FACTORS INFLUENCED INCIPIENT MELTING IN Hf-BEARING DS Ni-BASE SUPERALLOYS

含Hf的镍基高温合金存在Ni_5Hf相,它是影响合金初熔的主要因素之一。当该相的量大于1.0v-％时,在加热示差热分析(DTA)曲线上的1135-1160℃处出现Ni_5Hf相的熔化峰。P.D法定向铸锭上、下端存在明显的Ni_5Hf偏析,因而对初熔的敏感性明显不同。 Ni_5Hf相的初熔促进了共晶γ′和M_3B_2的熔化。低碳与无碳含Hf合金的初熔倾向更大,缓冷组织和固溶处理的升温速度对初熔是敏感的。 高于3V-％的初熔量将会降低合金的持久强度,其断裂主要沿Ni_5Hf初熔区和共晶γ′之间的界面扩展。采用1150℃8h预处理能消除全部的Ni_5Hf相,有效地控制含Hf合金的初熔。     

影响含Hf定向凝固合金初熔的某些因素

含Hf的镍基高温合金存在Ni_5Hf相,它是影响合金初熔的主要因素之一。当该相的量大于1.0v-%时,在加热示差热分析(DTA)曲线上的1135-1160℃处出现Ni_5Hf相的熔化峰。P.D法定向铸锭上、下端存在明显的Ni_5Hf偏析,因而对初熔的敏感性明显不同。Ni_5Hf相的初熔促进了共晶γ′和M_3B_2的熔化。低碳与无碳含Hf合金的初熔倾向更大,缓冷组织和固溶处理的升温速度对初熔是敏感的。高于3V-%的初熔量将会降低合金的持久强度,其断裂主要沿Ni_5Hf初熔区和共晶γ′之间的界面扩展。采用1150℃8h预处理能消除全部的Ni_5Hf相,有效地控制含Hf合金的初熔。     

一种高硼定向凝固合金的初熔行为及其对力学性能的影响

系统研究了高硼DZ444定向凝固合金的初熔行为及其对力学性能的影响。结果表明,在铸态合金中,枝晶间包含大量g/g′共晶、MC碳化物和由硼化物、Ni5Hf及h相组成的"团聚相"。在固溶处理期间,团聚相周围受硼化物显著影响的g基体首先发生初熔。硼化物不是初熔的形核点,但是对初熔的形成具有关键作用。较高的B含量,使得合金具有较低的初熔温度,介于1160~1170℃之间,明显低于正常合金。提升温度或延长保温时间,初熔现象变得更加严重。采用水淬方式,初熔倾向于凝固为典型的g枝晶和大量细小的沉淀相颗粒;而采用空冷方式时,初熔依次凝固为团聚相、g基体和g/g′共晶相,团聚相形貌与铸态时没有明显差异。完整热处理时,固溶温度由1210℃提升到1230℃,初熔略微增加,而当温度达1250℃时,初熔区尺寸和面积分数剧烈增大,对合金造成严重破坏。由于温度较低,合金的高、低温时效对初熔组织影响不是很大。随着初熔区尺寸和面积分数增加,初熔区消耗了大量的固溶强化元素,同时初熔区内部易萌生大量微裂纹,从而使合金的拉伸性能稍有下降,持久性能显著降低。     

Zr-Nb-Cu合金连续升温相转变动力学研究

采用热膨胀法对Zr-1Nb-0.01Cu合金在不同升温速率下α_(Zr)→α_(Zr)+β_(Zr)相转变动力学进行了研究。结果表明,α_(Zr)→α_(Zr)+β_(Zr)相转变起始温度与升温速率之间的关系为:Tα→α+β=744.4+9.4ln(R H),升温速率对合金相转变温度的影响要小于E110合金;结合JMA方程计算得到α_(Zr)→α_(Zr)+β_(Zr)相转变的激活能为149.1 kJ/mol,低于Zr-4和M5合金的相转变激活能。根据合金相转变动力学曲线,给出了合金连续升温过程α_(Zr)→α_(Zr)+β_(Zr)相转变的CHT连续升温相转变图。     

