深过冷铁钴基块体合金的细晶化研究

采用熔融玻璃净化与循环过热相结合的深过冷技术,对比研究了深过冷凝固和过冷熔体Cu模激冷凝固Fe₄₄Co₄₄Nb₇-B₄Cu₁块体合金的组织特征.结果表明,随过冷度增大,深过冷和深过冷Cu模激冷凝固组织均由最初的发达树枝晶逐渐演变为细小的粒状晶,并且Cu模激冷可显著减小树枝晶向粒状晶转变的临界过冷度.在相同过冷度下,过冷熔体Cu模激冷凝固组织更加细小均匀,其根本原因在于Cu模激冷和熔体深过冷的双重作用不仅提高了熔体的形核率并抑制了晶粒长大,而合金化元素在晶界聚集并不是阻碍晶粒长大的主要原因.     

物理变质处理对消失模铸造AZ91合金组织及力学性能的影响

在消失模铸造条件下,分别采用机械振动及熔体混合处理对AZ91合金的组织形貌和力学性能进行了研究,并分析了凝固过程中细化晶粒的机制。试验结果表明:采用机械振动及熔体混合处理都能显著地细化消失模铸造AZ91合金的组织,并显著提高力学性能。当机械振动激振力为1.5kN时,AZ91合金综合力学性能最优,其主要通过增大过冷度、枝晶臂熔断等作用细化组织。熔体混合处理其效果强于传统的熔体高温处理,熔体混合处理通过在混合液体中产生更多的核胚以增加形核位置,来细化枝晶组织。     

Ni-Cr-W基高温合金近液相线细晶铸造(英文)

通过近液相线‘铸造’的方法得到了细晶组织。采用感应加热方式将熔体处理至近液相线温度附近指定温度,然后让熔体直接在坩埚中炉冷。在具有一定形核过冷度的坩埚中得到了细晶组织并探索了晶粒细化的机制。可以将过冷熔体的凝固过程分为2个阶段:非平衡凝固阶段;近平衡凝固阶段。与近平衡凝固阶段相比,球形晶粒扰动发展的临界半径Rc在非平衡凝固阶段急剧减小。在非平衡凝固阶段,晶粒更倾向于以枝晶方式长大,晶粒的增殖通过枝晶的熔断产生并且最终的晶粒密度由该过程决定。在近平衡凝固阶段Rc急剧增大。Rc的增大与晶粒增殖阶段所确定大的核心密度导致凝固在球形晶粒扰动发展之前终止。因此,最终得到粒状晶粒形态。     

Grain Refinement of Ni-Cr-W Based Superalloy by Near Liquidus Casting

通过近液相线‘铸造’的方法得到了细晶组织。采用感应加热方式将熔体处理至近液相线温度附近指定温度,然后让熔体直接在坩埚中炉冷。在具有一定形核过冷度的坩埚中得到了细晶组织并探索了晶粒细化的机制。可以将过冷熔体的凝固过程分为2个阶段:非平衡凝固阶段;近平衡凝固阶段。与近平衡凝固阶段相比,球形晶粒扰动发展的临界半径Rc在非平衡凝固阶段急剧减小。在非平衡凝固阶段,晶粒更倾向于以枝晶方式长大,晶粒的增殖通过枝晶的熔断产生并且最终的晶粒密度由该过程决定。在近平衡凝固阶段Rc急剧增大。Rc的增大与晶粒增殖阶段所确定大的核心密度导致凝固在球形晶粒扰动发展之前终止。因此,最终得到粒状晶粒形态。     

不同热流模式下快速凝固Cu-Co不混溶合金枝晶-分相结构的转变（英文）

综合采用熔融玻璃净化、铜模喷铸及单辊旋淬技术,对比研究不同过冷度、不同冷速作用下Cu-Co不混溶合金的快速凝固行为。通过对凝固发生时的热流方式、形核及生长过程的分析,阐述合金非平衡组织由枝晶到分相结构的转变及其相应尺寸的变化规律。随过冷度增加,不混溶效应的增强导致柱状枝晶向分相结构转变。由于铜模喷铸时发生多点形核,凝固组织呈现为等轴枝晶并随冷速增加而不断细化。当喷铸试棒直径为4 mm时,不混溶效应形成的液滴由于长大不充分最终形成细小粒状分相组织。单辊旋淬薄带由于冷速最高,凝固过程瞬间完成,可有效抑制液相分离的发生,有利于胞状单相固溶体组织的形成。     

