Structure and mechanical properties of a eutectic high-temperature Nb-Si alloy grown by directional solidification

The structure and the short-term high-temperature strength of Ni-18.7 at % Si (Nb-Nb₃Si eutectic) alloys fabricated by vacuum electron-beam zone melting and induction melting in an argon atmosphere are studied. The structure of the samples prepared by vacuum electron-beam zone melting is characterized by the presence of primary Nb₅Si₃ intermetallic precipitates and the absence of its secondary precipitates. The structure of the samples prepared by induction melting in an argon atmosphere has two characteristic zones, namely, eutectic and eutectoid ones.     

Grain structure development and eutectic solidification of discontinuous magnesium borate whisker reinforced AA2024 matrix composite

Abstract

In the present research, the solidification behaviour of MBO whisker reinforced 2024 aluminium alloy matrix composite (MBO/AA2024) was studied by employing computer based thermal analysis, scanning electron microscopy and differential scanning calorimetry. The results show that the addition of MBO whiskers refines the grain structure in the matrix of the MBO/AA2024 composite. The nucleation of primary α-Al phase took place away from the whisker surfaces and started within the interstice of whiskers. The whiskers had almost no significant influence on the refinement of the eutectic phases.

Dans cette recherche, on a étudié le comportement de solidification du composite (MBO/AA2024) à matrice d’alliage d’aluminium 2024 renforcée par des barbes de MBO, au moyen de l’analyse thermique automatisée, de la microscopie électronique à balayage et de l’analyse calorimétrique à compensation de puissance. Les résultats montrent que l’addition de barbes de MBO affine la structure de grain dans la matrice du composite MBO/AA2024. La nucléation de la phase primaire α-A1 prenait place à l’écart des surfaces de la barbe et commençait à l’intérieur de l’interstice des barbes. Les barbes n’avaient presque pas d’influence sur l’affinement des phases de l’eutectique.     

定向凝固研究易切削钢中硫化物的生成行为

通过定向凝固实验研究了易切削钢中硫化物的生成过程,并建立了枝晶臂间距、硫化物的平均直径与凝固条件之间的关系.实验结果表明:随着温度梯度和拉速的增大,MnS的平均直径减小,数量增多;定向凝固过程中硫化物的完全析出温度大约在固相线温度下100℃左右,MnS形态的改变,主要受其组成元素的活度影响,即MnS形态的改变受其界面自由能的影响;对定向凝固实验中钢的枝晶臂间距、硫化物的平均直径与凝固条件的关系进行了拟合,拟合结果与前人研究结果相吻合.     

The solidification sequence in an 18-8 stainless steel,investigated by directional solidification

The directional solidification technique was applied in order to investigate the complicated solidification sequence in a commercial austenitic stainless steel which was known to yield a primary precipitation of § ferrite when cast into a 5 tons ingot. Three stages of solidification were found. The first precipitation of § ferrite was interrupted by precipitation of austenite and at the end of the solidification there was a transition back to precipitation of § ferrite. The competition between the first two stages is affected by the cooling rate and the nitrogen content. The precipitation of austenite from the melt results in the usual coring whereas ô ferrite forms with a very homogeneous composition, presumably due to rapid diffusion in this phase. On cooling austenite forms from the § ferrite and this reaction also results in coring, presumably due to rapid diffusion in § ferrite.     

Thermoelectric and morphological effects of peltier pulsing on directional solidification of eutectic Bi-Mn

We have carried out extensivein situ thermal measurements during Peltier Interface Demarcation (PID) during directional solidification of eutectic Bi/MnBi. We have observed that significant thermal transients occur throughout the sample as a result of the Peltier pulsing. We have separated the contributions of the Peltier, Thomson, and Joule heats, and studied them as a function of pulse intensity and polarity. The Joule and the combined Peltier and Thomson thermal contributions were determined as a function of time during and after the current pulses, close to the solid/liquid interface. Variations of the Bi/MnBi particle morphology clearly reveal the interface shape, changes in interface velocity, meltback, and temporary loss of cooperative growth, as a result of the pulsing. This paper is based on a presentation made at the symposium “Fluid Flow at Solid-Liquid Interfaces” held at the fall meeting of the TMS-AIME in Philadelphia, PA on October 5, 1983 under the TMS-AIME Solidification Committee.     

