Enhanced densification of cavitated dispersion-strengthened aluminum by thermal cycling

We report experimental data of creep cavity shrinkage for dispersion-strengthened-cast aluminum with about 23 vol pct submicron Al₂O₃ dispersoids, annealed isothermally or subjected to thermal cycling without applied stress. Thermal cycling is found to increase the rate of densification by a factor of 3 to 5.5 relative to isothermal annealing at the upper cycling temperature, allowing for recovery of full theoretical density in a shorter time. Isothermal densification is discussed in light of a diffusive cavity shrinkage mechanism, and a model considering thermal mismatch stresses is employed to rationalize the enhanced rate of densification observed during thermal cycling. Intermittent thermal-cycling densification is shown to improve creep life of dispersion-strengthened aluminum through the suppression of tertiary damage accumulation processes.     

Grain refinement through thermal cycling in an Fe-Ni-Ti cryogenic alloy

A thermal cycling technique which allows the grain refinement of Fe-12 Ni-0.25 Ti alloy from 40∼60 μm (ASTM #5∼6) to 0.5 ∼ 2 μm (ASTM #1∼18) in four cycles has been developed. The process consists of alternate annealing in γ range and (α + γ) range with intermediate air cooling. The transformation behavior, the change of microstructures and cryogenic mechanical properties on each cycling step are described. Due to the ultrafine grain size, the ductile-brittle transition temperature of this ferritic alloy in Charpy impact testing was suppressed below 6 K. In fracture toughness testing at 77 K, the mode of fracture was altered from brittle quasi-cleavage to complete ductile rupture through the grain refining.     

Evidence of transformation bursts during thermal cycling of a Pu-Ga alloy

The thermodynamics and kinetics of the fcc (delta) to monoclinic (alpha-prime) phase transformation and its reversion in a plutonium-gallium alloy have been studied using differential scanning calorimetry, resistometry, and dilatometry. Under ambient conditions, the delta phase is metastable in a Pu-2.0 at. pct Ga alloy. Thermal cycling to below the ambient temperature results in a partial transformation to the alpha-prime phase; this transformation is composition-invariant and exhibits martensitic behavior. Because this transformation results in an unusually large 25 vol pct contraction that cannot be fully accommodated by purely elastic adjustments, the transformation mode is expected to involve burst formation of individual alpha-prime particles. However, upon cooling, these individual bursts were not resolved by the above techniques, although signals corresponding to the overall accumulation of many alpha-prime particles were observed. On the other hand, upon heating, signals from differential scanning calorimetry, resistometry, and dilatometry showed a series of discrete changes occurring in periodic increments beginning at approximately 32 °C. These features correspond to the cooperative reversion of many alpha-prime particles to the delta phase; they appear to be the result of an interplay between the autocatalytically driven reversion of a cascade of individual martensite units and self-quenching caused by small changes of temperature or stress accompanying each individual transformation burst. The heat of the delta/alpha-prime transformation is estimated to be about +4 kJ/mol.     

Response to thermal cycling of tool materials under steel thixoforming conditions

Abstract

The principal failure mechanism of steel thixoforming dies is thermal fatigue owing to forging pressures much lower than those encountered in conventional forging. This makes a properly designed thermal fatigue test the best method to identify suitable tooling materials for the steel thixoforming environment. Samples of X32CrMoV33 hot work tool steel and CrNiCo alloy were cycled thermally between 450 and 750°C, every 60 s for a total of 1500 cycles. While the thermal stresses generated at the surfaces of the two materials were very similar, their responses to thermal cycling were markedly different. The X32CrMoV33 steel was softened by nearly 40% after only 400 cycles, raising serious concerns over its temper resistance under steel thixoforming conditions. The extensive oxidation and subsequent spalling of oxide scales suffered by the X32CrMoV33 hot work tool steel is also a major shortcoming. The performance of the CrNiCo alloy, on the other hand, was judged to be satisfactory with a much thinner heat affected zone and a much better oxidation resistance. Lack of evidence for heat checking in this alloy after 1500 cycles is an encouraging sign.     

Microstructure and thermal cycling behavior of nanostructured yttria partially stabilized zirconia (YSZ) thermal barrier coatings

Nanostructured yttria partially stabilized zirconia(YSZ) coatings were prepared by atmospheric plasma spraying(APS) using the conglomeration made by zirconia nanoparticle as the raw materials.The measurement methods,which consisted of scanning electron microscopy(SEM),transmission electron microscopy(TEM) and thermal cycling behavior,were used to character the morphology,composition and thermal oxidation behavior of the powder and the coatings.From the results,it was shown that the YSZ coating was the laminar structure,and the elements distribution in the bond and top coat were well-proportioned.The YSZ coatings were composed of fine grains with size ranging from 30 to 110 nm.The laminar layers with columnar grains were surrounded with unmelted parts of the nanostructured powder and some equiaxed grains.In the as-sprayed nanostructured zirconia coatings,there existed pores that were less than 1 μm.The cracks were observed on some of the crystal border.The cyclic oxidation experiment showed that the nanostructured coating had longer thermal cycling lifetime to exhibit the promising thermal cyclic oxidation resistance.The failure of the nanostructured TBC was similar to the failure of conventional APS TBC.     

Tensile ductility and fracture of superplastic Aluminum-SiC composites under thermal cycling conditions

The tensile and fracture creep behavior of aluminum-SiC composites has been evaluated under iso-thermal and cyclic heating conditions. The true strain to fracture under thermal cycling conditions is shown to decrease linearly with the logarithm of the applied stress. This trend is unexpected since the strain rate sensitivity exponent,m, is high (typical of superplastic materials) and constant over the range of stresses studied. The results obtained are explained by a fracture mechanics model in which the crack size increases by a strain-induced mechanism. The results are compared with similar behavior obtained in superplastic ceramics.     