镍基高温合金U720Li的熔化和凝固过程原位观察

本文采用高温激光共聚焦显微镜（HT-CLSM）和差热分析（DTA）研究了U720Li镍基高温合金的熔化和凝固行为。HT-CLSM熔化过程原位观察表明,升温至1122 ℃时在部分(γ + γ′)共晶颗粒前沿发生了初熔,但温度超过1173 ℃时该熔池才开始迅速扩大;升温至1195 ℃时枝晶干处开始出现离散的斑点状熔池,且该熔池随温度的升高缓慢扩大;升温至1235 ℃时(γ + γ′)共晶开始熔化且随温度的升高该熔池向枝晶干迅速扩展,最终在1333 ℃枝晶完全熔化。HT-CLSM凝固过程原位观察表明,降温至1315 ℃时熔体开始形核凝固,并在1180 ℃左右凝固结束;固相转化速率随温度的降低先缓慢增加,再迅速增加到最大值,然后迅速降低并在凝固后期基本为零。DTA分析表明,铸锭中γ′相开始发生溶解的温度约为1047 ℃,铸锭完全熔化的温度约为1362 ℃。将HT-CLSM原位观察结果与本文及我们前期的DTA分析结果进行对比,发现前者的测试结果较后者低30 ℃左右。     

一种含硼第三代镍基单晶高温合金的初熔及固溶处理研究

采用电子探针测试结合Thermo-Calc软件计算,研究了一种含0.001 wt%硼第三代单晶高温合金枝晶间区域的初熔过程。结果表明,初熔相为M2B,初熔温度为1265 ℃,远低于用差示扫描量热仪测得的共晶熔化温度1352 ℃。采用扫描电镜分析固溶处理过程中合金组织的演化过程,发现共晶组织中马赛克状共晶和扇状共晶率先溶解,粗大γ′最后溶解。初熔区域空冷可发生均质形核,重新凝固后的共晶组织呈圆形,初熔组织中粗大γ′组织较初始凝固的粗大γ′组织更细,而相比残余共晶组织,初熔组织中含有马赛克状共晶和扇状共晶。在通用分步固溶处理制度的基础上,通过低于硼化物初熔的首步处理和适当提高最终固溶温度（ST2）,可以有效避免初熔并完全消除共晶。     

一种含硼第三代镍基单晶高温合金的初熔及固溶处理

采用电子探针测试结合Thermo-Calc软件计算,研究了一种含0.001%(质量分数)硼第三代单晶高温合金枝晶间区域的初熔过程。结果表明,初熔相为M_2B,初熔温度为1265℃,远低于用差示扫描量热仪测得的共晶熔化温度1352℃。采用扫描电镜分析固溶处理过程中合金组织的演化过程,发现共晶组织中马赛克状共晶和扇状共晶率先溶解,粗大γ′最后溶解。初熔区域空冷可发生均质形核,重新凝固后的共晶组织呈圆形,初熔组织中粗大γ′组织较初始凝固的粗大γ′组织更细,而相比残余共晶组织,初熔组织中含有马赛克状共晶和扇状共晶。在通用分步固溶处理制度的基础上,通过低于硼化物初熔的首步处理和适当提高最终固溶温度(ST2),可以有效避免初熔并完全消除共晶。     

CuZnAlMnNi记忆合金慢升温的相变特性

利用差示扫描量热法研究了两种ＣｕＺｎＡｌＭｎＮｉ记忆合金以两种不同速率升温时的相变过程。发现升温速率为１０℃／ｍｉｎ时，ＤＳＣ曲线为单一大峰，而以０．１℃／ｍｉｎ的速率升温时出现许多连续或不连续的小峰。微观机理分析表明，诸多小峰是由合金内各微区相变点略有差异、相变不同步所形成的；单一大峰是在升温速率较快的条件下，各微区在较短时间内相继发生相变而形成的单一合成波。但相变热焓与升温速率无关。     

固相硼化反应阶段的升温速率对Mo_2FeB_2基金属陶瓷显微组织和力学性能的影响

采用反应硼化烧结法制备了Mo_2FeB_2基金属陶瓷。使用扫描电镜(SEM),X射线衍射仪(XRD)研究了固相硼化反应阶段800~1 010℃的升温速率对Mo_2FeB_2基金属陶瓷显微组织和力学性能的影响。研究结果表明:从800℃以不同的升温速率(1~9℃/min)升温升至1 010℃时,随着升温速率的升高,粘结相体积分数先增加后减少,硬质相平均晶粒尺寸和长径比逐渐减小,Mo_2FeB_2硬质相晶格常数a逐渐降低,c基本保持不变,硬质相形貌逐渐由长条状转变为近等轴状;随着升温速率的升高,金属陶瓷抗弯强度先升高后降低,硬度逐渐升高,断裂韧性逐渐降低。以5℃/min升温得到的金属陶瓷可获得较为均匀的显微组织,且具有较好的综合力学性能,其抗弯强度为2 367.5 MPa,硬度为88.2 HRA,断裂韧性为27.6 MPa·m~(1/2)。     