冷却速率对奥氏体不锈钢凝固过程影响的原位观察

通过采用激光共聚焦扫描显微镜对AISI304奥氏体不锈钢的凝固过程进行了原位动态观察研究.发现当冷却速率为0.05℃·s^-1时,奥氏体不锈钢以胞状晶方式凝固,其凝固模式为FA模式,即δ铁素体相先从液相中形核并长大,γ相在1 448.9℃时通过与液相发生包晶反应(L+δ→γ)在δ铁素体相界形成,当温度降到1 431.3℃时液相消失,δ铁素体相通过固态相变转变为γ相,富Cr贫Ni的残留铁素体位于胞状晶之间.当冷却速率为3.0℃·s^-1时,奥氏体不锈钢以枝晶方式生长,冷却到1346.4℃时包晶反应在液相与δ铁素体相界之间进行,其残留铁素体位于枝晶干,与冷却速率为0.05℃·s^-1时相比,其残留铁素体的数量增多,残留铁素体富Cr贫Ni的程度减轻.     

奥氏体不锈钢管路自动环焊工艺研究

本文采用自动环焊设备对发动机管路进行焊接试验.通过调整焊接电流和气体流量,摸索出适合壁厚0.8 mm的φ6奥氏体不锈钢管路自动环焊的焊接工艺参数.经外观检验和X光检验,证明不锈钢管路满足焊接要求,合格率达到95%以上,为实现航空发动机不锈钢管路的全自动焊接提供依据.     

Cr和Si元素对奥氏体不锈钢组织构成及凝固路线的影响

以316Ti奥氏体不锈钢为基础,设计不同Cr和Si元素含量的合金成分,结合Thermal-Calc热力学模拟计算与合金铸锭凝固组织形貌、成分分析,研究了Cr和Si元素对合金凝固组织构成的影响。研究结果表明,热力学计算能够获得奥氏体不锈钢中析出δ相的临界Cr和Si含量。合金凝固时的元素偏析和冷却过程中的“δ→γ”相变可对δ相析出预测产生一定影响。此外,本工作还针对δ相析出评价了两种凝固路线判据。      

Cr和Si元素对奥氏体不锈钢组织构成及凝固路线的影响

以316Ti奥氏体不锈钢为基础,设计不同Cr和Si元素含量的合金成分,结合Thermal-Calc热力学模拟计算与合金铸锭凝固组织形貌、成分分析,研究了Cr和Si元素对合金凝固组织构成的影响.研究结果表明,热力学计算能够获得奥氏体不锈钢中析出δ相的临界Cr和Si含量.合金凝固时的元素偏析和冷却过程中的δ→γ相变可对δ相析出预测产生一定影响.此外,本工作还针对δ相析出评价了两种凝固路线判据.     

镜面抛光技术在奥氏体不锈钢产品中的应用

冷轧BA表面不锈钢在抛光装饰行业中作为镜面板加工基板得到广泛应用.着重介绍了抛光行业对镜面不锈钢表面的判定标准,并从抛光原理、设备、抛光溶液、工艺技术等方面介绍了电解抛光、化学抛光、机械抛光以及化学机械抛光等目前抛光行业最常见的四种镜面抛光技术;同时从设备要求、镜面板质量、加工效率、对环保影响等多方面对四种抛光技术的优劣进行了对比.收集了抛光镜面板过程中容易出现的缺陷,对其原因进行了分析,并提出了相应的解决方案.     

Effect of superheat on the solidification structures of AISI 310S austenitic stainless steel

An experimental study was carried out to investigate the evolution of macrostructure and microstructure in AISI 310S stainless steel during solidification. Experimental findings suggested that the macrostructure and the microstructure of the cast material responded differently to variations in casting temperature. As the casting temperature decreased, the macro structure was refined, as expected, but the microstructure coarsened. A relationship was established between the proportion of equiaxed zone and superheat as follows: pct equiaxed zone =a +b ln (1/ΔT) wherea andb are constants. The relationship between grain width and superheat could be expressed by the equation $$gw = e^{\left( {c + d/\Delta T} \right)} $$ wherec andd are constants determined by the distance from the edge of the ingot. The relationship between primary arm spacing and superheat could be expressed by the equation $$\lambda _1 = p + q \ln \left( {1/\Delta T} \right)$$ wherep andq are constants determined by the distance from the edge of the ingot. The parameter “grain width ratio” has been introduced to describe the relationship between the shape and the nucleation and growth kinetics of the columnar grains.     