Ce及铜模快冷对AZ91合金组织及热稳定性的影响

采用铜模喷铸与等温热处理相结合,对比研究不同冷速及固溶处理条件下稀土Ce在AZ91镁合金中的存在形式及稀土相的演变行为,揭示Ce对快冷镁合金组织的细化及热稳定性的影响。结果表明,稀土Ce在凝固界面前沿富集,抑制晶粒生长并促进形核,导致镁合金组织细化,但含量过高时会形成针状Al_(11)Ce_3相,降低成分过冷,不利于组织细化。由于过冷度的增加及凝固潜热对形核抑制影响的减弱,铜模喷铸镁合金晶粒细化显著,并且过饱和固溶体的形成可有效避免针状稀土相的形成。经420℃固溶处理后,颗粒状Mg_(12)Ce化合物的析出可有效抑制高温晶粒长大,提高快冷镁合金细晶组织的热稳定性。     

深过冷Cu-20%Pb亚偏晶合金凝固组织的细化机制

采用熔融玻璃净化与循环过热相结合的方法研究Cu-20%Pb亚偏晶合金的凝固组织,用BCT-LKT枝晶生长模型对深过冷凝固过程的热力学参数进行了计算,分析了过冷Cu-20%Pb亚偏晶合金的凝固机制.结果表明,在低过冷度下,Cu-20%Pb亚偏晶合金凝固组织由粗大的Cu枝晶和Pb相组成.随着过冷度的增大,Cu枝晶和Pb相细化,Pb相分布均匀.但在不同的过冷度范围,凝固组织细化机制不同.过冷度小于200 K时,快速凝固阶段结晶潜热集中释放造成的温度回升引起枝晶大量重熔是枝晶细化的主要因素;过冷度大于200 K时,枝晶高速生长导致枝晶各部位的不均匀收缩及枝晶骨架间液相的高速流动会使枝晶受力产生碎断,枝晶重熔和枝晶碎断共同作用使得枝晶发生细化.     

深过冷BFe30-2合金凝固组织演化

用熔融玻璃净化与循环过热相结合的方法,研究了BFe30-2合金过冷熔体凝固组织的演化规律。研究表明:在过冷度39～203K的范围内,其凝固组织的形态有三次变化过程:第一次是在39～118K过冷度范围,随着过冷度的增加,凝固组织中枝晶明显细化且同时存在枝晶重熔形成的第一类粒状晶;第二次发生在118～174K过冷度范围,因枝晶熟化被抑制,形成的第一类粒状晶转变为高度细化的细枝晶;第三次发生在174～203K过冷度区间,组织因细枝晶再结晶转变为均匀的准球状晶粒。     

过冷Ni-1at%Fe合金晶粒细化及再结晶机理分析

通过熔融玻璃净化与循环过热相结合的方法研究了过冷Ni-1at%Fe合金的微观组织演化及再结晶。通过再辉后快淬和空冷实验对比,证明大过冷下的细化组织是再结晶的产物。结合无量纲过热度的计算,系统考察了合金凝固组织随过冷度的演化规律。研究发现合金的凝固组织在所获得的过冷度范围内发生了两次晶粒细化,第一次细化归因于枝晶的熟化和重熔导致的高度发达树枝晶碎断,第二次细化归因于应力积累导致的再结晶。     

亚偏晶Cu-Pb合金的深过冷快速凝固

用熔融玻璃净化与循环过热相结合的方法,研究了亚偏晶Cu-Pb合金过冷熔体凝固组织演化规律。研究表明:在试验过冷度10～302K的范围内,其凝固组织形态经历了两次变化过程:第一次是在10～207K过冷度范围,由粗大的枝晶重熔形成了第一类粒状晶;第二次发生在207～302K过冷度区间,合金组织中的第一类粒状晶转变为高度细化的细枝晶,组织因细枝晶再结晶向准球状晶粒转变。     

