强磁场作用下材料扩散型固态相变的微观机理

为探讨强磁场对物质原子尺度行为(电子运动、离子扩散)的影响,采用光学显微镜研究强磁场作用下Fe-0.12%C合金的扩散型固态相变;采用数字多用表测量强磁场作用下的纯铝板电阻研究其电子分布.结果表明:随磁感应强度增强,Fe-0.12%C合金室温显微组织中,铁素体晶粒平行于磁场方向伸长并呈链状排列的趋势增强,珠光体团的长轴方向平行于磁场方向伸长的程度也增强;纯铝板的电阻在平行于磁场方向放置时减小,垂直于磁场方向放置时电阻有增加趋势.这是由于组成金属晶体的自由电子和排列成晶格状的金属离子在磁场作用下受到洛伦兹力的作用,随磁感应强度增强,沿磁场方向的电子浓度、金属离子扩散有增强趋势,导致磁场作用下材料扩散型相变的室温组织出现形状各向异性.     

强磁场下冷却速率对Fe-0.76%C钢先共析铁素体组织的影响

研究了在12T强磁场下冷却速率对Fe-0.76%C钢中先共析铁素体的显微组织和性能的影响,结果表明:先共析铁索体晶粒的伸长方向与磁场方向的夹角随着冷速度的提高而增大,其原因是高速冷却时原子扩散减弱.在冷却速率相同的条件下,与非磁场热处理样品相比,强磁场热处理样品的先共析铁素体面积明显增加,宏观硬度下降,因为强磁场使Fe-0.76%C钢表现出更明显的亚共析钢特征.     

Effect of cooling rates on the microstructure of proeutectoid ferrite in Fe-0.76%C alloy under high magnetic field

研究了在12 T强磁场下冷却速率对Fe-0.76%C钢中先共析铁素体的显微组织和性能的影响,结果表明: 先共析铁素体晶粒的伸长方向与磁场方向的夹角随着冷速度的提高而增大,其原因是高速冷却时原子扩散减弱. 在冷却速率相同的条件下, 与非磁场热处理样品相比,强磁场热处理样品的先共析铁素体面积明显增加, 宏观硬度下降,因为强磁场使Fe-0.76%C钢表现出更明显的亚共析钢特征.     

强磁场对硅锰铸钢于A_1点附近等温珠光体相变的影响

通过开展12 T强磁下硅锰铸钢(50Si_2Mn_3)于A_1点温度附近不同等温时间的热处理试验,并对比强磁场与无磁场条件下等温珠光体相变试验结果,研究了磁场对珠光体转变量及组织形貌的影响。研究表明,对于50Si_2Mn_3在A1点以下较高温度范围内,强磁场可缩短珠光体相变的孕育期,促进硅锰铸钢珠光体转变,使其转变量明显增加;强磁场能够提高珠光体转变临界点,使50Si_2Mn_3在A1点以上等温过程中能够发生珠光体相变。     

强磁场下Al-Si过共晶合金组织细化原理研究

为了改善Al-Si合金性能,研究了强磁场对Al-Si过共晶合金组织的影响.依据热力学和晶粒形核理论,阐述了组织变化的原因.研究表明:当Al-Si过共晶合金在600℃施加强磁场并平行于磁场方向下凝固时,共晶组织被细化,但对初生硅相影响不大;磁场强度越大,细化效果越明显;强磁场降低了固态熵和磁自由能的影响,使共晶组织临界形核半径减小,而硅由于是逆磁质对其影响不大;强磁场使液态金属平行于磁场方向流动,进一步细化了组织.     

