. Increasing the thickness of the surface magnetite film reduces the tensile stress in the radial direction at the boundary of the core and the shell. As a result, the pellet strength is increased. On cooling roasted pellets with a hematite core and a magnetic shell in a neutral atmosphere, tensile normal stress appears in the tangential direction, tending to reduce the pellet strength.     

Theoretical modeling of gain boundary diffusion of hydrogen and its effect on permeation curves

A new grain boundary model is proposed. The model can be utilized to calculate the contribution of grain boundary diffusion to total mass transport in metals where lattice diffusion occurs interstitially and where the ratio of grain boundary diffusivity to that lattice is expected to be close to unity. The grain boundary segregation factor, small grain sizes and a large range of the ratio of grain boundary to lattice diffusivity can be accommodater. An analytical solution has been obtaine and applied to electrochemical permetion tests to give predictions of permeation rates enhanced by diffusion through grain boundaries. It is concluded that under most circumstances an average grain size of 10 μm can be used to determine if the transport of hydrogen in metals is enhanced by grain boundary diffusion.     

Solution-precipitation creep in glass ceramics

The deformation rate of polycrystalline ceramics that contain a residual glass phase is analysed in terms of material transport through the liquid phase. Molecules are transported and deposited in the direction of the positive normal traction gradient along the interfaces. This produces a change in the shape of the grains such that they become elongated in the direction of the tensile stress. It is suggested that the steady state kinetics of this process is controlled either by the rate of dissolution and precipitation reaction at the glass-crystal interfaces, or by the rate of transport through the liquid phase. An island structure of the grain boundary is assumed to model the atomistics of the transport process. The results of creep experiments in a β-spodumene glass ceramic are shown to be in agreement with the model for interface reaction controlled creep.     

Hydrogen induced grain boundary fracture in high purity nickel and its alloys—Enhanced hydrogen diffusion along grain boundaries

Embrittlement of nickel by solute hydrogen is usually accompanied by a change in fracture mode from ductile rupture to an intergranular mode. When hydrogen is supplied at the external surface, the kinetics of this embrittlement is shown to be more rapid than can be accounted for by lattice diffusion of hydrogen. The embrittlement kinetics are shown to be consistent with hydrogen diffusion along grain boundaries and the grain boundary diffusivity is derived over the temperature range 274–314 K. The effects of carbon and sulfur grain boundary segregation on the kinetics of grain boundary embrittlement were also investigated. Segregation of carbon decreases the extent of embrittlement while sulfur segregation increases the amount of embrittlement relative to that observed in pure nickel. These effects are interpreted in terms of the effects of segregated solutes on hydrogen grain boundary diffusivity and on the critical hydrogen concentration for intergranular fracture.     

The growth of grain boundary cavities under applied stress and internal pressure

Experiments were performed to investigate the growth of grain boundary cavities in nickel under internal pressure and applied stress. Under the experimental conditions the growth kinetics was found to be controlled by stress induced mass transfer. The growth rate was a nonlinear function of the applied stress and was found to depend on grain boundary orientation and grain boundary sliding. JAMES F. MANCUSO, formerly Graduate Student, Cornell University     

Isolation of carbon and grain boundary carbide effects on the creep and intergranular stress corrosion cracking behavior of Ni-16Cr-9Fe-xC alloys in 360 °C primary water

The influence of carbon and grain boundary carbides on intergranular stress corrosion cracking (IGSCC) of controlled-purity Ni-16Cr-9Fe-xC alloys in 360 °C primary water was investigated using constant load tensile (CLT) and constant extension rate tensile (CERT) tests. The CLT test results confirmed that carbon in solution decreases the creep rate by several orders of magnitude, while grain boundary carbides serve to increase the creep susceptibility. Although carbon increases the work hardening rate, it is demonstrated, using the Bailey-Orowan creep model, that the primary effect of carbon in solution is to delay the recovery process of climb at the grain boundary, thereby reducing the creep rate. Grain boundary carbides produce a negligible contribution to the internal stress and may increase the creep rate by acting as dislocation sources. Grain boundary carbide precipitation increases IGSCC resistance in 360 °C primary water containing 0, 1, and 18 bar hydrogen, providing the highest overall resistance to both environmentally induced creep and cracking. The magnitude of the beneficial effect of grain boundary carbides is extremely sensitive to hydrogen overpressure, with the largest influence observed for 1 bar hydrogen. The detrimental effect of hydrogen on IGSCC shows consistencies with aspects of both film rupture/slip dissolution and hydrogen embrittlement models.     