细晶粒钛合金薄板TIG焊温度场分析

采用数值模拟和试验相结合的方法分析了不同晶粒尺寸的细晶粒Ti-6Al-4V钛合金薄板常规钨极氩弧焊(TIG)焊接过程中的温度场.结果表明,不同晶粒尺寸的细晶钛合金的温度场相似,但是随着母材晶粒尺寸减小,晶界增多,热导率与比热容都相应减小,且比热容减小幅度较大.所以较细晶粒尺寸钛合金热传递速度较慢,同一温度的等温线的范围大于较粗晶粒钛合金的相应值,并且距离焊缝中心越远,这种差距越明显.较细晶粒尺寸钛合金距离焊缝中心相同点的峰值温度也明显高于较粗晶粒尺寸钛合金.     

Improving mechanical properties of 2219 aluminium alloy GTA welds by scandium addition

Abstract

The effect of scandium additions to 2219 aluminium alloy weld metal has been investigated. At low levels (0·16%) of scandium, in spite of grain refinement in the weld metal, improvement in the mechanical properties has been nominal. At higher levels of scandium (0·37%), a substantial improvement in the tensile properties has been obtained. Further improvement in mechanical properties has been achieved by adding small amounts of magnesium. Transmission electron microscopy revealed the presence of fine precipitate particles in the scandium containing weld metals. Electron probe microanalysis (EPMA) and SEM–EDX revealed extensive copper segregation to grain and subgrain boundaries. The presence of scandium reduces the severity of segregation by producing fine equiaxed grains in the weld metals and also by refining the grain substructure. The morphology and size of the high copper eutectic phase at grain boundaries and sub-boundaries have been found to be finer and well distributed in the case of scandium containing weld metals. EPMA linescans and quantitative analyses proved that the depletion of copper in the matrix is minimised as a result of the fine grained structure.     

B、Y及冷却速度对Ti-47Al-2Cr-2Nb合金组织的细化作用

研究了微量元素B、Y和冷却速度对Ti-47Al-2Cr-2Nb合金组织的复合细化效果。结果表明,在水冷铜坩埚磁悬浮真空感应熔炼和Y2O3型壳精密铸造条件下,B元素能够有效细化Ti-47Al-2Cr-2Nb铸件的晶粒尺寸,使其由粗大的柱状晶变为细小的等轴晶组织,但没有起到细化片层间距的明显作用。B、Y复合细化可以在B元素细化效果的基础上进一步细化片层团尺寸,同时显著细化片层间距。在壁厚为2～20mm范围内,Ti-47Al-2Cr-2Nb-0.8B-0.3Y铸件的宏观组织均为等轴晶;且随着厚度的增加,等轴晶尺寸逐渐增大。适当降低型壳预热温度也有助于获得细小的片层组织。     

深过冷Ni—50%Cu合金的晶粒细化

Ｎｉ－５０％Ｃｕ合金随过冷度的增加发生两次明显的晶粒细化过程,第一次产生于５０－１００Ｋ的过冷度范围内,第二次发生于临界过冷度２２０Ｋ之后,实验首次发现高冷度下细化合金的晶粒内含有枝晶亚结构。理论分析指出,枝晶重熔倾向的大小可用枝晶主干中最先析出的固相在再辉过程中的无量纲过热度来衡量,该无量钢过热度随原始过冷度的增加先增后减,其最大值对应于第一次细化发生的过冷度区间。热力学计算和组织分析表明,小过     

Microstructure and corrosion behaviour of Mg–2Gd–1Y–1Zn–0·2Zr (at-%) alloy processed by equal channel angular pressing

Significant grain refinement was achieved in a new Mg–2Gd–1Y–1Zn–0·2Zr (at-%) alloy through multipass equal channel angular pressing (ECAP) at 623 K. Corrosion behaviours of the ECAPed alloy were investigated by hydrogen evolution and electrochemical measurement in NaCl solution at room temperature. The results showed that a large number of intergranular phases were stretched and gradually broken above four ECAP passes, but the fine grained α-Mg phase was much easier to grow after 12 ECAP passes for dynamic recrystallisation. The corrosion resistance of the ECAPed Mg alloy in a fine grained state considerably increases, compared with that in the as cast state. After four ECAP passes, the corrosion potential, the pitting potential and the resistance value achieved ?1·55 V, ?1·39 V and 2·08 KΩ respectively. However, excessive ECAP passes reduced the corrosion resistance of the fine grained Mg alloy, due to grain coarsening and the gradual loss of barrier effect of intergranular phases.     