Modeling of micro- and macrosegregation and freckle formation in single-crystal nickel-base superalloy directional solidification

The formation of macrosegregation and freckles by multicomponent thermosolutal convection during the directional solidification of single-crystal Ni-base superalloys is numerically simulated. The model links a previously developed thermodynamic phase equilibrium subroutine with an existing code for simultaneously solving the macroscopic mass, momentum, energy, and species conservation equations for solidification of a multicomponent alloy. Simulation results are presented for a variety of casting speeds and imposed thermal gradients and for two alloy compositions. It is found that for a given alloy composition, the onset of convection and freckle formation occurs at a critical primary dendrite arm spacing, which agrees well with previous experimental findings. The predicted number and shape of the freckle chains in the unstable cases also agree qualitatively with experimental observations. Finally, it is demonstrated how the onset and nature of convection and macrosegregation vary with alloy composition. It is concluded that the present model can provide a valuable tool in predicting freckle defects in directional solidification of Ni-base superalloys.     

Effect of solidification conditions on the solidification mode in austenitic stainless steels

The effect of the solidification conditions on the solidification mode in the composition range in which the primary austenitic and ferritic modes compete is studied by varying the welding parameters in gas tungsten arc (GTA) welding and by comparing the results with those obtained from other laboratory experiments. A good agreement holds if the effect of the composition is described by the ratio Cr_eq/Ni_eq (Cr_eq = pct Cr + 1.37 × pct Mo + 1.5 × pct Si + 2 × pct Nb + 3 × pct Ti and Ni_eq = pct Ni + 0.31 × pct Mn + 22 × pct C + 14.2 × pct N + pct Cu) and the effect of the solidification conditions by the growth rate. The critical value of the ratio Cr_eq/Ni_eq corresponding to the transition from primary austenitic to ferritic solidification increases from 1.43 to 1.55 with increasing growth rate. The upper limit is valid in GTA welding at high welding speeds, while the lower limit corresponds to the practical conditions which exist in ingot and shaped casting. The validity and applicability of this solidification model are discussed.     

Hydrogen evolution during directional solidification and its effect on porosity formation in aluminum alloys

Most of the models for predicting porosity formation in aluminum alloy castings use a simple mass balance, such as the lever rule, to track hydrogen enrichment in the interdendritic liquid. However, the hydrogen concentration predicted by the lever rule is typically too low to satisfy the threshold concentration for pore nucleation based on classical nucleation and growth theory. As a result, important features of microporosity such as the size and spacing of pores cannot be treated properly. In this article, the hydrogen concentration during the directional solidification of an Al-4.5 pct Cu alloy is calculated, assuming hydrogen rejection during solidification and diffusion in the mushy zone. The calculation shows that the use of the lever rule greatly underestimates the hydrogen concentration at the eutectic front. This is due to the fact that the eutectic front also rejects hydrogen and that this is not considered in the use of the lever rule. Results of numerical simulations that consider hydrogen rejection and diffusion are compared with results obtained using the lever rule. The comparison indicates that actual hydrogen concentrations may be orders of magnitude higher than that predicted by the lever rule. It is suggested that the lever rule should not be used in predicting porosity nucleation. The model outlined in this article is used to propose and explain the formation of a wavelike distribution of pores during directional solidification.     

Wear resistance of composite material made of eutectic steel and bronze powders

The tribological properties of a hardened steel U8-composite pair in contact in which bronze is used as the matrix and eutectic alloy powder (steel 30Kh13 with 15 wt.% VC of eutectic structure) is used as a filler are studied under conditions of reciprocal sliding friction without lubrication. Composite and counterbody wear is not uniform and it depends on the amount of filler within the range 20–80%. A composite with 80% filler has good tribological properties and satisfactory lathe machining.Institute of Metal Physics, Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 69–73, January–February, 1994.     

Time-resolved in-situ analysis of phase evolution for the directional solidification of carbon steel weld metal

Kinetic information about the phase transformation, during solidification process and solid-state transformation is essential to the material processing, such as welding. In our research group, in-situ phase identification system consisting of undulator beam and imaging plate have recently been used. The welding torch is driven by stepping-motor in the system. Those make possible that phase transformation can be identified in real-time under the condition of directional solidification and the spatial resolution of 100 × 500 μm. The time-resolution is 0.3125 seconds. In the present work, combination of analyzing method: the in-situ phase identification system, morphological observation by high-temperature laser scanning confocal microscopy and observation of microstructure at room temperature by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process. Phase transformation process of hypereutectoid carbon steel, during welding was analyzed as an example of combination observation.