稳恒强磁场对Al-Cu扩散偶界面中间相形成和生长的影响

研究了稳恒强磁场作用下Al-Cu扩散偶界面中间相组成和扩散行为.结果表明:强磁场作用下扩散偶中间相厚度显著增加,并且随磁场强度增大,界面中间相组成发生变化.按照抛物线规律计算了强磁场作用下扩散偶中间相的扩散系数,发现在平行和垂直于磁场的两个方向上扩散系数有显著差异,垂直于磁场方向的扩散系数比平行于磁场方向更大.强磁场促进了Al-Cu扩散偶中Al和Cu原子的扩散,加速了界面中间相的形成和生长过程.磁场作用差异导致了扩散的各向异性.利用原子扩散理论初步分析了产生上述现象的原因.     

10T平行强磁场下Ni-纳米Al2O3复合共沉积的研究

将Ni-纳米Al2O3复合电镀过程移置于10T平行强磁场（B∥J）中，探讨了有无平行强磁场条件下电流密度对镀层形貌及镀层中纳米颗粒含量的影响规律。结果表明，在10T平行强磁场下，氧化铝颗粒在复合镀层中呈网格状分布，并且网格的尺寸随着电流密度的增加而增加；电流密度显著影响镀层中纳米氧化铝的含量，不论是否施加磁场，当电流密度为2A·dm^-2时镀层中颗粒含量最高。施加强磁场时，可使镀层在低电流密度下获得较高的纳米颗粒含量。     

磁场方向对碱腐蚀构建多晶硅绒面结构的影响

在零磁场和2T、4T磁场中用NaOH溶液腐蚀制备多晶硅绒面结构,样品板平面分别平行和垂直于磁场放置。用电子天平称重表征硅片的腐蚀程度、用奥林巴斯LEXT OLS4100共聚焦显微镜观察多晶硅片形貌、用Ocean Optics USB4000光谱仪测量多晶硅片的反射率、用WT-1200硅片测试仪测量样品的少子寿命,研究了磁场方向对碱腐蚀构建多晶硅绒面结构的影响。结果表明：随着磁感应强度的提高多晶硅片的腐蚀程度严重,绒面结构变得均匀和细腻,反射率降低;在磁感应强度相同的条件下碱液中沿着磁场方向运动的OH^-离子不受磁场力作用,而运动方向与磁场方向不完全一致的OH^-离子受磁场产生的Lorenz力作用。Lorenz力使板平面垂直于磁场方向的硅片样品腐蚀程度更加严重、绒面和断层状结构细腻程度更加显著、少子的寿命更长、反射率更低。磁感应强度为4T时反射率降低到14.5%,在用碱液腐蚀制备多晶硅绒面结构过程中施加强磁场,板平面垂直磁场方向放置硅片减反射效果更加显著。     

强磁场下Cu-50%Ag合金定向凝固过程中的组织及固液界面形貌演变

目的 研究强磁场下Cu-50%(质量分数)Ag合金定向凝固过程中的组织演变、固液界面形貌变化及溶质迁移行为,分析强磁场对金属凝固过程的作用机制,为强磁场下的金属材料制备提供理论借鉴和指导。方法 在不同的凝固速率与磁场条件下进行定向凝固和淬火实验,对合金的定向凝固组织、糊状区与固液界面形貌以及溶质分布行为进行考察。结果 强磁场破坏了凝固组织的定向生长,使凝固组织转变为枝晶与等轴晶共存的形貌;强磁场诱发了熔体对流,减少了糊状区中溶质的含量;强磁场改变了固液界面处的溶质分布和固液界面形貌,破坏了固液界面的稳定性。结论 强磁场通过洛伦兹力和热电磁力的共同作用,诱发了糊状区内液相的纵向环流,改变了固液界面及糊状区中的组织形貌与元素分布。     

磁场处理对LDPE及其碳纳米管复合材料电导特性的影响

在低密度聚乙烯(LDPE)及其碳纳米管(CNTs)复合材料的热成型加工过程中施加稳恒强磁场,研究了磁场处理对LDPE及CNT/LDPE复合材料直流电导特性的影响,并结合差示扫描量热、偏光显微镜与原子力显微镜分析探讨了磁场处理的作用机理。结果表明,磁场处理能导致LDPE的结晶度提高,体积电阻率增加;稳恒强磁场能在CNTs中"诱导"形成感应磁矩,使得CNTs沿平行于磁场方向在LDPE中取向,从而导致CNTs/LDPE复合材料沿平行于磁场方向的电导率增加,电导非线性特性提高;磁场处理导致CNT/LDPE复合材料电导率增加的幅度随CNTs掺量的增加而增大。     

