飞秒激光烧蚀0Cr18Ni9不锈钢精度的影响因素

为了研究影响飞秒激光烧蚀0Cr18Ni9不锈钢精度的因素,采用飞秒激光对0Cr18Ni9不锈钢进行了切割和打孔实验。利用光学显微镜、光学金相显微镜等设备,对不锈钢烧蚀区形貌和切缝显微组织进行检测,基于烧蚀过程中CCD实时采集到的不锈钢表面的激光光斑图样,采用COMSOL Multiphysic数值模拟软件,模拟了烧蚀过程中激光束的发散传播行为,并计算了光束发散角。结果表明:当激光重复频率为5kHz时,厚度为160μm的0Cr18Ni9不锈钢切缝和孔边缘被明显烧黑,切缝处晶粒明显长大,存在热影响区;烧蚀过程中,由飞秒激光超高功率密度所致的金属-空气混合等离子体使光束沿传播方向上发生散射,发散角在6°～10°之间。热影响区的存在和混合等离子体的行为是影响飞秒激光烧蚀0Cr18Ni9不锈钢精度的主要因素。     

高压和热处理对铝青铜微观组织的影响

 借助金相、扫描电子显微镜（SEM）/能谱仪（EDS）及X射线衍射（XRD）等方法,研究了高压和热处理对铝青铜微观组织的影响。结果表明：铝青铜经750 ℃温度、6 GPa压力处理后所获得的α相的晶粒尺寸较经750 ℃温度、常压处理后所获得的α相晶粒尺寸小;当铝青铜经高压处理后再经750 ℃保温2 min常压处理时,组织中出现细条状的α相,且热处理后的冷却速度越大,细条状α相的数量越多,颗粒状α相的数量越少。     

In situ SEM studies of the transformation sequence of multistage martensitic transformations in aged Ti-50.8 at.% Ni alloys

The transformation behaviour of the multistage martensitic transformation in aged Ti-50.8 at.% Ni alloys was investigated by differential scanning calorimetry (DSC) and in situ scanning electron microscopy (SEM). The specimens aged from 673 to 748?K for 3.6?ks under an unregulated heat treatment atmosphere exhibited the double-stage transformation during cooling. The quadruple-stage transformation was observed in the specimens aged at 773 and 798?K, and the triple-stage transformation appeared in the specimen aged at 823?K under an unregulated heat treatment atmosphere. The distribution and size of Ti₃Ni₄ precipitates were heterogeneous in these specimens. The single-stage transformation in the specimen aged at 848?K was similar to that of the solution-treated specimen. In the forward quadruple-stage transformation, the R-phase transformation occurred in the intermediate region and around the grain boundary. The first martensitic transformation, which corresponded to the M1 peak in the DSC cooling curve, took place in the intermediate region of grains via the R phase. The second transformation, which corresponded to the M2 peak, occurred around the grain boundary via the R phase. The final transformation, which corresponded to the M3 peak, arose directly from the B2 parent phase at the grain centre. The transformation sequence and areas described above were quantitatively verified by comparing the SEM observations with the DSC measurements. The transformation sequence of the triple-stage transformation was also discussed.     

Austenite grain growth kinetics and mechanism of grain growth in 12Cr ultra-super-critical rotor steel

ABSTRACT

In this paper, the austenite grain growth behaviour of 12Cr ultra-super-critical (USC) rotor steel was investigated by a series of heat treatments. The heat treatments at heating temperatures of 900°C–1250°C and holding time of 1?h–20?h were conducted in an electric box-type heating furnace. Experimental results showed that the sizes of austenite grain were affected by heating temperatures and holding time, and heating temperature was the dominant factor. In addition, the grain growth rate changed significantly before and after the turning points of 1050°C and 1250°C. Meanwhile, an austenite grain growth mathematical model was established at different heating temperature stages, and possession of the capability to accurately predict austenite grain size was confirmed. Furthermore, the microstructure of austenite grain in the heating process was observed by optical microscope (OM) and transmission electron microscopy (TEM), which revealed the mechanism of austenite grain growth. Analysis indicated that the change of quantity of precipitate particles with increasing heating temperature was the main reason for the difference in austenite grain growth.     

