双相钢中奥氏体形成部位的电镜观察

本文用透射电镜复型及薄膜方法研究了低碳钢在亚温(α十γ)区域加热时奥氏体的形成并讨论了预先冷轧变形对奥氏体化的影响。结果表明,原始组织为铁素体加珠光体的退火低碳钢在亚温区奥氏体化时,奥氏体不仅在铁素体晶界上形成,也可在铁素体晶粒内形成。预先冷轧变形使退火组织在亚温区奥氏体化时,珠光体球化大大加速,并且有更多的奥氏体在铁素体晶粒内形成,但是奥氏体形成过程的基本特征不变。     

热轧TRIP钢的加工工艺与残余奥氏体形成的关系

低合金TRIP钢的显微组织中残余奥氏体使其具有优良的强度和延性组合。分别介绍了热轧TRIP钢在奥氏体再结晶区、未再结晶区以及临界区变形对残余奥氏体形成的影响。分析了输送台上的冷却和卷取温度对残余奥氏体形成的影响。对低合金TRIP钢残余奥氏体的形成等物理冶金学的研究可以促进该类合金的开发和推广应用。     

奥氏体变形对22CrSH齿轮钢连续冷却相变的影响

为了研究奥氏体变形对22CrSH齿轮钢连续冷却相变的影响,在Gleeble 1500热模拟机上,将22CrSH钢在950℃变形0.4及未变形处理,然后连续冷却.利用膨胀曲线、光学显微镜、透射显微镜,结合各种腐蚀方法,分析了22CrSH钢相变行为及相变组织.研究表明:奥氏体变形使多边形铁素体加珠光体混合组织的临界冷速增大;当奥氏体变形及降低冷速时,大量的晶界仿晶型铁素体占据了奥氏体晶界,贝氏体相变向针状铁素体相变转变;变形使奥氏体在中温相变区稳定性增加,室温组织中M/A岛的数量增多.     

双相钢中的残余奥氏体

研究了热－机械处理中各种因素对双相钢中的残余奥氏体的影响，发现：双相区热处理前的冷加工使奥氏体成核密度提高，且使残余奥氏体分布均匀和含量增加；微观结构的观察表明，空冷过程中奥氏体颗粒在不断缩小，颗粒的尺寸效应和碳原子向奥氏体中偏聚均使奥氏体变得稳定。因此，恰当地控制冷加工量、冷却速度、加热温度和时间，可使双相钢含有数量和稳定度都合适的残余奥氏体。此外还发现，从双相区温度空冷所得马氏体必须经低温短时     

过共析钢在过冷奥氏体形变过程中的组织超细化

用Gleeble1500热模拟试验机进行单轴热压缩实验,研究了过共析钢在过冷奥氏体形变过程中的组织演变规律.结果表明:过冷奥氏体变形可以抑制网状渗碳体的形成,过冷奥氏体动态相变只得到珠光体组织,在继续变形过程中珠光体发生动态球化,得到超细化(α θ)复相组织.提高形变温度使过冷度降低,阻碍过冷奥氏体动态相变的进行;而降低应变速率使变形时间延长,有利于过冷奥氏体动态相变和珠光体动态球化的进行,但得到的超细化复相组织较为粗人.     

W1300无取向硅钢热变形行为的模拟分析

采用热模拟变形的方式在跨越奥氏体/铁素体转变的1100℃～800℃温度范围内观察了W1300无取向硅钢,在不同的模拟热轧变形流程中应力应变曲线的演变规律,并对比分析了热变形后的微观组织。研究表明,奥氏体区终轧之后相变得到的等轴铁素体组织难以再经加热粗化。铁素体区终轧后热轧组织的变形特征明显,内在的变形储存能可以推动高温卷取或常化退火过程中铁素体基体的静态再结晶,并获得粗大晶粒。铁素体与奥氏体变形抗力的明显差异会造成两相共存时热轧变形的不稳定性。     

