Coarsening rate of Β precipitates in Al-11Mg alloy

The coarsening rates of the Β precipitates in an Al-11 wt pct Mg alloy at 250 ‡C, 316 ‡C, and 330 ‡C aging temperatures were investigated. As predicted by the modified Lifshitz-Slyozov-Wagner (MLSW) theory of diffusion controlled particle coarsening, a linear dependence ofr ³ ont in the long time aging region was observed. Interfacial energies of 8.1, 1.8, and 1.7 J/m² for 250 ‡C, 316 ‡C, and 330 ‡C, respectively, were computed from these data using the MLSW theory. The high values of interfacial energy indicate that the Β precipitates are incoherent with respect to the Al matrix. The especially high value of interfacial energy and low activation energy for the precipitate coarsening at 250 ‡C suggest that significant short circuit diffusion occurs at this aging temperature.     

Coarsening kinetics during solidification of Ni-Al-Ta dendritic monocrystals

Several dendritic monocrystals of nickel-rich Ni-Al-Ta alloys were directionally solidified at about 0.25 m/h⁻¹ under a gradient of 8 × 10⁻³ K/m⁻¹. The solid-liquid interface was fossilized at a given moment by rapidly quenching the remaining liquid. In some specimens crystal pulling was interrupted for various lengths of time prior to quenching. The quenched solid-liquid interfaces were used for a convenient and rapid evaluation of: 1) isothermal coarsening kinetics of the dendritic solid at a temperature between the liquidus and the eutectic temperatures and; 2) dendrite coarsening kinetics during solidification. It was found that extension to the ternary Ni-Al-Ta system of a model previously developed for binary systems predicted isothermal dendrite coarsening kinetics in close agreement with experimental results. Agreement for coarsening kinetics during solidification was less good. An increase in tantalum or aluminum contents slowed down coarsening, yielding finer microstructures. At equal atomic percental increase in concentration, the effect of tantalum was more significant than that of aluminum.     

The grain boundary segregation of boron during isothermal holding

Boron segregation to grain boundaries was investigated by means of particle tracking autoradiography (PTA) in a low carbon and two Mo-bearing steels and in an Fe-30% Ni alloy. Non-equilibrium segregation took place after rapid cooling to a degree which increased as the temperature difference between the austenitization and subsequent holding temperatures was increased. The amount of deformation during isothermal holding had a similar effect. The maximum segregation produced in this way can be many times higher than that associated with equilibrium segregation. The observations support the view that boron atoms are transported to the boundaries by forming complexes with vacancies which migrate to the boundaries prior to annihilation. During holding at different temperatures, the boron intensity curves produced by the PTA method exhibit three types of time dependence. Two of these involve the appearance of a segregation peak, after which there is either complete or partial disappearance of the segregation. The former is associated with the back diffusion of boron into the depleted zone, a process that takes place at the highest holding temperatures and in the absence of precipitation. The latter involves the conversion of temporary segregation into grain boundary precipitates and is observed at intermediate temperatures. The third type is observed at the lowest temperatures; in this case the development of non-equilibrium segregation is converted into precipitation prior to the appearance of segregation peak. The temperature range associated with each type of curve depends on the relation between the kinetics of non-equilibrium segregation on the one hand and that of precipitation on the other.     

Deformation mechanisms of a rapidly solidified Al-8.8Fe-3.7Ce alloy

The mechanisms of deformation of a rapidly solidified and compacted Al-8.8Fe-3.7Ce (wt pct) alloy were investigated in the stress range 20 to 115 MPa and temperature range 523 to 623 K. The stress dependence of the steady state strain rates indicated a transition from diffusional creep to power law creep, the transition stress decreasing with increasing temperature from 70 MPa (σ/G = 3.1 × 10^-3) at 523 K to 40 MPa (σ/G = 1.9 × 10^-3) at 623 K. The activation energy in the power law creep regime was close to that of bulk self-diffusion in aluminum, while the activation energy in the diffusional creep regime was close to that of grain boundary self-diffusion in Al. The creep strain rates in the power law creep regime were found to be predicted much better by the substructure-invariant creep law (Sherby, 1981) than by the semi-empirical Dorn equation for Al, with the inclusion of a “threshold” stress. In the Coble creep regime, it was found that the cell/subgrain boundaries are inefficient vacancy sources/sinks and that their contribution to Coble creep is totally suppressed in this alloy. The Coble creep rates could be explained by using the average diameter of the powder particles as the effective grain size in the Coble creep equation.     

