Grain Coarsening of Cast Magnesium Alloys at High Cooling Rate: A New Observation

Most studies in the area of grain refinement have always taken for granted that higher cooling rate results in finer grains. However, when microstructural variation of the as-cast Mg with cooling rate was investigated using a specially designed V-shaped copper mold, the results were different. Although fast cooling during solidification led to microstructural refining in pure Mg, grain coarsening was observed at a higher cooling rate in Mg alloys that were inoculation treated with 1.0wt pctZr and 1.4wt pctCaO, and in the Mg-Al binary alloys. It is considered that the grain coarsening at higher cooling rate was attributed to the smaller constitutional undercooling zone formed at fast cooling due to the high temperature gradient in the three Mg alloys. These results can help in redefining the role of cooling rate in the grain refinement process.

     

In Situ Characterization of Deformation Behavior of Austenitic High Manganese Steels

The mechanical properties of high manganese steels are linked to their hardening mechanisms and their intrinsic behavior during deformation. The characterization of mechanical properties is influenced by the localization of plastic flow and the effect of this localization on the material. Depending on grain size, temperature, and extrinsic strain rate localization of strain, adiabatic heating, and hardening vary in spatial and temporal extent. Even at small strain rates the adiabatic heating of samples reaches temperatures more than 100 K over initial testing temperature due to the sharp localization and last but not least this heating is also dependent on the tested sample size. Furthermore, temperature influences the activated mechanisms of plastic flow. The characterization of temperature increase, strain distribution, and local hardening is pursued in tensile tests with application of infrared thermography. With those techniques it is possible to gather correlations between local strain and temperature. The analysis of dynamic strain ageing effects is also carried out by evaluation of the instantaneous strain rate, the strain rate in the gauge length, in dependence of stress in different alloys, as well as at different strain rate regimes. Thus it is possible to distinguish the onset of TRIP, TWIP and DSA.     

In Situ Observation of Acicular Ferrite Nucleation and Growth at Different Cooling Rate in Ti-Zr Deoxidized Steel

Metallurgical and Materials Transactions B - The effects of cooling rate on acicular ferrite (AF) nucleation, growth, and inclusion characteristics in Ti-Zr deoxidation steel were studied by...     

In Situ Observation of the Zr Poisoning Effect in Al Alloys Inoculated by Al-Ti-B

Metallurgical and Materials Transactions A - In situ synchrotron X-ray radiography observations of the Zr-poisoning phenomenon of an Al-20 wt pct Zn alloy inoculated by Al-5Ti-1B were carried out....     

In Situ Analysis of the Thermal Evolution of Electrodeposited Fe-C Coatings

Metallurgical and Materials Transactions A - Fe-C coatings with a high amount of carbon were electrodeposited from an iron-sulfate electrolyte. Citric acid as an additive in the electrolyte not...     

Application of In Situ Neutron and X-Ray Measurements at High Temperatures in the Development of Co-Re-Based Alloys for Gas Turbines

Co-Re alloy development is prompted by the search for new materials for future gas turbines which can be used at temperatures considerably higher than the current day single crystal Ni-based superalloys. The Co-Re-based alloys have been designed to have very high melting range, and they are meant for application at +373 K (+100 °C) above Ni-superalloys. They are significantly different from the conventional Co-based alloys that are used in static components of today??s gas turbines, and the Co-Re alloys have never been used for structural applications before. The Co-Re-Cr system has complex microstructure with many different phases present. Phase transformations and stabilities of fine strengthening precipitates at high temperatures remain mostly unexplored in the Co-Re alloys, and to develop basic understanding, model ternary and quaternary compositions were studied within the alloy development program. In situ neutron and synchrotron measurements at high temperatures were extensively used for this purpose, and some recent results from the in situ measurements are presented. In particular, the effect of boron doping in Co-Re alloys and the stabilities of the fine TaC precipitates at high temperatures were investigated. A fine dispersion of TaC precipitates strengthens some Co-Re alloys, and their stabilities at the application temperatures are critical. In the beginning, the alloy development strategy is very briefly discussed.     