影响纳米Cu固体材料性能的工艺参数研究

 采用正交试验方法，通过XRD，MHV2000型显微硬度计（数显）和基于浮力原理等测试手段研究了压制压力、保压时间、退火温度和退火时间对自悬浮定向流－冷压法制备纳米Cu固体材料性能（晶粒大小、密度及显微硬度）的影响。结果表明：对晶粒度而言，退火温度是显著性影响因素，同时表明纳米Cu固体具有较好的热稳定性；对密度而言，压制压力是显著性影响因素；对显微硬度而言，退火时间是显著性影响因素。     

Morphology of ablation craters generated by ultra-short laser pulses in dentin surfaces: AFM and ESEM evaluation

In this study, the surface morphology and structure of dentin after ablation by ultra-short pulses were evaluated using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). The dentin specimens examined were irradiated by a chirped-pulse-amplification (CPA) Ti:sapphire laser (800 nm) and the optimal conditions for producing various nanostructures were determined. Based on the ESEM results, it was possible to identify an energy density range as the ablation threshold for dentin. The laser-induced damage was characterized over the fluence range 1.3-2.1 J/cm². The results demonstrate that by selecting suitable parameters one can obtain efficient dentin surface preparation without evidence of thermal damage, i.e., with minimized heat affected zones and reduced collateral damage, the latter being normally characterized by formation of microcracks, grain growth and recrystallization in the heat affected zones.     

Nonlinear current－voltage characteristics of sintered tungsten－vanadium oxide

We have studied the densification behaviour, microstructure and electrical properties of WO_3 ceramics with V_2O_5 as the additive ranging from 0.5 to 15mol%. Scanning electron microscopic photos indicated that the grain size of WO_3－V_2O_5 specimens is smaller than that of pure WO_3. The addition of V_2O_5 to WO_3 showed a tendency to enhance the densification rate and to restrict the grain growth. Electrical properties of all specimens were measured for different electrodes at different temperatures. The formation of the grain boundary barrier layer was confirmed by the non-ohmic I－V behaviour. The nonlinear coefficient was obtained at the current density J=0.01, 0.1 and 1mA/cm^2 for a series of WO_3－V_2O_5 samples. The V0.5mol% specimen showed an abnormal phenomenon that the nonlinear characteristics appeared at 350℃ and disappeared at lower and higher temperatures. This implies that it could be applied as a high-temperature varistor. The double Schottky barrier model was adopted to explain the phenomena for the WO_3-V_2O_5 varistors.     

Anomalous porosity in certain aluminum alloys

Metallographic examination was used to study the formation of porosity in an aluminum alloy A1-4%Cu with grain size in ranges 1000-200 and 120-85 , in technical grade aluminum Al (grain size 80-60 ), and in type D1 alloy (grain size 30-10 ) during a thermal cycling treatment, each cycle of which included aging in water at 100 ° C. The character of porosity depends on alloy composition and grain size. In coarsely granular materials the pores are formed inside grains, have clearly defined form, and reach a size of 10–20 after 60 cycles. The pore size in finely granular materials is smaller by one order of magnitude; the pores are formed predominantly along grain boundaries with some pores formed in the grain interior. Pores formed in alloy D1 are of an irregular form and grow to sizes exceeding the grain size. Data on changes in the specimen length, diameter, and volume were obtained. The relative increase in specimen volume reaches 0.5–1.0% per cycle. In spite of differences revealed by metallographic examination, the phenomena taking place in specimens with large and small grain size are of the same nature.     

The effect of severe plastic deformation and annealing conditions on mechanical properties and restoration phenomena in an ultrafine-grains Fe-28.5%Ni steel

The effect of annealing condition on the microstructure evolution, together with mechanical properties of Fe-28.5Ni steel processed by one and six cycles of accumulative roll bonding (ARB), was explored. The evolution of microstructure was studied by means of Electron Backscattered Diffraction method. The ARB-processed specimens were annealed for 30 minutes between 500°C and 600°C, and the effect of these annealing processes was elaborated. Results showed a significantly reduced martensite phase during ARB process cycles. Moreover, a refined and stabilised ultrafine structure was obtained in 6-cycle ARB-processed specimen. Results also show that by annealing the 1-cycle specimen at 550°C, no significant microstructural change and mechanical properties variation are identified. As such, no significant microstructural change and mechanical properties variation were notified for annealing of 6-cycle ARB-processed specimen at 500°C. However, annealing at 600°C in both conditions led to a complete recrystallisation of the deformed structure. Annealing at 550°C in 6-cycle ARB-processed specimen showed partial recrystallisation as well as an abnormal grain growth characteristic. The abnormal grain growth was seen by annealing of 1-cycle ARB-processed specimen at 600°C. The changing features of dislocations were discussed in these specimens in terms of changing the dynamics of low- and high-angle grain boundaries. Tensile test results also showed a significant increase in the yield/ultimate tensile strengths with the application of ARB cycles. Although ARB process led to a rapid reduction of total elongation, full restoration during high-temperature annealing returned this value back to the initial, non-deformed, condition.     