X70管线钢的局部脆化

研究了X70管线钢模拟热影响区的局部脆化现象和临界再热粗晶区粒状贝氏体的形成及其对韧性的影响规律.结果表明,二次峰值温度为780℃时,M-A组元在原奥氏体晶界成链状分布;二次峰值温度为840℃时,原粗晶区晶界位置生成了细晶贝氏体带,及大颗粒的M-A组元;二次峰值温度为900℃时,M A组元弥散分布,原粗晶区的奥氏体晶界消失.在二次加热过程中奥氏体主要在原粗晶区晶界形核长大.X70钢的粗晶区出现再加热脆化现象,它的脆化温度区间是一个比Ac1-Ac3更窄的温度区间,主要在T2P=Ac1-840℃温度范围内.临界再热粗晶区脆化的主要原因是在晶界上有链状分布的M-A组元.     

形变对氰化过程的影响及其工业应用

本文探讨了室温予变形对氰化过程的影响。结果指出:冷变形显著提高表面碳氮浓度并使奥氏体晶粒和碳氮化合物细化;但是予先冷变形对氰化层深度的影响较为复杂,这可能与变形方式以及冷变形产生的在不同方向的位错组态有关。用正电子湮灭谱仪测定了氰化过程中冷变形引起的晶体缺陷的变化。试验表明虽然高温下再结晶进展十分迅速,但由于碳氮原子的渗入和扩散可将位错钉扎,淬火后的位错密度较高。在自行车另件生产上的实际应用表明,予先冷变形可使氰化另件的接触疲劳寿命提高50％到一倍,并使某厂轴档热处理合格率由不足30％提高到80％以上。     

奥氏体高温转变区二段冷却速率对铁素体相变的影响

采用Gleeble热力模拟机对C-Mn钢热压缩变形后过冷奥氏体高温转变区进行二段冷却速率控制,通过冷却过程中施加微小应变,并根据应力-温度曲线,结合金相组织观察,研究了二段冷却速率对铁素体相变开始温度和相变组织的影响。结果表明:在过冷奥氏体高温转变区冷却相同时间,相对于连续冷却,当前段快冷,后段缓冷时,铁素体相变开始温度下降,相变的铁素体体积分数增加;当前段快冷速率为100℃/s时,铁素体相变开始温度下降幅度能达到100℃,铁素体体积分数增加近1倍。因此,应用前置式超快冷,并随后缓冷的冷却方式有助于提高铁素体转变量,并降低铁素体相变的温度,以细化铁素体晶粒。     

低温临界区退火时间对22MnB5钢组织和性能的影响

为了研究22MnB5钢在退火过程中的组织演变规律,细化热冲压成形后马氏体板条束,通过扫描电镜(SEM)、能谱分析、电子背散射衍射(EBSD)分析技术和拉伸实验等方法,研究了不同低温临界区退火时间对22MnB5钢显微组织和力学性能的影响,并阐述了不均匀奥氏体在退火过程中的转变机制及合金元素对粒状珠光体形成的影响.研究表明,经低温临界区不同退火时间保温及随后等温处理后,得到不同的珠光体形态,在770℃保温0.5 h,并在700℃等温处理后,得到铁素体基体上分布颗粒状碳化物的粒状珠光体组织;随着临界区保温时间的延长,奥氏体转变逐渐均匀,使部分奥氏体在随后的等温过程中发生共析转变,得到多边形铁素体+片层状珠光体组织.粒状珠光体组织有利于细化淬火后的马氏体板条束,提高综合力学性能.     

Microfluidic rheology of non-Newtonian liquids

We investigate the rheological properties of a non-Newtonian glass-former liquid within lithographically defined microchannels in the range of temperatures above the vitrification region. The non-Newtonian behavior of the fluid, as evidenced by rotational rheology, is well described by a power law dependence of the viscosity on the shear rate. Taking into account such non-Newtonian character in the equations for the microfluidic motion, we relate the penetration dynamics into capillaries with the liquid rheological properties. The temperature dependence of the viscosity, determined over 1 order of magnitude in the temperature range 286-333 K and for shear rates between 0.07 and 1 s-1, can be described by a Vogel-Fulcher-Tamman law, consistent with the fragile nature of the investigated compound. Microfluidics is a promising analytical approach for the investigation of the rheology of non-Newtonian fluids within confined microenvironments.     

