An upper bound on strain rate for wedge type fracture in nickel during creep

It is shown that measurements of the rate of grain boundary sliding using internal friction can be used for calculating an upper bound in strain-rate, above which wedge type of intergranular fracture will not occur. A transition in mode of fracture, from ductile-transgranular at high strain-rate to wedge-intergranular at a lower strain-rate, in Ni is adequately predicted by internal friction measurements. At a still lower strain-rate, a shift from wedge type to “r” type of intergranular fracture occurs. This transition apparently agrees well with the change in the overall creep mechanism from power-law creep to diffusional creep. C. GANDHI, formerly Research Associate, Department of Materials Science and Engineering, Cornell University, Ithaca, NY.     

A framework for modeling creep in pure metals

The process of creep in pure metals is modeled as the cooperative interaction of three phenomena: the thermally activated, force-dependent release of dislocation segments from obstacles; the substructural refinement of the microstructure due to plastic deformation; and the diffusion-controlled coarsening of the substructure. Key parameters are given as approximate generic values which can be varied. It is shown that for a wide range of parameters, the model reproduces the key features of the creep of pure metals: a steady-state stress exponent near 5 is recovered, and the key microstructural-length scale is related by a power law close to the reciprocal of stress (this dependence is not a strong function of temperature at a given stress). In addition, the activation energy of steady-state creep is nearly that of self-diffusion. Thus, the model reproduces the well-known phenomenology of puremetal steady-state creep. However, the present model is based on separate microstructural phenomena, which can be independently refined and studied. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10⁻⁴s⁻¹ to 400 s⁻¹. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.     

热交换器用铝合金复合管材的制备新技术

采用自行设计制造的包覆铸造装置成功制备出尺寸为Φ140 mm/Φ110 mm的4045/3003铝合金包覆铸锭,通过反向热挤压将包覆铸锭制备成铝合金复合管材.通过OM、SEM、拉伸实验、剪切实验对界面组织和性能进行了分析和测定.结果表明,利用该装置制备的铝合金包覆铸锭表面质量良好,界面清晰,无气孔、夹杂,界面处元素发生互扩散,并形成约20μm的过渡层,平均抗拉强度为103.3 MPa,抗剪切强度为80.2 MPa,两种合金实现冶金结合.反向热挤压后得到的复合管材,界面处保持了铸态时的层状结构特点.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al-Mg-Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.     

Liquid metal cooling: A new solidification technique

This paper describes a new and highly efficient directional solidification process using liquid metal as a coolant. A laboratory version of this process is reviewed in detail. The selection of a coolant is also discussed. Process thermal characteristics such as thermal gradient, growth rate and cooling rate are measured and compared with established directional solidification processing. Microstructural refinement of primary dendrites, secondary dendrite arms and MC carbides is demonstrated in the case of Ni-base super-alloys. Special advantages of the process, such as a lack of interdependence of growth rate and thermal gradient are discussed. Liquid Metal Cooling (LMC) offers a wide range of rate-gradient combinations and i therefore the most flexible directional solidification process discovered to date. The high levels of thermal gradient which are available make LMC a natural choice for the growth of eutectics. Alternatively, growth rates of dendritic materials can be substantially increased leading to significantly finer microstructures. Thus, LMC promises to be a highly useful process.     

Liquid metal cooling: A new solidification technique

This paper describes a new and highly efficient directional solidification process using liquid metal as a coolant. A laboratory version of this process is reviewed in detail. The selection of a coolant is also discussed. Process thermal characteristics such as thermal gradient, growth rate and cooling rate are measured and compared with established directional solidification processing. Microstructural refinement of primary dendrites, secondary dendrite arms and MC carbides is demonstrated in the case of Ni-base super-alloys. Special advantages of the process, such as a lack of interdependence of growth rate and thermal gradient are discussed. Liquid Metal Cooling (LMC) offers a wide range of rate-gradient combinations and i therefore the most flexible directional solidification process discovered to date. The high levels of thermal gradient which are available make LMC a natural choice for the growth of eutectics. Alternatively, growth rates of dendritic materials can be substantially increased leading to significantly finer microstructures. Thus, LMC promises to be a highly useful process. Formerly with Pratt & Whitney Aircraft.     

