磁场热处理后多晶Ni2 MnGa铁磁形状记忆合金组织的变化

采用定量金相、X射线衍射等手段，研究了在恒定磁场下加热到高于Tc或Af温度保温30min后水淬或空冷对多晶Ni2MnGa组织的影响。结果表明，在相同的热处理工艺下，外加磁场均能促进马氏体相的形成。同时，磁场热处理使马氏体孪晶变体的位向分布发生明显改变，产生不同程度的择优排列(磁织构)；在经过优化的工艺条件下磁场热处理能使多晶Ni2MnGa的磁驱动形状回复应变得以提高。     

两种热处理40CrNi2Si2MoVA钢的微观组织与力学行为EI

观察和比较了油淬热处理和等温热处理４０ｃｒＮｉ２Ｓｉ２ＭｏｖＡ钢的显微组织。研究表明，两种热处理后钢均得到回火马氏体＋贝氏体＋残余奥氏体的混合组织，但贝氏体及残余奥氏体形态有所不同。等温热处理后钢的韧性相较多而对韧性有益，主要表现为裂纹起始功的提高。     

Q-I-Q-T热处理新工艺下两种奥氏体化温度对60Si2CrVAT弹簧钢疲劳强度的影响

研究了淬火-等温-再淬火-回火(Q-I-Q-T)热处理新工艺下两种不同奥氏体化温度对60Si2CrVAT弹簧钢疲劳强度的影响,得到新工艺显微组织为马氏体/贝氏体及少量薄膜状残余奥氏体的复相组织。结果表明:低温奥氏体化状态较高温状态,马氏体、残余奥氏体的量减少,贝氏体的量增加。由于高温奥氏体化状态晶粒粗化,晶界处淬火应力集中,疲劳裂纹主要在晶界界隅处萌生;低温奥氏体化状态晶粒细化,疲劳裂纹主要由内部夹杂物引起。低温奥氏体化状态疲劳强度高于高温奥氏体化状态。     

外磁场对Ni-Mn-Ga磁性形状记忆合金相变应变及显微组织的影响

研究了外磁场对Ni52Mn24.6Ga23.4(%,原子分数)单晶马氏体相变及其相变应变的影响,并对磁场增强相变应变的微观机制进行了探讨。研究结果表明无外加磁场时,NiMnGa合金发生马氏体相变时可产生约0.3%的收缩形变,沿单晶[100]方向施加外磁场,其相变应变随磁场的增加而呈近线性增加。当外磁场强度为6.37×105A/m时,应变量达到最大值(3.5%)。磁场作用下冷却形成的马氏体虽然孪晶亚结构不变,但自协作组态消失,并伴随有孪晶板条的增厚。磁场对马氏体相变应变的增强效应来自于磁场作用下的马氏体变体的择优取向。     

再制造大型热轧支承辊的堆焊层开裂失效分析

针对再制造大型热轧支承辊堆焊层的服役早期开裂现象,通过检测分析堆焊层硬度、显微组织和断口形貌,确定堆焊层开裂失效机制是低周接触疲劳破坏.由于堆焊层内残余奥氏体含量过多,马氏体相含量相对不足,堆焊金属低硬度、低强度,降低了再制造热轧支承辊面堆焊层的抗接触疲劳性能.通过高温回火热处理促使堆焊层残余奥氏体向马氏体转变,调控辊...     

机械装备的失效分析(续前) 第8讲 失效诊断与预防技术(4)

正3.4库存或运输中的失效诊断3.4.1残余内应力引起开裂失效(1)工件淬火热处理后其内部会产生较大的残余内应力,同时马氏体形成时的相互碰撞还会产生大量的显微裂纹,如果不及时回火,可能会导致工件开裂失效。(2)工件焊接后不但会产生较大的焊接应力,有时还会产生硬而脆的马氏体组织,若焊后不及时     

中断正火热处理工艺的研究与应用

对铸钢中断正火热处理工艺进行了研究。结果表明:铸钢件利用铸造余热进行中断正火法热处理后,其基体组织是下贝氏体+马氏体+少量残余奥氏体。因而使铸钢件的强度和韧性获得最佳匹配,并使铸钢件的耐磨性也相应得到提高。     