In Situ Observation of High Temperature Creep Behavior During Annealing of Steel

Previous studies on creep suggested a close relationship between polycrystal grain size, substructure, and creep rate. At present, however, our understanding of the influence of polycrystal grain size, substructure, and thermal stress on creep deformation behavior seems rather insufficient, especially as there is a general lack of in situ data on structural changes during creep. In this study, the effects of thermal stress, austenite grain size, and cooling rate on slip deformations in C?CMn?CAl steel during annealing were investigated systematically on the basis of in situ observations using high temperature laser scanning confocal microscopy. Finally, a kinetics model based on thermal expansion anisotropy and temperature difference was developed to explain these interesting experimental results. The in situ investigation of slip deformation during annealing greatly contributes to the understanding of high temperature creep behavior.     

Primary Recrystallization Textures in Dilute Fe-C Alloys

Iron, iron-0.012 pct C, and iron-0.032 pct C alloys subjected to a multistage rolling-annealing sequence display significantly different recrystallization textures after a final decarburizing anneal for 100 hours at 870 °C in dry hydrogen. A (110) [001] texture which developed by primary recrystallization was observed in the 0.012 C alloy. The unalloyed iron and 0.032 C alloy exhibited major components, respectively,(111〈011〉 and 111–211). Control of the alloy carbon level during the entire processing is considered to be critical to the development of the various textures.     

Mechanics of Intermittent Plasticity Punctuated by Fracture During Shear Deformation of Mg Alloys at Near-Ambient Temperatures

Microstructure evolution of basal-textured Mg alloy AZ31B (Mg: Al: Zn; 96: 3: 1 wt pct) during simple shear deformation at near-ambient temperatures was studied by plane-strain machining. Using Schmid factor calculations in conjunction with quantitative electron microscopy, it was found that plastic deformation in AZ31B in the primary deformation zone of machining commences by extension twinning followed by basal slip. Characteristics of twinning in individual grains were described by correlating the direction of twinning with the principal stress state. The implications of these deformation mechanics for the microstructure inherited by the freshly generated surfaces in shear-based material removal processes are examined. These include the identification of extensive surface texture reorientation at machined surfaces via extension twins, limits on surface integrities wrought by fracture events that punctuate plastic deformation, and their relationship to the cutting tool geometry.     

In Situ Measurement of Nonlinear Shear Modulus of Silty Soil

A new field test method to evaluate in situ nonlinear shear modulus of soils was developed. The method utilizes a drilled shaft as a cylindrical, axisymmetric source for shear loading of soil at depth. The applicability of the test method was studied by conducting small-scale, prototype experiments at a “calibration” field site in Austin, Texas. Numerous conventional in situ and laboratory measurements were performed to characterize the soil at the field site. The “small-scale” nature of the tests involved using a 381?mm (15?in.) diameter, 3.7?m (12?ft) long drilled shaft. Experimental results from this field study provided an opportunity to compare laboratory and field measurements of the G?log?γ and G/Gmax?log?γ curves. This comparison was used to investigate the accuracy of common procedures relating field and laboratory modulus reduction curves. Nonlinear modulus measurements were performed at depths of 1.8?to?2.1?m (6?to?7?ft) in a silt (ML). The field G/Gmax?log?γ curve for this soil at low confining pressures are in general agreement with the laboratory curve from an intact specimen as well as empirical curves.     

3D In Situ Imaging of Aluminium Alloys During Solidification

X-ray tomography has become a widely used 3D characterisation technique in materials science either using laboratory tomographs or large X-rays facilities. The advantage of large X-rays facilities is the high photon flux which allows fast tomography to be performed. With fast acquisition CCD cameras, it is possible to obtain a 3D image of the material in less than 30?s with a spatial resolution of the order of 1 or 2???m. We will show that this allows to study the influence of copper addition on the formation of intermetallics formation in Al?CSi?CFe?CCu alloys: we will show that copper seems to have beneficial effects. With CMOS cameras it is possible to reduce acquisition times down to values of about 0.15?s. We will show that this allows to obtain in situ 3D information on the early stage of solidification in aluminium alloys, only accessible up to now using radiography : the solidification front rate was measured on several individual dendrites an Al?C20wt%Cu alloy and estimated to be around 10???m/s.     