TC4-Fe合金连续升温过程的相变研究

通过连续升温热膨胀法(DIL)研究了Ti6Al4V-0.55Fe合金在连续升温过程中的α相回溶（α+β→β）的热膨胀行为和显微组织演化。本文采用了1、5、10、15 K/min的四种升温速率对Ti6Al4V-0.55Fe合金进行热膨胀实验,结果发现：不同升温速率的α相回溶曲线都展现出典型的“S”型曲线,表明α相回溶是一种由形核长大控制的过程。通过Kissinger-Akahira-Sunose（KAS）方法和Kolmogorov-Johnson-Mehl-Avrami(KJMA)模型分别得到α相回溶转变平均相变激活能E和随着α相回溶体积的增大所对应的Avrami指数n,可分为三个阶段,即相变初期（0相似文献   

匀速加热过程中第二相的扩散溶解

通过求解匀速升温条件下的扩散方程，建立了匀速加热过程中扩散控制的第二相溶解模型，给出了基体中的溶质分布及第二相溶解速度的解析表达式．以Al—5.8％Cu合金为例的计算结果表明，第二相的形态和加热速率显著影响基体中的溶质分布和第二相的溶解速度．Al—5.8％Cu合金的热分析和液淬实验结果验证了所建模型的合理性.     

金属Cu熔化及晶化行为的计算机模拟

采用分子动力学方法对500个金属Cu原子的模型体系在熔化及晶化过程中的结构组态、能量变化进行了计算机模拟研究．原子间作用势采用FS势．模拟结果表明：在连续升温过程中，金属Cu在1444K熔化；在较慢冷却条件下，液Cu在1014K结晶；在较快冷速条件下，液Cu形成非晶态．从能量的角度分析了模拟过程中温度变化速率对结果的影响，并给出了体系微观结构与能量变化关系的物理图像．     

La0．67Mg0．33Ni2．5Co0．5贮氢合金的制备和MH电极性能研究

采用高频感应熔炼方法制备了PuNi3型La0.67Mg0.33Ni2.5Co0.5合金;用X射线衍射分析和电化学方法研究了添加不同Mg含量以补偿Mg元素烧损时合金的组织结构和电化学性能。X射线衍射分析(XRD)表明,铸态合金由．PuNi3型主相和少量的CaCu5型第二相组成,铸态合金经1223K和10h退火处理后,CaCu5型第二相可明显减少,其中Mg增加10％时得到纯度较高的PuNi3型组织。电化学测试表明,增加适当Mg含量和进行退火热处理能明显提高和改善合金电极容量、循环稳定性和大电流放电性能。与AB5型和。482型Laves相贮氢合金比较,PuNi3型La0.67Mg0.33Ni2.5Co0.5贮氢合金具有电极容量高及优良的大电流放电性能。     

亚共晶Al-5.8%Cu合金的非平衡熔化行为

分析了Al-5.8%Cu亚共晶合金在不同加热条件下的DSC试验结果，研究了具有不同凝固组织的试验合金的非平衡熔化行为。研究表现，其熔化开始温度随加热速度的增大略有升高；共晶熔化激活能与凝固组织相关，慢速凝固组织的熔化激活能大于快速凝固组织。凝固组织中存在的非平衡共晶在熔化前的加热过程中大部分溶解，剩余部分参与共晶熔化，溶解的非平衡共晶的量受溶质扩散面积和加热速度的共同作用，随加热速度的减小和扩散表面积的增大而增大。     

Vaporization and precipitation characteristics of cerium nitrate precursor droplets heated by monochromatic irradiation