Effects of high magnetic field strength and direction on pearlite formation in Fe–0.12%C steel

The effect of a magnetic field on the formation of pearlite in a Fe–0.12%C steel was investigated. The results show that pearlite colonies elongate and align along the field direction, and that this tendency increases with increasing magnetic field strength. The possible preferential nucleation of ferrite between existing ferrite grains aligned along field direction at the later stage of proeutectoid transformation promotes carbon diffusion into the austenite areas between the ferrite chains accounts for the phenomena. Moreover, the field effect is dependent on the specimen position with respect to the field direction.     

珠光体对ZG120Mn13钢拉伸断裂过程的影响

为探究珠光体降低高碳高锰钢机械性能的原因,本文采用金相组织分析、机械性能测试和断口微观形貌分析等实验方法,研究了奥氏体基体上含体积分数23%珠光体的ZG120Mn13高碳高锰钢的拉伸性能及其裂纹形核和扩展过程.结果表明:通过时效处理,在奥氏体基体上析出的条状、颗粒状以及沿晶界连续分布的珠光体将使ZG120Mn13钢的强度和塑性大幅度下降.机械性能的降低与其力学行为有关,当基体为单一奥氏体时,裂纹将在大量孪生变形后,在孪晶界、孪晶与晶界交界处形核,并沿孪晶界长大而相互连接、扩展.而奥氏体基体上存在珠光体时,裂纹主要在珠光体团内形核,并通过相邻珠光体间奥氏体的塑性耗竭、切断而得以扩展.     

Effect of high magnetic field on the processing of pearlitic steels

This paper compares data from various sources concerning the impact of high magnetic field (HMF) on changes that occur in pearlite with respect to microstructure, phase transition, and mechanical properties. HMF raises the transformation temperatures of both ferrite and pearlite. This effect can be enhanced by increasing the carbon content. Other alloying elements may influence austenite decomposition temperature, Curie temperature, and magnetic moment, thus either increasing or decreasing the effect of HMF on phase transformation temperature. By altering the transformation thermodynamics, HMF increases the volume fraction of proeutectoid ferrite, decreases that of cementite, and decreases the lamellar spacing. HMF introduces the microstructure anisotropy and aligns proeutectoid ferrite grains parallel to the direction of the HMF. This effect becomes smaller when the cooling rate is higher. By affecting both phase transformation and the alignment of grains, HMF affects the morphology and microstructure of proeutectic ferrite and pearlite, and, consequently, their mechanical properties and corrosion resistance.     

Correlation of a plastic zone length near the tip of a mode I crack in an orthotropic material with anisotropy parameters

We derived equations that relate the length of a plastic zone near a mode I crack tip in a plate made of an orthotropic material with yield strength levels in the direction of the anisotropy axes. The case of crack orientation along one of the anisotropy axes is examined, with the latter being determined by the strain hardening of a material at the stage preceding the crack nucleation. The growth of yield strength along the axes lying in the plane of the plate is shown to result in smaller sizes of the plastic zone. An increase in yield strength in the direction of the normal to the above plane leads to an increase in its length. Ukrainian Regional Research and Design Institute of Civil Engineering, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 32–37, July–August, 1999.     

