基于MoSi2／316L不锈钢连接的烧结工艺及其梯度过渡层优化

采用放电等离子烧结(SPS)技术,以MoSi2/316L不锈钢梯度材料作为过渡层连接MoSi2 与316L不锈钢,优化了具体的连接工艺和梯度过渡层.结果表明优化得到的连接工艺为:以100 ℃·min-1速度升温至1000 ℃,保温10 min,再以6 ℃·min-1速度冷却至600 ℃后随炉冷却.其中升温时的烧结压力为50 MPa,降温时为40 MPa;优化得到的梯度过渡层结构为:梯度层层数n=7,梯度层厚度d=0.8,梯度层各层MoSi2的体积分数分别为:40%,50%,60%,70%,80%,90%,95%;利用优化后的梯度过渡层结构和连接工艺,经SPS烧结可实现MoSi2与316L不锈钢的连接.梯度层组织致密、均匀,界面结合良好,未出现明显的裂纹或其他缺陷.     

烧结气氛对MIM316L不锈钢微观组织和性能的影响

在Ar、Ar+H2、N2、N2+H2四种气氛下对MIM 316L不锈钢进行了烧结,讨论了烧结气氛对样品C、O、N含量、烧结密度、晶粒形貌和力学性能的影响.结果表明四种气氛烧结下样品C、O、N含量可控制在许可范围内.烧结气氛的露点显著影响着合金的致密化和最终力学性能;N2+H2气氛烧结下样品的抗拉强度和延伸率分别可达到765 MPa和32 %;Ar和Ar+H2气氛烧结下样品致密化程度高(可达理论密度的98 %),孔隙细小、圆滑、分布均匀,晶粒尺寸大约为50μm;样品的力学性能如下极限拉伸强度630 MPa、屈服强度280 MPa、延伸率52 %、硬度HRB 69.5.     

烧结工艺对316L不锈钢组织与性能的影响

通过研究烧结气氛和烧结温度对冷等静压态316L不锈钢组织和力学性能的影响,探究了烧结的致密化过程,并初步分析了挤压之后不锈钢的组织与性能。发现真空条件下获得的力学性能均比Ar气氛下烧结的好;在N2气氛烧结的不锈钢抗拉强度为803.5 MPa,屈服强度为407.2 MPa,但是断后延伸率仅为33.7%。在真空气氛下进行烧结,随温度的升高,孔隙率下降、孔隙尺寸减小并发生球化;通过对比烧结温度的影响,得出在1 380℃进行烧结获得的力学性能最好,抗拉强度为578.4 MPa,断后伸长率为52.0%,并且晶粒比较细小。经过挤压处理,不锈钢晶粒进一步细化,抗拉强度为675.6 MPa,屈服强度为305.4 MPa,断后伸长率为45.6%。     

添加金属粉末对粉末冶金316L不锈钢性能的影响

研究了分别添加20％（质量分数）铜粉、锡粉，铝粉对粉末冶金3161，不锈钢性能的影响。在烧结温度为1100℃、烧结气氛为分解氨的条件下，对烧结材料的硬度、密度和显微组织进行了检测和分析。结果表明：添加20％铝粉可显著提高不锈钢粉末的压制性，但铝粉会与不锈钢基体发生强烈的化学反应，生成Fe2Al，恶化材料的性能；添加20％锡粉可显著提高材料的硬度；添加20％铜粉对材料的硬度影响不大。添加大量低熔点金属粉末的液相烧结不能显著提高材料的密度。     

高温水环境下温度对316L不锈钢应力腐蚀开裂的影响

在高温水环境中,采用慢应变速率拉伸实验方法研究了温度对316 L不锈钢应力腐蚀开裂的影响规律,并通过扫描电镜(SEM)对试样断口形貌进行分析.结果表明:在高温水环境中,温度为200~345℃时316 L不锈钢具有应力腐蚀开裂敏感性;材料脆性指标随温度升高而增大,应力腐蚀开裂敏感性增强,断口分析与之吻合;250℃是316 L不锈钢发生应力腐蚀开裂的敏感温度,断口边缘形貌呈现明显脆性断裂特征.     