A novel experimental setup using radiative heating has been used to simulate the high heating rates of solution precursor plasma spray process and understand the thermophysical phenomena inside cerium nitrate precursor droplets. CO₂ laser having a heating rate comparable to that of the plasma has been adapted to heat pendant precursor droplets generated at the tip of a micro-needle. The process has been imaged with a high speed video camera. The images have depicted two distinct phases during the entire heating process. In the first stage, the surface evaporation is predominant while the second phase is dominated by precipitation phenomena. Inside the droplet during the heating process formation of explosive bubbles resulting in severe instability and shape deformation has been observed. The droplet deformation was found to be dependent on heating rate and solute concentration in the precursor solution. High laser power (high heating rate) makes the droplet prone to deformation while high concentration precursor solution being highly viscous increases the resistance against instability and deformation. It was also observed that the precipitate transitions through a series of chemical transformations from the initial gelatinous phase into the final solid agglomerate during the last part of the heating process. The SEM images of the precipitate showed a porous morphology. However high concentration precursors with low heating rate resulted in dense microstructure. It was also observed that the micro-structure exhibited irregular patterns, which is evidence of the presence of multiple nucleation sites. The number of nucleation centers increased with the increase of solute concentration and laser power, while XPS analyses confirmed the presence of Ce³⁺ and Ce⁴⁺ in the residue.     

Al-5.8%Cu合金半固态浆料的控制熔化制备法

研究了具有枝晶组织的Al-5.8%Cu合金在控制熔化过程中固相形态的演变,以及加热速率、保温时间对固相形态演变的影响.分析了影响熔化组织中固相形态的因素,提出由枝晶组织直接获得适合半固态成形的球晶组织的控制熔化工艺,并制备了固相颗粒圆整、尺寸较小、分布均匀的半固态浆料.     

超高功率电炉无氧状态下冶炼低磷、硫钢

主要介绍超高功率电弧炉在无氧状态下冶炼低磷、硫钢的工艺特点。采取矿石+铁皮的氧化方法造泡沫渣,利用熔化末期、氧化初期大法量去磷,从而达到工艺要求。     

Joule heating of solid wires in MAG welding

There are numerous studies^1-12 concerning the melting rate of welding wire and various investigations have been carried out on welding current, welding current waveform, wire extension, polarity, chemical composition, wire diameter, arc length, shielding gas composition and melting rate. Joule heating of bulk wire has been a large contributory factor to the melting rate of solid wire for carbon steel (hereafter referred to as either wire or solid wire) and Joule heating is a crucial factor in aspects of melting stability. However, there are few case studies concerning the detailed measurement of Joule heating at the contacts between the contact tube and wire and the calculation of melting rate.     

马氏体回火过程中组织转变量预测的实验研究

利用45号碳素钢，实验研究了不同加热速度及保温温度和保温时间条件下马氏体回火过程的组织变化，结果发现，即使在高速加热过程中，马氏体也能发生回火组织转变．通过数据处理，利用回火参数λ建立了能比较准确、方便地预测回火组织转变量的动力学方程，数值模拟计算结果表明，马氏体回火组织转变量计算结果与实验结果吻合。     

Study on Heat Generation Mechanism and Melting Behavior of Droplet Transition in Resistive Heating MetalWiresEI北大核心CSCD

With the development of space technology, the ability of manufacturing in space is a necessary guarantee for a long-term space mission. To achieve the repair and maintenance of spacecraft structure in space, a metal additive manufacturing method named resistance heating metal wire additive manufacturing process has been proposed in this work. During the experiments, the wire and the base plate are short-circuited, the current output from the programmable power source flows through the wire and the base plate to generate resistance heat, and then the wire begins to melt and transfer to the base plate. A real-time synchronization system has been used to record the current, voltage and image of metal wire synchronously, to study the melting process of metal wire by resistance heating. The direct current and pulse current with different amplitudes which were supplied by programmable power source have been used to study the effect of the current style and value on the melting process and transition behavior of metal wire. The change characteristic of the resistance in the wire and base plate has been analyzed during wire melting, to study the relationship between the current resistance and the wire state. The effect of gravity on the wire melting process has been studied by the wire transfer experiments at different space locations. The results show that when the metal wire was heated by the constant current, the total heat of metal melt could be controlled by controlling the current value, but it was difficult to precisely control the heating speed and the heat input. When using pulse current heating, both the heating speed and the heat input could be precisely controlled by pulse frequency and pick value. In the melt transfer stage, the constant current provides a fixed force on the molten wire, but the pulse current makes the molten wire swing by the intermittent force. The real-time resistance of metal wire during heating could be used to reflect the melting state of wire in both current styles. On the ground environment, the surface tension and electromagnetic contraction force make the melting wire against the gravity and transfer to the base plate, which illustrated the feasibility of using this process in space environment.