GCr15轴承钢高温变形后控冷工艺的研究

轴承钢棒材轧后温度较高导致的网状碳化物析出严重影响我国高质量轴承钢生产.在热模拟试验机上对GCr15轴承钢进行了试验研究,分析了不同控冷工艺参数对GCr15轴承钢二次碳化物的析出和珠光体转变的影响.研究表明,GCr15轴承钢经980℃高温变形后快速冷却,随着冷却速度的增加,晶界处二次碳化物由半网状分布、短棒状分布到最后弥散析出,珠光体球团直径和片层间距减小,并有退化珠光体生成.轴承钢中退化珠光体组织的出现,是由于其热变形后快速冷却,抑制了先共析碳化物在冷却过程中的过早析出造成的.较合理的冷却工艺是GCr15轴承钢高温变形后快速冷却到700℃,再以3℃/s的冷却速度进行冷却.     

连续缓慢冷却贝氏体转变特征及合金元素分布的影响

利用热膨胀仪对实验用预硬型塑料模具钢不同冷却速率进行热模拟,观察组织形貌,并用电子探针显微分析仪对不同组织区域进行定点分析,发现Mo,Si在抑制珠光体转变及粒状贝氏体转变过程中起重要作用.在冷速≤0.015℃/s 冷却条件下,组织转变较为复杂,转变过程中有珠光体出现.由于内部合金元素微观分布不均匀,贝氏体组织分为片状和...     

An improved method for magnetic identification and localization of cracks in conductors

A SQUID magnetometer can be used to measure the magnetic field produced by flaws in a two-dimensional, conducting plate carrying a current. Identification of the flaw-induced magnetic field is difficult because of the large magnetic field associated with the edges of the plate and the current in the leads that connect the plate to the power supply. We have developed a technique by which the wire and edge fields can be cancelled prior to mapping the magnetic field. In this technique, a similar unflawed conducting sheet is placed adjacent to the flawed plate, with a connection between the sheet and the plate at one edge, and with the opposite edges of the sheet and of the plate connected to the two conductors of a coaxial cable. Thus, an applied current will flow along one conductor of the cable, across the cancelling sheet, cross into the flawed plate, return along the plate, and then return to the power supply along the other conductor of the coaxial cable. As a result of this geometry, there is no magnetic field from the lead-in wires because they are coaxial, and the magnetic field due to the edges of the plate is cancelled by the opposing magnetic field of the edges in the adjacent sheet. The extent of cancellation is determined primarily by the separation between the plate and the cancelling sheet, by the thickness of the plate, and by macroscopic inhomogeneities in their electrical conductivities.     

中厚板加速冷却和直接淬火时冷却能力研究

为了确定加速冷却或直接淬火时实现预期的冷却速率所需的对流换热系数,利用MSC.MARC有限元分析软件对Q345B中厚钢板冷却过程中温度场进行了数值模拟计算.确定了实现直接淬火条件下不同厚度(20 mm)钢板的理论极限冷却速率所需的对流换热系数为15 000 W/(m2·℃),并分析了冷却速率与对流换热系数、钢板厚度之间关系.研究表明,对于同一厚度、材质中厚钢板,其冷却速率随对流换热系数的增加而增大.超快速冷却或直接淬火时,带钢冷却速率随对流换热系数增加而显著增加;对流换热系数大于15 000 W/(m2·℃)时,厚度(30 mm)钢板的冷却速率基本不变,达到其物理极限冷却速率;换热系数增加,厚度方向上温度梯度增加.     

Heat transfer enhancement in magnetic cooling by means of magnetohydrodynamic convection

In this work, we analyze the potential of magnetohydrodynamic (MHD) convection to increase heat transfer during magnetic cooling. To do this, we consider a section of an active magnetic regenerator, namely a flat gadolinium plate, immersed in an initially stagnant heat transfer fluid (NaOH) which is placed in a cuboid glass cell. To create the MHD flow, a small electric current is injected by means of two electrodes and interacts with the already present magnetic field. As a result, a Lorentz force is generated, which drives a swirling flow in the present model configuration. By means of particle image velocimetry and Mach–Zehnder interferometry, the flow field and its impact on the heat transfer at the gadolinium plate are analyzed. For the magnetization stage, we show that a heat transfer enhancement by about 40% can be achieved even with low currents of 3 mA.