烧结方法对MoSi2显微组织和力学性能的影响

以MoSi2为试验对象，详细研究了等离子火花放电烧结法（SPS）和热压烧结法（HP）对其显微组织和力学性能的影响。结果表明，在相同的烧结温度下，SPS烧结制备的MoSi2材料，其显微组织比热压烧结要细小得多，且SiO2含量较少，纯度较高，致密度比热压烧结提高7％，显微硬度提高3倍以上，抗压强度提高2倍左右，但断裂韧性比后者稍低。     

烧结工艺对Ti/Al2O3复合材料性能的影响

本文利用放电等离子烧结技术探讨了烧结温度和保温时间对40%Ti(体积分数)/Al2O3(体积分数)复合材料性能的影响.实验结果表明复合材料的性能受烧结温度的影响最为显著,过度的延长保温时间会使晶粒发生异常长大,使得复合材料性能降低.烧结温度1 300℃,保温8 min,制备的复合材料力学性能最佳,其弯曲强度、断裂韧性、显微硬度和相对密度分别为1002.22 MPa、19.73 MPa*m1/2、18.14 GPa和99.74%.     

放电等离子烧结参数对TiC/Fe复合材料密度和硬度的影响

采用放电等离子烧结技术(SPS)原位合成了TiC颗粒增强铁基复合材料。利用X射线衍射(XRD)、扫描电镜(SEM)及洛氏硬度计等分析测试手段,研究了烧结温度、保温时间和烧结压力对材料微观结构、密度和硬度的影响。研究结果表明,随着烧结温度的升高,复合材料的密度和硬度先增大后减小,并于1 150℃时取得最大值;随着保温时间的延长,增强颗粒TiC的尺寸增大;复合材料的密度和硬度随保温时间延长呈现先增加后减小的趋势,在保温时间为5 min时取得最大值;复合材料的密度和硬度随烧结压力的变化具有与保温时间相似的规律,压力为40 MPa时,密度和硬度取得最大值。     

ZrO2/Si3N4颗粒增强MoSi2基复合材料的显微组织和力学性能

采用放电等离子烧结法(SPS)制备了不同体积分数的MoSi2及其复合材料,研究了复合材料的显微组织和力学性能.结果表明:10%ZrO2/20%Si3N4/MoSi2复合材料的致密度、显微硬度、抗压强度、断裂韧性分别为92.3%、15.17 GPa、2105 MPa、6.61 MPa·m1/2.与20%ZrO2/MoSi2复合材料相比,断裂韧性下降2.9%,显微硬度和抗压强度分别提高了22.8%,13.4%;与20%Si3N4/MoSi2复合材料相比,断裂韧性提高了5.3%,显微硬度和抗压强度相近;经500℃氧化300 h,氧化增重与ZrO2和Si3N4单独增强的相近,均是纯MoSi2的1/10左右,抗氧化效果显著.     

冷加工对316L不锈钢力学行为和组织的影响

采用室温拉伸及硬度测试研究了不同的冷变形量对316L不锈钢室温力学性能及硬度的影响,并通过OM、TEM对冷变形后组织结构的观察,分析讨论了不同变形后力学性能及硬度的变化机制.结果表明,冷变形使材料的强度和硬度得到大幅度提高,但塑性有所降低.冷变形量为25％时,钢的屈服强度可达到745 MPa,同时伸长率达到19.3％.随冷变形量的不同,该钢加工硬化能力不同.变形量低于2.5％时,强度、硬度增加的速度较快,而变形量高于约2.5％后,强度、硬度增加的速度却相对较小,其原因是变形机制不同.另外,冷变形后钢的屈服强度与硬度有着相似的变化规律,由此提出了由冷变形后硬度变化预测冷变形后拉伸屈服强度的方程.     