Grain boundary void nucleation in astroloy produced by room temperature deformation and anneal

Dyson and co-workers have shown that the creep life of a nickel base superalloy can be greatly shortened if the material is strained at room temperature before the creep test is carried out. They found that a prestrain followed by a short annealing time produces small grain boundary cavities, and it is the presence of these prenucleated voids which so seriously degrades service life at elevated temperatures. The present work explores the relationship between microstructure and prestrain void nucleation. Samples of the nickel base superalloy astroloy were given various heat treatments which led to significantly different microstructures. It was found that voids resulting from a prestrain-anneal treatment form preferentially at the ends of carbides on grain boundaries oriented roughly parallel to the prestrain tensile axis or rolling direction. Void spacing in the various microstructures is proportional to (but larger than) carbide spacing. The growth of these cavities during annealing is attributed to the presence of tensile residual stresses arising from the difference in deformability between grain boundary regions and the relatively soft matrix. Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL     

Microstructural stability of thermal-mechanically pretreated type 316 austenitic stainless steel

Studies of the microstructural stability of Type 316 austenitic stainless steel were performed for a wide range of thermal-mechanical pretreatments in the limited aging temperature range of 550° to 760°C. The pretreatments were selected in order to investigate the effects of varying solution treatment temperature, amount of cold reduction by rolling, initial grain size, and initial precipitate distribution. Large variations in both phase stability and recrystallization behavior can be effected by appropriate pretreatments. Cold work accelerates precipitation of M₂₃C₆ carbide and the intermetallic compounds (Laves, χ, and σ phases). Both the amount and kinetics of σ phase formation are especially enhanced by recrystallization occurring in the aging temperature range. It is suggested that this occurs due to ready σ nucleation at slowly moving (recrystallizing) grain boundaries together with enhanced growth rates due to diffusion along the boundary. Fine grain size enhances phase instability by providing additional nucleating sites and decreased diffusion paths for precipitate forming elements, but in the grain size range studied (ASTM No. 3.5 to No. 13) the effect is not as significant as the effect of cold work, particularly when recrystallization occurs during the aging treatment. Fine grain size and pretreatments which precipitate the carbides prior to the final cold working step enhance recrystallization kinetics relative to solution treated and cold-worked materials. This is apparently due to stabilization of the cold-worked substructure in the solution treated samples by precipitation of carbide and Laves phases on the dislocations and stacking faults.     

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M₂₃C₆ type carbides, having high chromium content, and M₆C type carbides, having high molybdenum content, co-exist on the grain boundaries. M₂₃C₆ type carbides, however, are quantitatively predominant. Furthermore, M₆C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.     

High temperature creep damage in steels and superalloys

The nucleation and growth of cavities was examined in steel bicrystals (Fe-3%-Si, X 8 CrNiNb 16 13) and in the ODS superalloy Inconel MA 754 (Inconel MA 754 (78% Ni; 20% Cr; 0.5% Ti; 0.3% Al; 0.6% Y₂O₃). Cavity density distributions were measured on metallographic sections and on cleaved grain boundaries as a function of time, strain, temperature and stress. Nucleation and growth laws were obtained by evaluating the distributions with appropriate models. For the fcc and bcc bicrystals, it was found that cavities nucleated continuously at sulfide and carbide particles during creep. They grew by grain boundary diffusion. But the growth rate was delayed with increasing creep strain due to cavities which nucleated in the surroundings of existing cavities. For the ODS alloy, however, many round cavities preexisted on quasi-boundaries consisting of the aggregate of coarse oxide and carbide particles. They grew initially by diffusion, but with increasing creep time (cavity size), the growth mechanism switched from growth controlled by grain boundary diffusion to growth controlled by power law creep. Implications for life predictions are discussed.     

Mechanisms of deformation-induced grain boundary chromium depletion (sensitization) development in type 316 stainless steels

Deformation accelerates the development of grain boundary chromium depletion (GBCD), or sensitization, in type 316 austenitic stainless steels (SS). Quantitative assessment of the degree of sensitization (DOS) using the electrochemical potentiokinetic reactivation (EPR) test indicates that the acceleration in GBCD is a function of the amount of strain in the material and temperature of isothermal sensitization treatment. A systematic increase in strain from 0 to 20 pct yields a continuous increase in EPRDOS values below 700°C, while at higher temperatures, a threshold strain of 6 to 10 pct is required to cause accelerated GBCD development. Straining SS above 20 pct also produces higher amounts of chromium depletion, though the (intergranular) sensitization susceptibility of the material could not be quantitatively evaluated due to the presence of grain matrix or transgranular corrosion. Classical C-curve precipitation-sensitization behavior was also noted for strained and unstrained materials, though strain moved the C-curves to the left. Microstructural evaluation of sensitization revealed a systematic increase in grain boundary and twin boundary corrosion on EPR attack surfaces with strain, which corroborated the deformation-induced acceleration of EPRDOS. A time-temperature-strain dependence of transgranular corrosion was also identified on EPR-etched samples strained above 20 pct. These were also reflected in transmission electron microscope (TEM) observations of higher grain boundary carbide precipitation on strainedvs unstrained specimens and site-specific carbide precipitation on deformation sites in the material. Kinetic and thermodynamic modeling of deformation effects on carbide precipitation and depletion development in type 316 SS indicated that strain induces a reduction in the activation barrier to diffusion (Q) _a and thermodynamic barrier to nucleation (ΔG ^*) during the precipitation-depletion process. The lowering ofQ _a with strain caused chromium diffusivity and depletion development to be accelerated in strainedvs unstrained materials and appears to be due to increased dislocation pipe diffusion with strain. Reduction of ΔG ^* with strain was related to an increase in the free energy change of the grain boundary (ΔG) _gb and accelerated carbide precipitate nucleation in deformed SS. The effect of strain on the kinetics and thermodynamics of the precipitation-depletion process decreases with increasing temperature.     