Effect of initial grain size on texture evolution and magnetic properties in nonoriented electrical steels

The magnetic properties of nonoriented electrical steels are influenced by the grain size and crystallographic texture. The technologies used to control the grain size in nonoriented electrical steels are approaching their limits. However, there is still some room for improvement of the magnetic properties through texture control. Hot-band annealing is known to be one of the most effective processing stages for texture modification. In this study, two types of initial grain sizes prior to cold rolling are obtained by different hot-band annealing. The effect of initial grain size on texture evolution and magnetic properties in nonoriented electrical steels containing 2% Si is examined. The specimens having different initial grain sizes have significantly different textures in the cold-rolled state and the annealed state. During the recrystallization stage, new grains formed in the coarse-grained specimens have stronger Goss but weaker γ-fibre texture than those in the fine-grained specimens. During the grain growth after complete recrystallization, the coarse-grained specimens still have weaker γ-fibre texture than the fine-grained specimens. The magnetic induction of the coarse-grained specimens is always higher at the same temperature than that of the fine-grained specimens. The core loss of the coarse-grained specimens is lower at the same temperature than that of fine-grained specimens. However, the improvement of the core loss becomes less pronounced as the annealing temperature increases.     

Effect of grain size and arrangement on dynamic damage evolution of ductile metal

Plate-impact experiments have been carried out to examine the effect of grain size and grain arrangement on the damage evolution of ultrapure aluminum. Two groups of samples, "cross-cut" and "longitudinal-cut," are obtained from the rolled aluminum rod along different directions. The peak compressive stress is approximately 1.25 GPa-1.61 GPa, which can cause incipient spall damage that is correlated to the material microstructure. The metallographic analyses of all recovered samples show that nearly all damage nucleates at the grain boundaries, especially those with larger curvature. Moreover, under lower shock stress, the spall strength of the "longitudinal-cut" sample is smaller than that of the "crosscut" sample, because the different grain sizes and arrangement of the two samples cause different nucleation, growth, and coalescence processes. In this study, the difference in the damage distribution between "longitudinal-cut" and "cross-cut" samples and the causes for this difference under lower shock-loading conditions are also analyzed by both qualitative and semi-quantitative methods. It is very important for these conclusions to establish a reasonable and perfect equation of damage evolution for ductile metals.     

Modelling of austenite grain growth kinetics in a microalloyed steel (30MSV6) in the presence of carbonitride precipitates

Austenite grain growth kinetics in the presence of secondary precipitates of a microalloyed steel (30MSV6) was studied employing quantitative metallographic techniques. Austenitizing experiments were carried out at 1,000, 1,100 and 1,200?°C. According to the experimental data, abnormal grain growth behaviour is observed at 1,100?°C while it is normal at 1,000 and 1,200?°C. TEM observation represents multicomponent carbonitride precipitate, (Ti,V)(C,N), in the as-received steel with a mean radius of 35?nm. A mathematical model is proposed considering austenite grain growth along with dissolution and coarsening kinetics of the multicomponent precipitates. The austenite grain growth model for short-term non-isothermal and subsequent long-term isothermal heating stages was developed using a statistical approach. The model includes an algorithm to estimate the size distribution of austenite grains. Precipitate mean field dissolution and Lifshitz–Slyozov–Wagner coarsening models were integrated in the proposed model to calculate the pinning pressure retarding the grain boundary movement. The mean austenite grain size and the grain size distribution (normal and abnormal) calculated by the model are in good agreement with experimental data.     

Examination of Dislocation Mechanism on Grain Boundary Sliding in High Angle Grain Boundaries by Stress Change Test

In order to understand the factors dominating grain boundary sliding, stress change tests and stress reversal tests were conducted on aluminum bicrystal specimens with high angle grain boundaries. It is found that small change in stress results in the remarkable change in the rate of grain boundary sliding. Stress reversal tests showed that grain boundary slide hardening does not work for the sliding to the direction opposite to that before the stress reversal. Mean internal stresses for the dislocations contributing to grain boundary sliding are experimentally measured by a method consisting of stress change and annealing. It is found that the velocity of dislocations in the grain boundary is in proportion to the mean effective stress.     