Residual stresses and interface strengths in MoSi2/stainless steel 316L joints produced by metallic-glass brazing

The residual stresses in molybdenum disilicide (MoSi₂)/stainless steel 316 L joints were evaluated using X-ray diffraction (XRD) and instrumented indentation techniques. These joints were produced by high-temperature brazing using a cobalt-based metallic glass (METGLAS 2714A). Joining was completed in two different ways, either by feeding excess braze into the braze gap upon heating or by constraining the MoSi₂/stainless steel assembly with an alumina (Al₂O₃) fixture during the heating cycle. These steps were necessary to ensure the production of a high-quality void free joint. The indentation results show higher tensile residual stresses in the stainless steel for the joint with the external constraint, in comparison to the unconstrained state. In contrast, the compressive residual stresses in the MoSi₂ (as measured by XRD) were lower in the constrained state relative to the unconstrained state. These results and a lack of residual stress balance indicate that the stress state in the braze is significantly different under the two joining conditions and the volume of the braze plays an important role in the development of the residual stresses. Push-out tests carried out on these joints gave higher joint strengths in the unconstrained as compared to the constrained condition. The results of this study have important implications on the selection of the appropriate joining process (use of constraint vs extra braze).     

The effect of niobium morphology on the fracture behavior of MoSi2/Nb composites

The morphology of the niobium reinforcement added to MoSi₂affected the fracture behavior (and hence toughness) of MoSi₂/20 vol pct Nb composites. The addition of discontinuous random niobium in the form of particles or short fibers deflected cracks that propagated through the MoSi₂ matrix. However, this did not result in any improvements in toughness (estimated from the area under flexural stress-displacement curves), as matrix cracks preferentially prop-agated through the Nb/MoSi₂ interphase region. The addition of aligned niobium fibers, ori-ented perpendicular to the direction of matrix crack propagation, directly participated in the fracture of the composite. Depending on the diameter of Nb embedded in the MoSi₂matrix, these fibers either fractured in a brittle manner or ruptured in a ductile manner. Small (400-μn) diameter continuously aligned Nb fibers fractured by brittle cleavage during testing. Therefore, the addition of these fibers was not as effective in improving the toughness of MoSi₂as the addition of larger (800-μm) diameter continuously aligned Nb fibers, which ruptured in a ductile manner. It was observed that the larger diameter fibers had separated from the matrix through the propagation of cracks in the reaction zone adjacent to the fibers and that these cracks formed prior to yielding of these fibers. In contrast, the smaller diameter fibers remained well bonded to the matrix and, thus, were constrained by the MoSi₂matrix from yielding. This resulted in brittle fracture behavior of the Nb fiber. There appeared to be an effect of aspect ratio on the fracture of the ductile embedded fibers. Shorter length 400-μm-diameter fibers separated from the matrix, behavior similar to the continuous 800-μm-diameter fibers and not the continuous 400-μm-diameter fibers. Formerly Graduate Research Assistant, Materials Engineering Department, Rensselaer Polytechnic Institute.     

Role of reinforcement in sintering of SiC/316L stainless steel composite

Abstract

Austenitic stainless steels with improved corrosion resistance are gaining wide popularity. However, their applications are limited because of their poor tribological properties. The present work was undertaken to improve the overall performance of 316L stainless steel by reinforcing it with SiC. During the processing of the 316L SS composite, the 316L SS matrix was found to interact strongly with the SiC at 1100°C resulting in the formation of low melting Fe–SiC phase. An attempt to process SiC/316L SS composite above this temperature resulted in complete melting of the composite compact.     