低温热泵用涡旋压缩机性能的试验研究

制定低温热泵用涡旋压缩机试验方案,对研制的原型机进行性能测试。试验结果表明：在冷凝温度不变的情况下,随着蒸发温度的降低,原型机的制热量有所减少,但减少的速度低于普通热泵系统用涡旋压缩机;压缩机的电功率有所增加,但增加的幅度不大,且压缩机的排气温度也有所降低,故在低温工况下采用准二级压缩热泵用涡旋压缩机比采用普通热泵用涡旋压缩机可以更有效地提高空气源热泵的低温制热性能,是寒冷地区用小型空气源热泵比较适宜采用的压缩机。     

连续排水边界条件下土体一维流变固结解析解

基于四元件流变模型,通过引入连续排水边界条件研究了单级加载下的土体一维流变固结问题。利用分离变量法和Laplace变换技术得到瞬时荷载下的解析解,进而通过积分的方法得到单级加载下的解析解。随后对排水边界和流变模型进行了退化,得到Terzaghi边界下及三元件流变模型下的一维流变固结解答,通过与现有解答对比初步验证了该文解答的正确性。最后,对不同界面参数、流变参数以及加载速率对土体固结特性的影响进行了分析。结果表明:基于连续排水边界得到的固结解答与基于Terzaghi双面排水得到的固结解答的差异主要在固结前期,且两者的差异随界面参数α、β取值变大而减小。流变固结与线弹性固结差异主要在固结后期,流变固结需要更长时间土体才能完全固结。此外,土体固结速率随加载速率增大而增大。     

Numerical study of a novel cascading adsorption cycle

A novel cascading adsorption cooling cycle for refrigeration purposes is proposed in this paper. This cycle consists of two zeolite adsorbent beds and a silica gel adsorbent bed. The working refrigerant for the three adsorbers is water. The zeolite adsorbent bed is configured as the high temperature stage while the silica gel adsorbent bed acts as the low temperature stage. Both heat and mass recovery are carried out between the two zeolite adsorbent beds. In addition, heat is also exchanged between the zeolite adsorbent and the silica gel adsorbent beds. A lumped model is assumed for this cascading cycle. The COP for the base case is found to be 1.35, which is much higher than the COP of an intermittent cycle (about 0.5) and a two-bed combined heat and mass recovery cycle (about 0.8). However, its specific cooling power (SCP) of 42.7 W/kg is much lower than that of the intermittent cycle. The numerical results indicate that an optimal middle temperature exists for a prescribed driven temperature. The optimal COP increases with an increase in the driven temperature. However, when the driven temperature increases beyond 503 K, there is negligible change in the COP.     

Thermoelastic analysis of a partially insulated crack in a strip under thermal impact loading using the hyperbolic heat conduction theory

In this paper, the transient temperature and thermal stresses around a partially insulated crack in a thermoelastic strip under a temperature impact are obtained using the hyperbolic heat conduction theory. Fourier and Laplace transforms are applied and the thermal and mechanical problems are reduced to solving singular integral equations. Numerical results show that the hyperbolic heat conduction parameters, the thermal conductivity of crack faces, and the geometric size of the strip have significant influence on the dynamic temperature and stress field. The results based on hyperbolic heat conduction show much higher temperature and much more dynamic thermal stress concentrations in the very early stage of impact loading comparing to the Fourier heat conduction model. It is suggested that to design materials and structures against fracture under transient thermal loading, the hyperbolic model is more appropriate than the Fourier heat conduction model.     