Effect of Alloying Elements on Nb-Rich Portion of Nb-Si-X Ternary Systems and In Situ Crack Observation of Nb-Si-Based Alloys

To find a new route for microstructure control and to find additive elements beneficial for improving high-temperature strength, a systematic investigation is performed on hypoeutectic Nb-15 at. pct Si-X ternary alloys containing a transition element, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt, or Au. Information on phase equilibrium is classified in terms of phase stability of silicide phases, α Nb₅Si₃, Nb₄SiX, and Nb₃Si, and the relationship between microstructure and mechanical properties both at room temperature and high temperature is investigated. All the additive elements are found to stabilize either α Nb₅Si₃ or Nb₄SiX but destabilize Nb₃Si. A microstructure of Nb_ss/α Nb₅Si₃ alloy composed of spheroidized α Nb₅Si₃ phase embedded in the Nb_ss matrix is effective for toughening, regardless of the initial as-cast microstructure. Also the plastic deformation of Nb_ss dendrites may effectively suppress the propagation of longer cracks. High-temperature strength of alloys is governed by the deformation of Nb_ss phase and increases with higher melting point additives.     

In Situ Observation of P91 High-Alloy Steel Solidification Characteristics at Different Cooling Rates

To clarify the effect of different radial cooling intensities on the formation of central cracks in large round bloom continuous casting, it is necessary to study the solidification characteristics and dynamics of P91 high-alloy steel at different cooling rates (CRs) to improve the central defects. In this article, the solidification characteristics of P91 high-alloy steel at different CRs are studied by using the Thermo-Calc software, high-temperature laser confocal microscopy, scanning electron microscopy, and optical microscopy. Meanwhile, the growth kinetics of δ-Fe and γ-Fe phases under different CRs are determined. The results show that the solidification path of P91 high-alloy steel is L(L + δ-Fe) → (L + γ-Fe + δ-Fe) → (δ-Fe + γ-Fe). The δ-Fe and γ-Fe phase precipitation process is divided into two stages. Stage I is the nucleation and rapid growth phase, in which a high undercooling is required. Stage II is the slow growth stage, where the undercooling decreases and remains constant. The initial growth linear velocities of the δ-Fe phase are 0.51, 2.72, and 2.09 μm s⁻¹ at CRs of 10, 50, and 100 °C min⁻¹, respectively, while those of the γ-Fe phase are 0.10, 1.42, and 1.41 μm s⁻¹.     

Effect of Strain Rate on Deformation Behavior of Semi-Solid Dendritic Alloys

The behavior of semi-solid Sn-Pb alloys was studied in compression between two parallel plates. Small dendritic samples were deformed at cross-head speeds leading to initial strain rates ranging from 1.3 × 10^-3 s^-1 to 1.2 × 10³ s^-1 in the semi-solid state at a temperature just above the eutectic. At the lower rates of deformation, breakdown of the dendrite structure occurs, at strains of 0.2 to 0.4, and a high degree of segregation of the liquid phase occurs. For higher rates the segregation no longer occurs to such a great extent and the alloy deforms more homogeneously. Some related experiments involving compression over a filter are presented to obtain stress-strain relations in bulk compression for later analysis. The behavior in compression of alloys in the semi-solid state may be used as a refining process in the low strain-rate range where segregation of the liquid is large. It may also prove useful in the high strain-rate range as a forming method. M. SUERY, formerly Visiting Scientist, Materials Processing Center, Massachusetts Institute of Technology.     

Reactions Between Silicon and Graphite Substrates at High Temperature: In Situ Observations

Graphite as a refractory material has found wide application in many process steps to produce photovoltaic silicon. In the current study, the melting behavior of silicon in contact with different grades of graphite was investigated. The infiltration of silicon into graphite was found to be highly dependent on the internal structure of the graphite substrate. It was confirmed that the heating history of silicon in contact with a graphite substrate strongly influences the melting behavior, which is likely attributed to a gas–solid reaction that forms SiC at less than the liquidus temperature of silicon and alters the surface properties of the graphite. It was also observed that a concentration of CO greater than 5 pct in the inlet gas leads to SiC formation on the surface of the silicon and severely hinders melting.     

In Situ Observation for Abnormal Grain Coarsening in Vacuum-Carburizing Process

An in situ observation method was developed to investigate abnormal grain coarsening which occurs around the surface of steel during the vacuum-carburizing process. In this method, diffusion of carbon atoms in the vacuum carburizing was simulated by a cementite and steel diffusion couple. Abnormal grain coarsening, which appeared around the cementite and steel interface, was observed by a confocal scanning laser microscope. With this method, it was observed that when holding time was 60 seconds, the temperature at which the abnormal grain coarsening appeared in a specimen was higher when carburized than the temperature when not carburized. On the contrary, when holding time was 120 seconds, the temperature at which the abnormal grain coarsening appeared in a specimen was 10 K to 20 K lower than that in a non-carburized specimen. The validity of the observed results was confirmed by the calculated NbC fraction using Nb solubility and measured carbon content.     