A comparison between creep of laminated aluminum and the transverse creep behavior of a unidirectional boron-aluminum composite

The transverse creep behavior of a unidirectional 30 vol pct boron/1145-0 aluminum composite material was investigated over the temperature range 573 to 773 K. The creep curve of the composite exhibited primary, steady state, and tertiary stages of creep, as did the unreinforced laminated matrix; however, the primary stage of creep was consid-erably less pronounced in the composite than in the matrix. The minimum (or steady state) creep rate of the composite was less than that of the laminated matrix alone below a transition stress Σ = 1.74 × 10⁻⁴ E whereE is Young's Modulus of aluminum. Above this transition stress, the minimum creep rate of the composite exceeds that of the unreinforced matrix; further, the strain to failure of the composite generally decreased when the applied stress was above the transition stress. The temperature dependence of the minimum creep rate was the same for both the composite and the laminated matrix. Failure in the composite was initiated by debonding of the filament-matrix interface and it is suggested that debonding of the interface contributes in an additive way to the creep of the composite and to a greater extent at high stresses, leading to the transition stress observed. Formerly graduate student     

A comparison between creep of laminated aluminum and the transverse creep behavior of a unidirectional boron-aluminum composite

The transverse creep behavior of a unidirectional 30 vol pct boron/1145-0 aluminum composite material was investigated over the temperature range 573 to 773 K. The creep curve of the composite exhibited primary, steady state, and tertiary stages of creep, as did the unreinforced laminated matrix; however, the primary stage of creep was consid-erably less pronounced in the composite than in the matrix. The minimum (or steady state) creep rate of the composite was less than that of the laminated matrix alone below a transition stress Σ = 1.74 × 10⁻⁴ E whereE is Young's Modulus of aluminum. Above this transition stress, the minimum creep rate of the composite exceeds that of the unreinforced matrix; further, the strain to failure of the composite generally decreased when the applied stress was above the transition stress. The temperature dependence of the minimum creep rate was the same for both the composite and the laminated matrix. Failure in the composite was initiated by debonding of the filament-matrix interface and it is suggested that debonding of the interface contributes in an additive way to the creep of the composite and to a greater extent at high stresses, leading to the transition stress observed.     

Effect of creep strain on microstructural stability and creep resistance of a TiAi/Ti3ai lamellar alloy

Creep of a TiAl/Ti₃Al alloy with a lamellar microstructure causes progressive spheroidization of the lamellar microstructure. Microstructural observations reveal that deformation-induced spheroidization (DIS) occurs by deformation and fragmentation of lamellae in localized shear zones at interpacket boundaries and within lamellar packets. Deformation-induced spheroidization substantially increases the interphase interfacial area per unit volume, demonstrating that DIS is not a coarsening process driven by reduction of interfacial energy per unit volume. Creep experiments reveal that DIS increases the minimum creep rate (ε_min) during creep at constant stress and temperature; the activation energy (Q _c ) and stress exponent (n) for creep are both reduced as a result of DIS. Values ofn andQ _c for the lamellar microstructure are typical of a dislocation creep mechanism, while estimated values ofn andQ _c for the completely spheroidized microstructure are characteristic of a diffusional creep mechanism. The increase in (ε_min) associated with DIS is thus attributed primarily to a change of creep mechanism resulting from microstructural refinement.     

Martensite formation,strain rate sensitivity,and deformation behavior of type 304 stainless steel sheet

The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed. Formerly Visiting Scientist, Department of Metallurgical Engineering, Colorado School of Mines.     