低碳中合金钢破碎机锤头的研制

研究了一种新型的破碎机锤头,对其采用合金化处理并经过热处理,实验表明:该低碳中合金耐磨钢的硬度大于51HRC,冲击韧性大于141J/cm2。经热处理后的显微组织为回火马氏体+残余奥氏体+弥散的碳化物,比高锰钢破碎机锤头性能有很大提升。     

钢材相似显微组织的分析与鉴别(续)

在日常金相检验中,常常会观察到钢的非平衡组织中存在颜色和形态比较相似的显微组织,初学者往往容易出现误判。笔者以常用钢铁材料为例,采用常规光学显微镜,通过对组织形态、分布、颜色以及热处理工艺进行仔细比对,总结归纳了铁素体与残余奥氏体、碳化物与残余奥氏体、低碳板条马氏体与羽毛状上贝氏体以及片状马氏体与下贝氏体等几种相似显微组织的鉴别方法,以供金相检验人员参考。     

缸套渗碳淬火工艺的改进

渗碳钢20crMnTi执行一次淬火工艺后,硬度值偏低,在48—50HRC之间,金相组织为粗针状马氏体＋35残余奥氏体＋未溶碳化物,因为粗针马氏体与残余奥氏体的存在,使得渗碳件的强度及表面硬度降低。执行二次淬火工艺后表面硬度达到了60HRCP以上,组织为隐晶马氏体＋弥散的颗粒状碳化物,该工艺有效的减少了残余奥氏体量,提升了渗碳件的硬度及耐磨性。     

磁控溅射Ni-Mn-Ga薄膜的磁致应变

采用直流磁控溅射的方法,在NaCl基底上沉积了Ni-Mn-Ga薄膜,对薄膜进行了形貌观察、微区成分及结构分析,并测量了薄膜的磁致应变.结果表明,薄膜表面可见大小不一的团簇颗粒,具有明显的岛状结构,表明Ni-Mn-Ga薄膜的形成为典型的核生长型机制.热处理前的薄膜具有部分非晶存在,热处理后薄膜晶化为多晶形态.无约束薄膜在磁场下呈现负的磁致应变,在1.3T磁场下,其最大应变值可达-0.008%,并且可以完全恢复.     

Electromagnetic control of convective heat transfer during electron beam evaporation: model experiments

The present paper aims to demonstrate that melt-flow during electron beam evaporation can be effectively controlled by using external magnetic fields to considerably reduce the convective heat transfer. We discuss the various effects of a static magnetic field, a static field combined with an applied electrical current, and a rotating magnetic field. We perform model experiments using GaInSn in eutectic composition as a test liquid. The liquid metal is heated locally at its free surface by an electric resistance heater. The results of the measurements are compared to prediction of numerical simulations.     

Spherical porous hydroxyapatite granules containing composites of magnetic and hydroxyapatite nanoparticles for the hyperthermia treatment of bone tumor

Spherical porous granules of hydroxyapatite (HA) containing magnetic nanoparticles would be suitable for the hyperthermia treatment of bone tumor, because porous HA granules act as a scaffold for bone regeneration, and magnetic nanoparticles generate sufficient heat to kill tumor cells under an alternating magnetic field. Although magnetic nanoparticles are promising heat generators, their small size makes them difficult to support in porous HA ceramics. We prepared micrometer-sized composites of magnetic and HA nanoparticles, and then supported them in porous HA granules composed of rod-like particles. The spherical porous HA granules containing the composites of magnetic and HA nanoparticle were successfully prepared using a hydrothermal process without changing the crystalline phase and heat generation properties of the magnetic nanoparticles. The obtained granules generated sufficient heat for killing tumor cells under an alternating magnetic field (300 Oe at 100 kHz). The obtained granules are expected to be useful for the hyperthermia treatment of bone tumors.     

The influence of an external magnetic field on the magnetic part of the specific heat of a ferromagnetic substance

An equation is derived for the magnetic part of the specific heat of iron using the Weiss Molecular Field Theory. Numerical values are obtained, using a computer, for the magnetic part of the specific heat with an external magnetic field. The results show that an external magnetic field has a considerable influence on the magnetic specific heat at the Curie temperature.     