高碳铬不锈轴承钢孪晶碳化物研究

研究了高碳铬不锈轴承钢“孪晶碳化物”（直线状和链状碳化物）的影响因素及形成原因,结果表明：加热温度达到1140℃,退火后开始出现沿晶界分布的链状碳化物;加热温度≥1160℃,退火后出现大量直线状和链状两种形态的碳化物。材料从高温直接冷却时,温度≥1080℃并且冷却速度≤80℃／h可能析出链状碳化物,并且温度越高冷却速度越慢析出的可能性就越大。直线状碳化物形成原因为：材料加热温度过高．晶粒长大的过程中晶界迁移时偶然发生堆垛错误形成了生长孪晶,在随后的退火过程中碳化物向奥氏体挛晶界面沉淀而形成,是真正意义上的孪晶碳化物。链状碳化物是由于材料过热或者局部过热,在随后冷却过程中碳化物沿奥氏体晶界析出而形成的,本质上是一种网状碳化物。     

Isolation of carbon and grain boundary carbide effects on the creep and intergranular stress corrosion cracking behavior of Ni−16Cr−9Fe−xC alloys in 360 °C primary water

The influence of carbon and grain boundary carbides on intergranular stress corrosion cracking (IGSCC) of controlled-purity Ni−16Cr−9Fe−xC alloys in 360 °C primary water was investigated using constant load tensile (CLT) and constant extension rate tensile (CERT) tests. The CLT test results confirmed that carbon in solution decreases the creep rate by several orders of magnitude, while grain boundary carbides serve to increase the creep susceptibility. Although carbon increases the work hardening rate, it is demonstrated, using the Bailey-Orowan creep model, that the primary effect of carbon in solution is to delay the recovery process of climb at the grain boundary, thereby reducing the creep rate. Grain boundary carbides produce a negligible contribution to the internal stress and may increase the creep rate by acting as dislocation sources. Grain boundary carbide precipitation increases IGSCC resistance in 360 °C primary water containing 0, 1, and 18 bar hydrogen, providing the highest overall resistance to both environmentally induced creep and cracking. The magnitude of the beneficial effect of grain boundary carbides is extremely sensitive to hydrogen overpressure, with the largest influence observed for 1 bar hydrogen. The detrimental effect of hydrogen on IGSCC shows consistencies with aspects of both film rupture/slip dissolution and hydrogen embrittlement models. J.L. HERTZBERG, formerly Graduate Student Research Assistant, Department of Materials Science and Engineering, University of Michigan     

Combined matrix/boundary precipitation strengthening in creep of Fe-15 Cr-25 Ni alloys

High temperature creep behavior of carbide precipitation strengthened Fe-15 Cr-25 Ni alloys with different carbon content have been investigated. Grain boundary carbides obstruct dislocation annihilation at the grain boundary and, therefore, increase the dislocation density near the grain boundary. This gives rise to formation of a hard grain boundary region and significantly increase creep resistance of the alloy. The grain boundary precipitation strengthening and combined matrix/boundary strengthening are modeled following the concept of hard-soft composite structure, and a unified creep equation is derived by taking account of back stress from intergranular carbide particles, “boundary obstacle stress”. The models and analysis show that grain boundary precipitation strengthening is predominant for soft matrix but decreases with the increase of matrix strength, indicating the existence of coupled matrix/boundary strengthening.     

Effect of surface diffusion on the creep of thin films and sintered arrays of particles

An analysis is presented that illustrates the effect of surface diffusion on the creep of a uniform, sintered array of cylinders. The analysis is also appropriate for describing the creep of thin films bonded to substrates when there is no interfacial diffusion. The first part of the paper presents an analytical solution which can be obtained when it is assumed that a constant flux of matter out of the grain boundary is maintained. This solution illustrates the important physical phenomenon behind the problem in that a finite surface diffusivity causes a back stress to be developed owing to the enhanced surface curvatures in the region of the grain boundary. In the latter portion of the paper, this analytical solution is used to generate numerical solutions for more practical boundary conditions, and to illustrate the effects of finite boundary lengths.