Effect of pulsed laser parameters on in-situ TiC synthesis in laser surface treatment

Commercial titanium sheets pre-coated with 300-μm thick graphite layer were treated by employing a pulsed Nd:YAG laser in order to enhance surface properties such as wear and erosion resistance. Laser in-situ alloying method produced a composite layer by melting the titanium substrate and dissolution of graphite in the melt pool. Correlations between pulsed laser parameters, microstructure and microhardness of the synthesized composite coatings were investigated. Effects of pulse duration and overlapping factor on the microstructure and hardness of the alloyed layer were deduced from Vickers micro-indentation tests, XRD, SEM and metallographic analyses of cross sections of the generated layer. Results show that the composite cladding layer was constituted with TiC intermetallic phase between the titanium matrix in particle and dendrite forms. The dendritic morphology of composite layer was changed to cellular grain structure by increasing laser pulse duration and irradiated energy. High values of the measured hardness indicate that deposited titanium carbide increases in the conditions with more pulse duration and low process speed. This occurs due to more dissolution of carbon into liquid Ti by heat input increasing and positive influence of the Marangoni flow in the melted zone.     

Focused ion beam preparation of atom probe specimens containing a single crystallographically well-defined grain boundary

Needle-shaped atom probe specimens containing a single grain boundary were produced using the focused ion beam (FIB) of a two-beam FIB/SEM (scanning electron microscope) system. The presented specimen preparation approach allows the unprecedented study of a grain boundary which is well characterised in its crystallographic orientation by means of the field ion microscope (FIM) and the tomographic atom probe (TAP). The analysis of such specimens allows in particular the determination of solute excess atoms at this specific grain boundary and hence the investigation of the segregation behaviour. The crucial preparation steps are discussed in detail in the present study for the Sigma 19 a {331} 110 grain boundary of a 40 at.ppm-Bi doped Cu bi-crystal. Transmission electron microscope (TEM) images and TAP analyses of the atom probe tips demonstrate unambiguously the presence of the selectively prepared grain boundary in the apex region of some of the specimens.     

Sintering and electrical conductivity of gadolinia-doped ceria

Bulk specimens of Ce_0.9Gd_0.1O_2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H₂ mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (～0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.     

Unidirectional laser oblique illumination (ULOI): a comparison with partial dark field illumination (PDFI) in the optical microscopy of metals

Some metallographic observations of the surface of polycrystalline specimens in an optical microscope illuminated with a HeNe laser beam; at oblique incidence; are compared with observations with white light also at oblique incidence. It is shown that similar effects are obtained in both cases; however; the width of the maximum in the dispersed intensity is narrower in the case of illumination with the laser beam. In addition; this kind of illumination permits us to distinguish distinct angular directions of the surface roughness.     

Nondislocation etch pits in gallium arsenide obtained by vapor-phase epitaxy

The effect of the vapor-phase epitaxy conditions on the formation of small nondislocation etch pits (SEP) in autoepitaxial gallium arsenide layers was investigated by the oblique-section metallographic technique. The investigations showed that SEP are found in n-type low-dislocation layers and are located directly in the junction region. The width of the SEP region depends on the concentration and type of dopant in the source, the type of substrate, and also on the concentration of transport agent at the system inlet. The formation of SEP in the layers involves impurity segregates of the second phase.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 60–64, September, 1979.We thank V. A. Moskovkin and L. P. Porokhovnichenko for providing the specimens for the investigations.     

Grain growth and static recrystallization kinetics in Co–20Cr–15W–10Ni (L-605) cobalt-base superalloy

The grain size evolution of cold-rolled L-605 cobalt-base superalloy during ultra-rapid annealing is investigated in this paper. Cold-worked specimens undergo static recrystallization, leading to grain refinement or grain coarsening depending on the annealing time and temperature. The kinetics of grain growth is found to be independent of the initial deformation. The evolution of grain size can be simply described by a grain growth model for high temperatures and long annealing times, and the mobility of interfaces is estimated by modelling. Fast annealing treatment process is a very promising technique to customize grain size and enhance mechanical strength. In particular, the reduction of annealing time is an efficient method to produce a refined microstructure through static recrystallization.