TWIP钢应变诱导孪晶原位观察

利用装配拉伸台的共焦激光扫描显微镜原位观察了一种TWIP钢(Fe-30Mn-3AI-3Si)变形过程中的应变诱导孪晶(SIT).结果表明,孪晶诱导应变阀值对应变速率不敏感,均为4.0%左右;孪晶萌发于晶界,沿晶粒扩展至晶界(或孪晶界)终止;按生长过程孪晶可分为三类:层片状、透镜状以及"耳"状,形态差异源于变形过程中TWIP钢多晶体取向以及沿孪生特定位向的调整幅度;孪晶可以交互穿越,低应变速率下,先形成初生孪晶,而后出现次生孪晶并与前者交错,高应变速率下两个体系的孪晶同时形成;断裂时裂纹可以沿孪晶界扩展.利用SIT模型计算了孪晶层片间距,与TEM观测结果吻合.     

Atmospheric Oxygen Transfer During In Situ Oxygen Uptake Rate Measurements

In situ apparent oxygen uptake rate (OUR) measurements of laboratory bioreactors open to atmospheric air are practical and useful, but atmospheric oxygen transfer into the reactor may significantly underestimate the actual values of OUR. The effects of the atmospheric oxygen transfer were quantitatively assessed using an oxygen mass transfer model. A factor Ψ representing the degree of underestimation of actual OUR by the in situ apparent OUR (OURa) technique was introduced. The Ψ factor for the primary settling tank effluent of a local water pollution control plant was found to be unity in the beginning of reaction period when the microbial activity was high (OURa = 27 mg/L?h), but decreased to 76% at the end of reaction period when the microbial reaction rate was low (OURa = 2 mg/L?h).     

Rate of reduction of FeO in slag by Fe-C drops

The rate of reduction of FeO in slags by Fe-C drops plays an important role in several metallurgical processes, including iron bath smelting. In this study, the rate of this reaction was determined by measuring the volume of CO generated as a function of time, and the reaction was observed by X-ray fluoroscopy. The drops entered the slag in a nearly spherical shape, remained as single particles, and for the major portion of the reaction remained suspended in the slag surrounded by a gas halo. The rate was found to decrease with carbon content for alloys with low sulfur contents. The rate decreased significantly with increasing the sulfur content. Based on the results and a comparison of the calculated rates, for the possible rate-controlling mechanisms, a kinetic model was developed. The model is a mixed control model including mass transfer in the slag, mass transfer in the gas halo, and chemical kinetics at the metal interface. At high sulfur contents (>0.01 pct), the rate is primarily controlled by the dissociation of CO₂ on the surface of the iron drop. At very low sulfur, the rate is controlled by the two mass-transfer steps and increases as the gas evolution from the particle increases. Formerly with Carnegie Mellon University     

RE-Mg-Ni系储氢合金高倍率放电性能研究现状

RE-Mg-Ni系储氢合金具有超晶格结构,其主相晶格单元是由一定比率的AB5单元和AB2单元沿c轴交替层叠排列而成。该类型合金自问世以来便以其高容量、易活化的优势受到人们的广泛关注,然而其循环稳定性及高倍率放电性能不尽人意。人们通过大量研究有效提高了其循环稳定性,使其基本满足了商业化要求。但是要将基于该负极材料的镍氢电池应用在混合动力汽车上,仍需改进其高倍率放电性能。系统分析了元素替代、多元合金化、制备工艺、化合物复合、表面处理等手段对RE-Mg-Ni系储氢合金晶体结构及高倍率放电性能的影响。其中元素替代是一种重要且有效的手段,文中分析了不同稀土元素及B侧元素的作用机制,结果表明,B侧组分采用Ni,Co,Mn,Al的储氢合金具有较好的性能。多元合金化是一种复杂的过程,不同元素间可能存在一定的协同作用,研究其作用机制也是下一步的工作重点。通过优化实验方案,综合使用多种改性手段,可以得到高倍率放电性能良好的RE-Mg-Ni系储氢合金,使其基本满足电动工具用镍氢电池的要求,并可望在以后的研究中进一步提高其高倍率放电性能,使其满足混合动力汽车用镍氢电池的要求,实现良好的经济和社会效益。