The theory of polymer dynamics

Recent years have brought exciting theoretical advances to understanding the behavior of macromolecular systems under nonequilibrium conditions. Developments in diffusion-controlled reactions of polymers are bringing molecular insights to reactive blending technologies, and improved theories relating to associating polymers should aid in the design of thickening agents and coatings. Recent progress in molecular theories of flow and deformation may facilitate the design of branched polymers with tailored rheological properties and improved adhesives.     

Temperature and stress fields evolution during spark plasma sintering processes

Numerical modelling of Spark Plasma Sintering (SPS) processes is essential to evaluate temperature and stress distributions that can result in sample inhomogeneities. Most of the available literature, however, produced analysis in static conditions. In this work, we focused our attention on the time evolution of current density, temperature and stress distribution during a SPS process using a new approach that includes a PID control in the algorithm, allowing a realistic simulation of experiments performed using a temperature controller. Controlled temperature experiments have been simulated and discussed, with special interest focused on the time evolution of the process. The results showed that stress gradients inside the samples (~40%) are much greater than the temperature gradients (~2%), suggesting that heterogeneities in the microstructure can also be caused by the stress gradient. During the evolution of the process, a peak in stresses is experienced by the alumina sample at the beginning of the cooling stage, caused by differences in contraction between the sample and the die. It has been proved that, using a controlled cooling stage, these peaks in the stresses can be easily eliminated.     

Densification and Sintering Kinetics of Rare Earth (α′+ β′)-Sialon Composites

The sintering processes of Re-(α' + β')-sialon composites (Re=Sm, Dy, Yb) have been investigated by using a specially designed high temperature dilatometer. The initial densification of various samples starts at about 1200℃, and the maximum shrinkage rate of these sialon composites occurs at about 1500℃. The light rare earth sialon has a noticeably tower densification temperature and a higher final shrinkage. The sirtering kinetics of Re-(α' +β')-sialonare much more complex. The Kingery's liquid phase sintering model appears to be applicable,but the mechanism of mass transport in stage two appears to be changeable. The controlling factor shiffs from solution-precipitation to diffusion when densification process proceeds from the earlier part to the later part of this stage.     

A generalized analysis for cascading single fluid vapor compression refrigeration cycles using an entropy generation minimization method

This paper focuses on cascading an ideal vapor compression cycle and determining the optimal intermediate temperatures based on the entropy generation minimization method. Only superheating and throttle losses of the cycle are considered since they are inherent to the ideal vapor compression refrigeration cycle. The second law equations have been developed in terms of specific heats and temperature ratios with the intent of reducing involved property modeling. Also the entropy generation was expressed in terms of a single independent variable and minimized to develop an advanced rule for selecting optimum intermediate temperatures. Results for a cascade system operating between reduced temperatures of 0.684 and 0.981 with R-134a as the working fluid are presented. The approximate method presented here predicted the optimum intermediate reduced temperature for a two-stage system to be 0.88, a difference of 2% from the optimum. The method presented was a much better predictor of the optimum temperature than the geometric mean method which was 0.82, a difference of 5% from the optimum. The entropy generation distribution of the optimum solution was investigated. For a two-stage system, the lower stage and higher stage entropy generation was 44% and 56%, respectively. In comparison to the single stage, the two-stage reduced losses by 78%.     

Experiments on interaction of liquid tin with solid copper

Abstract

The reaction of liquid tin with solid copper has been studied by heating small volumes ofpure tin on copper coupons at various temperatures and times, and evaluating the resulting reaction metallographically. Three reaction temperatures were used:260,400,and450 ° C. Specimen geometry was chosento simulate a typical solder joint. The reaction was observed to occur in two stages: an initial fast stage with copper/liquid tin interface movement rates from 0.2 μm s^-1 at260 ° C to 0.8 μm s^-1 at 450 ° C, followed by a much slower stage. It was concluded that the first stage corresponds to direct dissolution of copper in liquid tin up to or beyond the liquidus concentration for the reaction temperature used.This is followed by the formation of an intermetallic compound layer atthe copper/liquid interface. Subsequent copper dissolution then occurs by solid state diffusion through the compound layer, a much slower process than direct dissolution.