Synthesis of MoSi2-TiSi2 pseudobinary alloys by reactive sintering

MoSi₂-TiSi₂ pseudobinary alloys are synthesized from mixtures of elemental powders of molybdenum, silicon, and titanium by reactive sintering under a pseudoisostatic pressure of 150 MPa. When the titanium content in the alloy increases from 0 to 33 at. pct, the density of the alloy decreases from approximately 6 to 4 g/cm³, while the relative density is more than 95 pct independent of the titanium content. Vickers hardness of the alloy is approximately 800 when the alloy consists of a monophase structure of α-MoSi₂ or β-MoSi₂. However, the hardness increases to approximately 950 when the alloy consists of a dual-phase structure of (α-MoSi₂+β-MoSi₂) or (β-MoSi₂+γ-TiSi₂). The oxidation resistance of the alloy at 773 K is approximately tripled when the titanium content increases from 0 to 1.7 at. pct, but the effect of the titanium content on the oxidation resistance becomes less remarkable as the titanium content increases.     

球磨方式对TiC/316L复合粉末压缩性及烧结行为的影响

应用双对数压制方程和相关烧结模型,分析和探讨不同球磨方式对TiC/316L复合粉末的压制特性及烧结行为的影响,并采用扫描电镜观察复合粉末的形貌。结果表明,采用湿磨方式获得的复合粉末,其破碎程度不够,很多316L不锈钢颗粒特别是颗粒较小的不锈钢颗粒变形不大,仍然呈球形,不利于其后续的压制和烧结;采用直接干磨获得的复合粉末,其表面新鲜断裂面多,粉末表面活性大,有利于粉末的烧结致密化,但其加工硬化明显,压制成形性能较差;与其他球磨方式相比,采用先干磨再湿磨的球磨方式获得的复合粉末不仅具有较好的成形性能,而且粉末烧结激活能低,以该粉末为原料可以获得较高相对密度的烧结体。     

316L/X65双金属复合管焊接接头组织与性能

 采用钨极氩弧焊和手工电弧焊焊接316L/X65双金属复合管。利用光学显微镜、能谱仪、扫描电镜、力学性能测试及电化学测试等分析手段研究了复合管焊接接头的微观结构、化学成分、力学性能及电化学腐蚀性能。结果表明,过渡层焊缝的化学成分受到稀释较小,过渡层熔合线附近出现了元素迁移,不锈钢层焊缝与母材的化学成分基本一致;扩散层为类马氏体＋残留奥氏体,过渡层和不锈钢层焊缝均为奥氏体＋少量铁素体;在试验参数下,焊接接头各项力学性能优良、无缺陷;覆层焊缝与母材的电化学腐蚀性能相差极小。     

The effect of nb morphology on the cyclic oxidation resistance of MoSi2/20 vol pct Nb composites

The effect of Nb morphology on the 1200 °C cyclic oxidation resistance of MoSi₂/20 vol Pct Nb composites was investigated. Niobium was incorporated into MoSi₂ as particles, random short fibers, and continuous aligned fibers. After oxidation, it was found that all the composites had lost weight and essentially disintegrated. This was attributed to spalling of both the Nb₂O₅ scale and the MoSi₂ matrix. The spalling of the matrix was a result of cracks originating in the oxidized Nb and propagating through the MoSi₂ matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb₂O₃ and (2) the difference in thermal expansion between Nb₂O₅ and MoSi₂. However, it was found that the addition of smaller diameter Nb reinforcements tended to retard the disintegration of the composites. This was attributed to the effect of reinforcement size on CTE mismatch cracking. D.E. Alman, Formerly Graduate Student, Materials Engineering Department, Rensselaer Polytechnic Institute     

Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract

Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densities of up to 7·44 g cm^-3 with tensile strength of 482 MPa, hardness value of 153 HV10 and 15% elongation were obtained with ball milled plus 3 wt-%Si addition. Low levels of interconnected porosities (~4%) were recorded within the temperature range 1250-1300°C suggesting the possibility of good corrosion resistance.

The sintered microstructures consisted of ferrite and austenite (duplex structure), complex silicide and eutectic phases within grains and at grain boundaries, pools of liquid (rich in Si) and some medium and small pores preventing full density to be achieved despite the liquid phase formation.