Numerical and experimental investigation of the grain refinement of liquid metals through cavitation processing

An investigation was carried out on the grain refinement of molten AA5754 Aluminum alloy through intensive shearing. The results show intensive shearing via cavitation decreases the grain size significantly. The above hypothesis for structure refinement was evaluated and an experiment was performed to ensure the creditability of this assumption. Finally, it was simulated by computational fluid dynamics (CFD) modeling. It was understood that shearing is the responsible mechanism for creation of cavitation bubbles and further collapse of them. It was also concluded the pressure which generated from the collapse of the bubble is well enough for braking the oxide layer and wetting them. It was proved that breaking of the oxide layer wets the impurity particles upon collapse of cavitation bubbles and provides additional nuclei and additional grain refinement. The suggested mechanism includes improved wetting by breaking the oxide layer through fatigue via continuous hitting of the micro-jets, local undercooling upon the collapse of cavitation bubbles, and pre-solidification inside fine capillaries.     

Effects of grain refinement on cast structure and tensile properties of superalloy K4169 at high pouring temperature

In order to improve the filling ability of large complex thin wall castings, the pouring temperature should be increased, but this will result in the grain coarsening. To overcome this problem, two kinds of grain refiners of Co-Fe-Nb and Cr-Fe-Nb ternary alloys, which contain high stability compound particles, were prepared. The effects of the refiners on the as-cast structures and tensile properties of the K4169 superalloy with different casting conditions were studied by analyzing specimens 110 mm long and 20 mm in diameter.Results showed that the mixture addition of the two refiners in the melt of K4169 can reduce the columnar grain region and decrease the equiaxed grain size greatly. After refinement, the amount of Laves phase decreases and its morphology changes from island to blocky structure. The carbides in the fine grain samples are fine and dispersive. Meanwhile, the porosity in specimens is decreased due to grain refinement. As a result, the yield strength, ultimate strength and the elongation of the specimens are increased. The grain refinement mechanisms are also discussed.     

过冷Fe75Ni25合金晶粒细化机制的研究

采用熔融玻璃净化结合循环过热的方法，对过冷Fe75Ni25合金的晶粒细化机制进行了系统的研究，在所获得的过冷度范围内，发现了两次晶粒细化现象，基于理论计算和对实验结果的分析，系统考察了合金的晶粒细化机制，结果表明，第一次晶粒细化（56K〈△T〈145K），是由于重熔引起的枝晶骨架的破断；而第二次晶粒细化（△T〉209K），则是快速凝固收缩产生的应力导致枝晶碎断，并在应变激活能和晶格畸变能的共同作用下引发再结晶的结果。     

Microstructure refinement of AZ91D alloy solidified with pulsed magnetic field

The effects of pulsed magnetic field on the solidified microstructure of an AZ91D magnesium alloy were investigated. The experimental results show that the remarkable microstructural refinement is achieved when the pulsed magnetic field is applied in the solidification of AZ91D alloy. The average grain size of the as-cast microstructure of AZ91D alloy is refined to 104 μm. Besides the grain refinement, the morphology of the primary α-Mg is changed from dendritic to rosette, then to globular shape with changing the parameters of the pulsed magnetic field. The pulsed magnetic field causes melt convection during solidification, which makes the temperature of the whole melt homogenized, and produces an undercooling zone in front of the liquid/solid interface by the magnetic pressure, which makes the nucleation rate increased and big dendrites prohibited. In addition, primary α-Mg dendrites break into fine crystals, resulting in a refined solidification structure of the AZ91D alloy. The Joule heat effect induced in the melt also strengthens the grain refinement effect and spheroidization of dendrite arms.