Fatigue and fracture of high-alloyed steel specimens subjected to purely thermal cycling

High-alloyed steel specimens subjected to hundreds of thermal cycles (heating up in a furnace to about 900 °C, quenching in water, resting in air) are considered. Contrary to standard “low-cycle fatigue” tests with a cyclically varying applied mechanical load, the driving “force” is a cyclic temperature field with self-equilibrating residual stress states. Despite the cyclic character of this temperature field and no applied load, a significant monotonie change of the shape and metallurgical structure of the specimens can be observed depending strongly on the material and the initial geometry. Precipitations along the grain boundaries and remarkable residual stress states are responsible for internal cracks and damage. An experimental program is reported, and metallurgical and mechanical interpretations are presented.     

The effect of rapid thermal cycling on the creep properties of alpha iron

The effect of rapid, low amplitude thermal cycling on the creep properties of alpha iron was investigated. It was found that creep rates under thermal cycling conditions are lower than the creep rates calculated from steady-state isothermal creep experiments. This relative decrease of the creep rate was dependent on the dwell time and the applied stress. The maximum decrease in creep rate occurred when the time at the lower temperature was very short. In single cycle creep experiments it was found that an abrupt temperature decrease was followed by a delay period of zero creep rate. When the temperature was again increased, a period of inverse transient creep was observed. It is concluded that the inverse transient creep is responsible for the relatively lower creep rates observed under thermal cycling conditions. Investigations of the dislocation substructure with the electron microscope did not show any significant changes attributable to thermal cycling. The lowering of the creep rate is tentatively explained on the basis of dislocation pinning by point defects during the cooling part of the cycle which inhibits subsequent dislocation motion during the rest of the cycle and hence decreases the overall creep rate. D. EYLON, formerly with the Department of Materials Engineering, Technion     

改变循环方式,提高小型沸腾炉热效率

本文介绍了循环流化床锅炉的特点。通过对循环流化床锅炉的两种不同循环方式（高倍率外循环和低倍率内循环）的比较，指出对于小型循环流化床锅炉，应采用低倍率内循环方式，其具有操作简单，锅炉出力大，锅炉结构简化，故障率较低，循环倍率低，运行电耗小等优点。     

Fatigue fracture of the welded joints of commercially pure titanium after thermal cycling

The results of investigation are given on the structural state and variation of mechanical properties characteristic of welded joints of commercially pure titanium subjected to static and cyclic loading after thermal-cycling treatment (TCT). On the basis of the pattern of variation in relative specimen deflection, fatigue-damage growth is estimated quantitatively for specimens cyclically deformed to the point where initiation and propagation of a main crack begins.     

NiTi and NiTi-TiC composites: Part 1. transformation and thermal cycling behavior

The transformation behavior of titanium-rich NiTi containing 0 vol pet, 10 vol pct, and 20 vol pct equiaxed TiC particles was studied by differential scanning calorimetry. The thermoelastic phase transformation of the unreinforced matrix exhibits multiple steps. Upon multiple transformation cycles, the rhombohedral phase (R phase) appears and all transformation temperatures decrease. The TiC particles inhibit the R phase and also lower some of the transformation temperatures. These effects can be explained by the internal misfit stresses resulting from both thermal expansion and transformation mismatch between matrix and reinforcement. The measured transformation enthalpy of bulk and reinforced NiTi is discussed in light of a thermodynamical model, taking into account the elastic energy stored upon cycling. The model indicates that a significant fraction of the matrix is stabilized and thus does not contribute to the transformation enthalpy. Formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology.     

Effect of thermal cycling on the mechanical properties of 350-grade maraging steel

The effects of retained austenite produced by thermal cycling on the mechanical properties of a precipitation-hardened 350-grade commercial maraging steel were examined. The presence of retained austenite caused decreases in the yield strength (YS) and ultimate tensile strength (UTS) and effected a significant increase in the tensile ductility. Increased impact toughness was also produced by this treatment. The mechanical stability of retained austenite was evaluated by tension and impact tests at subambient temperatures. A deformation-induced transformation of the austenite was manifested as load drops on the load-elongation plots at subzero temperatures. This transformation imparts excellent low-temperature ductility to the material. A wide range of strength, ductility, and toughness can be obtained by subjecting the steel to thermal cycling before the precipitation-hardening treatment.     

带烟气回流W型辐射管热过程数值模拟

基于商业软件FLUENT同时,采用RNG k-ε模型和非预混燃烧模型,建立了带烟气回流W型辐射管燃烧器的燃烧及传热的三维数学模型,对辐射管内湍流和燃烧状况进行了模拟,并得到了出口NOx排放的计算结果;通过与相应实验数据的对比,验证了模型的可靠性.分析了辐射管内流体流动,传热和燃烧的特征,研究了燃气流量及空气预热温度对辐射管使用性能的影响;结果表明:燃气流量的提高,会增大辐射管壁面温度的不均匀性,而提高预热温度则相反;两者与辐射管加热能力呈正相关,但都会使得出口烟气内的NOx含量增大.     

Creep of udimet 500 during thermal cycling: II. The time to failure

The effect of rapid temperature cycling on the time to failure of Udimet 500 superalloy was investigated. It was found that the time to failure is determined only by the minimum creep rate and not by the thermal history. For this reason the time to failure under thermal cycling conditions is longer than the time to failure calculated from isothermal data.