Deformation characteristics of Ti-6Al-4V alloy under isothermal forging conditions

An investigation was undertaken to determine the influence of forge temperature, ram rate, and starting microstructure on the deformation characteristics of isothermally forged Ti-6A1-4V alloy. Both yielding and finish forge pressures were measured in the practical range of forge temperatures and ram rates. With the absence of die-chilling, the results obtained can be quantitatively related to the hot deformation properties. The yielding and finish forge pressures and the stress-strain relationship are strongly dependent on forge temperature, ram rate, and initial microstructure. Although the forge pressures do not vary significantly with strain, an apparent yield-drop was observed, particularly in the β preforms. On the basis of the above experimental findings and the activation analysis, the rate-controlling deformation process under isothermal forgings is discussed with respect to the dynamic softening. Additional observation was made on dynamic recrystallization in the temperature range of 1500°F (1089 K) to 1650°F (1172 K). Structural features and tensile properties of isothermally forged material are also presented.     

Deformation characteristics of Ti-6Al-4V alloy under isothermal forging conditions

An investigation was undertaken to determine the influence of forge temperature, ram rate, and starting microstructure on the deformation characteristics of isothermally forged Ti-6A1-4V alloy. Both yielding and finish forge pressures were measured in the practical range of forge temperatures and ram rates. With the absence of die-chilling, the results obtained can be quantitatively related to the hot deformation properties. The yielding and finish forge pressures and the stress-strain relationship are strongly de-pendent on forge temperature, ram rate, and initial microstructure. Although the forge pressures do not vary significantly with strain, an apparent yield-drop was observed, particularly in the Β preforms. On the basis of the above experimental findings and the activation analysis, the rate-controlling deformation process under isothermal forgings is discussed with respect to the dynamic softening. Additional observation was made on dynamic recrystallization in the temperature range of 1500‡F (1089 K) to 1650‡F (1172 K). Structural features and tensile properties of isothermally forged material are also pre-sented.     

Hardening mechanisms in a dynamic strain aging alloy,HASTELLOY X,during isothermal and thermomechanical cyclic deformation

Isothermal cyclic deformation tests were conducted on HASTELLOY X with a total strain range of ±0.3 pct at several temperatures and strain rates. Cyclic hardening exhibited a broad peak between about 200 °C and 700 °C, with a maximum near 500 °C of about 80 pct increase in stress amplitude, Δσ/2, at failure. The present work examines the mechanisms contributing to this marked cyclic hardening. Cr₂₃C₆ precipitation on dislocations contributed to hardening, but only with sufficient time above about 500 °C. The substantial hardening rate at lower temperatures or shorter times was attributed to solute drag. The contribution of solute drag was evidenced in tests at both 400 °C and 600 °C by a continually decreasing strain rate sensitivity of the AcrJ2. Solute drag alone produced very considerable cyclic hardening. The increase in AcrJ2 after 1000 cycles at 427 °C was 75 pct of the maximum observed at higher temperatures where carbides did precipitate. Additionally, thermomechanical tests were conducted between various temperature limits, but with the same ±0.3 pct mechanical strain range. Hardening was bounded by isothermal behavior at the temperature limits of the thermomechanical cycles, except for tests between 400 °C and 600 °C which exhibited extreme hardening. However, microstructural examination did not suggest a cause. Specimens subjected to thermomechanical cycles appeared similar to those isothermally cycled at the maximum temperature of the thermomechanical cycle, including those from the 400 °C to 600 °C tests.     

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.     