Low stress and superplastic creep behavior of Zn-22 Pct Al eutectoid alloy

The temperature, grain size, and stress dependence of steady-state strain-rates were investigated for Zn-22 pct Al eutectoid alloy using double shear type specimens. Tests were performed on specimens of grain size from 1.3 m to 3.7 μm over a range of temperature from 450 to 525 K. The applied stresses were in the range between 10⁻¹ and 4.0 x 10¹ MPa with the resulting true strain-rates ranging from 10⁻⁷ to 10⁻²s⁻¹. The alloy exhibited two distinct regions of constant stress dependence with stress exponents 1 and 2.2. The transition stresses between these two regions were between 3 × 10⁻¹ and 7 x 10⁻¹ MPa. The true activation energies associated with these regions were found to be 95.9 ±2.1 and 69.9 ±2.1 kJ/mol, respectively. The grains remained equiaxed following large deformation, although there is evidence of excessive grain growth and “grain clustering.” Specimens deformed in both regions showed few or no dislocations within the grains. There was no evidence of subgrain formation or dislocation pile-up at the grain boundaries. It is possible that dislocations found during deformation were annihilated when stress was withdrawn or lost during thin foil preparation. There was clear evidence of primary stage in both regions. However, it is suggested that whereas the primary stage in Region II is due to dislocation multiplication process, at low stresses the primary stage in Region I may be due to elastic bowing of the existing dislocations in the structure. At low stresses Nabarro-Herring diffusional creep was found to be the rate controlling mechanism. At intermediate stresses, superplastic creep was found to be the dominant mechanism. The transition between these two mechanisms, as well as Coble creep and the ranges of manifestation for superplastic creep, are analyzed and the importance of the preexponential term A, in the dimensionless relation:γκT/DGb = A(b/d) ^p(Τ/G)ⁿ and the role of the concurrent grain growth are emphasized. A. K. S. YU, formerly with the Materials Science Section, Department of Mechanical Engineering, University of California. An erratum to this article is available at .     

温度对GH4169合金蠕变行为及机制的影响

在研究了温度对镍基高温合金GH4169蠕变行为及机制的影响基础之上,分析了其断口形貌和蠕变断裂机理。实验结果表明,随着蠕变温度的升高,GH4169合金的稳态蠕变速率逐渐升高,蠕变寿命显著降低,即该合金有极强的温度敏感性。蠕变过程中,γ″相长大聚集,并向δ相转变,随着蠕变温度的升高,γ″相向δ相转变速度加快,晶内的γ″相数量减少,δ相所占体积增加,尺寸增大,次生裂纹数量减少,尺寸减小。当蠕变温度为650 ℃时,断口中存在较多亮白色撕裂棱,韧窝尺寸大小不一,有少量析出物和碳化物;当温度提高到670 ℃时,韧窝尺寸减小,以浅韧窝为主,且出现解理面;当温度提高到690 ℃时,只存在少量韧窝,且δ相的数量显著增多,出现解理台阶,断裂方式为解理断裂或准解理断裂。      

Effect of strain rate on stress corrosion cracking in duplex type 304 stainless steel weld metal

The influence of strain-rate on the stress-corrosion cracking properties of wholly austenitic Type 304 base metal and duplex austeno-ferritic Type 304 weld metal in boiling MgCl₂ was investigated using constant extension rate tensile testing techniques. Transgranular SCC in both base and weld metals is preferred at low strain-rates, while intergranular cracking in the base metal and interphase cracking along the austenite-ferrite interface in the weld metal are preferred at higher strain-rates. Promotion of the intergranular stress-corrosion cracking in the base metal and “interphase-interface” stresscorrosion cracking in the weld metal with increases in strain-rate may be mechanistically analogous. Stress-induced alterations in the grain or interphase boundary defect structure may make these regions preferentially susceptible to dissolution. W. A. BAESLACK III, Lt., USAF, formerly with Rensselaer Polytechnic Institute, Troy, New York     

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10⁻⁵ s⁻¹ to 3×10⁻³ s⁻¹. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10⁻⁵ s⁻¹ and 3×10⁻³ s⁻¹ and beyond 923 K for 3×10⁻⁴ s⁻¹ strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.     

SOM法金属氧化物制取金属新技术

以MgF2-CaF2为熔盐电解质,利用固体透氧膜(SOM)法直接电解还原Ta2O5和丁TiO2制备了金属钽和钛,分析了制样压力和电解温度对阴极产物形貌的影响以及电流的变化规律.结果表明:SOM法直接制备金属钽和钛的电解速度快,电流密度高,合理的制样压力为250～332 MPa,电解电压为3.2～3.8 V,电解温度在1 373～1 432 K之间.该方法较FFC有诸多优点,用于对我国各类稀有难熔金属矿资源进行开发利用,具有好的发展前景.     

A critical strain model for the creep fracture of nickel-base superalloys

Fracture toughness samples of NIMONIC 115 were creep tested at 704°C in Mode I (tension) and Mode III (torsion) loading. In Mode III loading the rupture lives were two orders of magnitude shorter than in Mode I. The effects of loading mode are shown to agree with predictions based on a critical strain fracture model. Earlier test results with a number of different superalloys also are consistent with a strain controlled fracture model. Improved resistance to crack growth during creep at intermediate temperatures can be achieved by increasing Young’s modulus, yield strength, grain size and the critical strain value.