磁场热处理对NdFeB非晶快淬粉末的晶化与磁性的影响

研究了外中磁场对非晶Ｎｄ５．５Ｆｅ６６Ｂ１８．５Ｃｒ５Ｃｏ５，Ｎｄ４．５Ｆ３７７Ｂ１８．５和Ｎｄ１０．５Ｆｅｎ７０Ｂ７．０Ｚｒ２．５Ｃｏ１０粉末的晶化与磁性的影响，发现在热处理过程中加磁场可促进淬非晶粉末的晶化，使相转变在较低的温度下进行，讨论了各向异性复合纳米永磁材料可能的制备方法。     

Forced convection of a temperature-sensitive ferrofluid in presence of magnetic field of electrical current-carrying wire: A two-phase approach

This research examines laminar forced convection of a temperature-sensitive magnetic nanofluid flowing within a horizontal tube through the two-phase mixture model. The ferrofluid flowing in the tube is exposed to the magnetic field generated by electrical current-carrying wire(s) along the tube, and the effect of such magnetic field is studied on heat and mass transfer phenomena. It is observed that due to the dependency of magnetization on temperature, the cold fluid flowing at the central regions of the tube is attracted more significantly towards the source of the magnetic field, which results in creation of secondary flow. Such mixing in the flow, subsequently, disturbs the thermal and hydrodynamic boundary layers, especially at the vicinity of the magnetic field source, leading to better heat transfer rate and also higher pressure drop. Furthermore, increasing the strength of the magnetic field leads to greater enhancement in heat transfer, while increasing the Reynolds number decreases the effectiveness of the magnetic field on the ferrofluid flow and heat transfer. Moreover, placing two wires above and under the tube can enhance the heat transfer even more significantly, such that the average convective heat transfer coefficient in this case is about 34.5% higher than that of the case without magnetic field.     

In vitro heat generation by ferrimagnetic maghemite microspheres for hyperthermic treatment of cancer under an alternating magnetic field

Ferrimagnetic materials can be expected to be useful as thermo seeds for hyperthermic treatment of cancer, especially where the cancer is located in deep parts of body, as they can generate heat by magnetic hysteretic loss when they are placed in an alternating magnetic field. Recently, it was reported that ferrimagnetic maghemite (γ-Fe₂O₃) microspheres 20–30 μm in diameter prepared in aqueous solution can show excellent heat generating ability. However, these microspheres have many cracks on their surfaces. In this study, the preparation conditions for the microspheres was further optimized in order to obtain crack-free ferrimagnetic microspheres, and the in vitro heat generation of the obtained microspheres was measured in an agar phantom under an alternating magnetic field. Crack-free γ-Fe₂O₃ microspheres 20–30 μm in diameter were obtained successfully. Their saturation magnetization and coercive force were 68 emu g⁻¹ and 198 Oe, respectively. Their heat generation under an alternating magnetic field of 300 Oe at 100 kHz was estimated to be 42 W g⁻¹. The microspheres showed in vitro heat generation when they were dispersed in an agar phantom and placed under an alternating magnetic field. It is believed that these microspheres may be useful for the in situ hyperthermic treatment of cancer.     

磁流体在交变磁场中的热效应研究

制备了平均粒径分别为4、6 和8nm 的Fe3O4 粒子,将其分散在不同的介质中,在63kHz、7kA/m的交变磁场中研究其热效应。实验结果显示,粒径较大的Fe3O4 粒子具有较高的产热比功率(SAR),Fe3O4 粒子经过表面处理后SAR显著增加,而且SAR随表面活性剂不同而不同,还显示SAR与磁场强度的平方成正比。     

Numerical simulation of free convection of MHD non-Newtonian nanofluid within a square wavy enclosure using Meshfree method

Abstract

In this article, flow and heat transfer of nanofluids inside a wavy square enclosure filled with non-Newtonian (shear thinning) nanofluid under magnetic field has been simulated numerically. Single-phase model is used. The governing equations have been solved numerically using element free Galerkin method. The results are obtained for isotherms, streamlines, and average Nusselt number for various values of Hartmann number, Rayleigh number, nanoparticles volume fraction, and power-law index. Here, the main objective is to explore the effect of power-law index, Rayleigh number, Hartmann number, and volume fraction on average Nusselt number. It is found that in the absence of magnetic field, Nusselt number drops on increasing the value of power-law index whereas in the presence of magnetic field, heat transfer rate increases with increase in power-law index. With the increase in Rayleigh number and volume fraction of nanoparticles, the heat transfer rate increase in all cases. This type of problem has a direct application in the coolant systems, solar collector where the structure used is wavy in order to increase the rate of heat flow. Here, EFGM is efficiently applied for simulation due to irregular domain, which creates a novelty in the work.