帘线钢等温和控冷过程氧化铁皮的形成

制备了80级帘线钢盘条在900℃空气等温5、8、12和16min以及900℃等温后分别以4、12和(4+12)℃/s的冷速冷却形成的氧化铁皮。用激光拉曼光谱仪确定了氧化铁皮是由FeO层、Fe3O4层和极少量的Fe2O3层组成。用金相显微镜观测了氧化铁皮的形貌、各层厚度和钢基体的显微组织。结果表明:由于氧化铁皮中产生了CO和CO2气体,900℃空气等温形成的氧化铁皮,除5min的样品外,都出现不同程度的裂缝或孔洞;随等温时间的延长,FeO中不断析出先共析Fe3O4,FeO层的急剧增加导致氧化铁皮总厚度增加。随冷速的提高,FeO层及氧化皮总厚度减少,钢基体中索氏体含量增多。以12℃/s冷却形成的氧化铁皮和基体组织,既有利于机械剥壳,又有利于盘条冷拉拔。     

Al—10Ce中间合金对ZL102共晶Al—Si合金的变质研究

分析研究了Al—10Ce中间合金对ZL102共晶铝硅合金的变质处理效果。结果表明,Al—10Ce中间合金用量、熔体中的杂质以及冷却速度对变质效果的影响很大,Al-10Ce中间合金用量为0．9％左右时达到最佳变质效果,提高Al—Si的纯度可以获得变质组织,冷却速度超过70℃／min时才具有明显的变质效果。     

Al-10Ce中间合金对ZL102共晶Al-Si合金的变质研究

分析研究了Al-10Ce中间合金对ZL102共晶铝硅合金的变质处理效果.结果表明,Al-10Ce中间合金用量、熔体中的杂质以及冷却速度对变质效果的影响很大,Al-10Ce中间合金用量为0.9%左右时达到最佳变质效果,提高Al-Si的纯度可以获得变质组织,冷却速度超过70℃/min时才具有明显的变质效果.     

Al-5Ti-B对过共晶Al-18Si合金的变质效果

采用Al-5Ti-B变质剂对过共晶Al-18Si合金进行反向变质处理,用光学显微镜观察合金的组织与形貌,研究变质剂加入量、变质温度和冷却速度对初晶硅的尺寸、形态和面积分数以及共晶组织的影响。研究表明:当Al-5Ti-B加入量(质量分数)为0.3%时,变质处理后Al-18Si合金中的初晶硅和共晶硅尺寸明显减小,初晶硅的面积分数减小;与其相比,变质剂加入量增加到0.6%时,初晶硅尺寸变化不明显,但共晶硅进一步细化;随冷却速率降低,变质处理后Al-18Si合金中初晶硅相的数量减少,但Si颗粒尺寸明显增大,并且共晶硅细化;与Al-18Si合金在720℃变质相比,该合金在780℃变质处理时,初晶硅的尺寸增大,但初晶硅的面积分数显著减小;合金在850℃变质处理后初晶硅的尺寸、面积分数都比720℃变质处理后明显减小;随变质温度升高,Al-Si合金中的共晶硅明显细化。     

Coarsening of Al<Subscript>3</Subscript>Sc precipitates in an Al-0.28 wt pct Sc alloy

The Ostwald ripening of Al₃Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with aging time, t, by electrical resistivity. The coarsening kinetics of Al₃Sc precipitates obey the t ^1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with t are consistent with the predicted t ^−1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al₃Sc interface energy, γ, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of γ using coarsening data obtained from TEM and electrical resistivity measurements. The value of γ estimated from the LSW theory is 218 mJ m⁻², which is nearly identical to 230 mJ m⁻² from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined to be (7.2±6.0)×10⁻⁴ m² s⁻¹ and 176±9 kJ mol⁻¹, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.     

Coarsening kinetics of alloy platinum-palladium particles on oxide substrates

Coarsening of small (5 nm initial radius) platinum-50 at % palladium particles supported on flat alumina substrates has been studied at temperatures from 600 to 900°C under oxidizing conditions.The mode of coarsening appears to be nucleation inhibited. At 800 and 900°C interparticle transport of PtO₂ is rate-controlling, whereas at 600 and 700°C transport of either platinum species or palladium species appears to result in the same coarsening rates.In samples heat treated at 600 and 700°C agglomerations of the solid oxide PdO are present on the substrate along with the alloy particles.