Assessment of solidification characteristics of carbon-inoculated Mg-3%Al melt by thermal analysis

The Mg-3%Al melt was inoculated by carbon with different holding time. The effect of holding time on grain refining efficiency was evaluated. The solidification characteristics of the carbon-inoculated Mg-3%Al melt with different holding time were assessed by computer-aided cooling curve analysis. The results showed that Mg-3%Al alloy could be effectively refined by carbon inoculation. Slight fading phenomenon occurred with increasing the holding time to 60 min. Carbon inoculation could significantly influence the shape of cooling curves of Mg-3%Al melt. The nucleation starting and minimum temperatures increased. The recalescence undercooling and duration decreased to almost zero after carbon inoculation. The grain refining efficiency of carbon inoculation could be assessed by the shape of the cooling curve and solidification characteristic parameters including nucleation starting and minimum temperatures, recalescence undercooling and duration.     

Influence of fading on characteristics of thermal analysis curve of compacted graphite iron

In general,during the production of compacted graphite iron (CGI),the active residual magnesium reduces and the effect of inoculation fades after magnesium treatment.In this paper,characteristics of the thermal analysis curve of CGI are compared with those of ductile iron and grey cast iron.The fading effect on the compacted graphite percentage and thermal analysis curve were also studied.Results indicate that the undercooling of CGI is as low as that of ductile iron,but CGI shows evident recalescence.In fading process,the magnesium element acts with oxygen.For a decrease in magnesium content,both the compacted graphite percentage and the austenitic liquidus temperature increase.The temperature of eutectic undercooling (TEu) decreases before the flake graphite appears.After that,TEu increases quickly,up to as high as 20 ℃,and then gradually decreases.The evolution of recalescence degree is opposite to that of TEU.     

Effect of strontium and solidification rate on eutectic grain structure in an AI-13 wt% Si alloy

The influence of strontium addition and solidification rate on eutectic grain structure in a near-eutectic AI-Si alloy was investigated. The characteristic temperature of eutectic nucleation (TN),minimum temperature prior to recalescence (TM),and the growth temperature (TG) during cooling were determined by quantitative thermal analysis. All characteristic temperatures were found to decrease continuously with increasing Sr content and solidification rate. Microstructural analysis also revealed that the eutectic grain size decreases with increasing Sr content and solidification rate. Such eutectic grain refinement is attributed to the increased actual under-cooling ahead of the liquid/solid interface during solidification.     

The influence of silicon content on microstructure and hardness of Mo-Si alloys

Mo-Si alloy sheets with different silicon content were fabricated by powder-metallurgical and thermo-mechanically processing. The effect of Si content and annealing temperature on microstructure and hardness of the Mo-Si alloys were studied by using optical microscopy (OM), transmission electron microscopy (TEM) and Vickers hardness tester, respectively. The results indicated that the presence of Si can effectively refine the grain sizes and improve the hardness of Mo-Si alloys. Si can also increase the recrystallization temperature of alloys and play a significant role in restraining the grain growth at high temperatures. Increasing the annealing temperature, the microstructure of Mo-Si alloy sheets is gradually coarsened and changed from fibrous to equiaxed structure, causing reduction in hardness. The hardening effect in the Mo-Si alloys came from the refined grain strengthening, solid solution strengthening, and second phase particle strengthening, which are closely dependent on the Si content and annealing temperature.     

Influences of melt treatment on grain sizes and morphologies of AZ91D alloy

Influences of the three melt treatment processes （namely, refinement with carbon inoculation, electromagnetic stirring and ＂refinement ＆ electromagnetic stirring＂ processing） on the microstructures of AZ91D alloy were explored experimentally. The results indicate that the micron carbon powders inoculation processing with 0.1% （mass fraction） addition level makes the grain size of the primary phase （a-Mg） decrease to approximate 40% that of the initial primary crystal, from about 530 μm to 200 μm around. The electromagnetic stirring processing not only decreases the grain sizes sharply, but also transfers the grain shape of a-Mg from coarse dendritic to nearly spheralitic with the shape factor from about 0.1 to approximate 0,8. And the ＂refinement ＆ electromagnetic stirring＂ processing improves grain shape of a-Mg further and refines grain size to about 72 μm, less than 15% that of the initial primary crystal.     

Effect of Composition and Stirring on the Grain Structure of Aluminium Alloys solidified in Ingots

Abstract

Solidification experiments have been performed on a series of industrial aluminium alloys in a rectangular vertical ingot mould, in which an electromagnetic linear motor activated a recirculating flow. Results include macrographs, grain size measurements, and high resolution temperature records on 14 thermocouples. The refinement effect is clearly demonstrated. It is strong when stirring changes from zero to some moderate level, then its increment decreases asymptotically. In most of the temperature records, the recalescence behaviour due to the formation of equiaxed crystals can be followed in detail. Under good stirring conditions, equiaxed crystallisation starts simultaneously in all the volume elements and all of them follow the same thermal history, which explains the very good uniformity in grain structure. Non-uniform or coarse grain structures which are observed when stirring conditions are poor can be correlated to dissimilarities in the thermal history of the different volume elements. A theoretical model explains these observations and allows the effects of stirring and of composition on the equiaxed grain size to be predicted.     

Effect of strontium on the grain refining efficiency of Mg–3Al alloy refined by carbon inoculation

The effect of Sr on the grain refining efficiency of the Mg–3Al alloy refined by carbon inoculation has been investigated in the present study. A significant grain refinement was obtained for the Mg–3Al alloy treated with either 0.2% C or 0.2% Sr. The Al–C–O particles were found in the sample refined by 0.2% C, and the element O should come from reaction between Al₄C₃ nuclei of Mg grains and water during the process of sample preparation. The grain size of the sample refined by carbon inoculation was further decreased after the combined addition of Sr. The grain size decreased with increasing Sr content. Much higher refining efficiency was obtained when the Sr addition was increased to 0.5%. Sr is an effective element to improve the grain refining efficiency for the Mg–Al alloys refined by carbon inoculation. The number of Al₄C₃ particles in the sample refined by the combination of carbon and Sr was more than that in the sample refined by only carbon. No Al–C–O–Sr-rich particles were obviously found in the sample refined by the combination of carbon and a little (<0.5%) Sr addition.     

Grain refinement of Al-Cu alloy in low voltage pulsed magnetic field

The effect of a low voltage pulsed magnetic field (LVPMF) on grain refinement of Al-Cu alloy was investigated at different solidification stages. The cooling curve was also studied to investigate the grain refinement mechanism of LVPMF. The fine grains are obtained by applying the LVPMF during the nucleation stage. The LVPMF has no obvious influence on the solidification structure when it is applied during liquid phase stage or crystal growth stage. Application of LVPMF increases the nucleation temperature of the isomorphous transformation, and also decreases the recalescence magnitude of the alloy. The refining mechanism was proposed that the LVPMF provides extra energy for nucleation, which decreases the energy barrier and the critical radius for nucleation, leading to high nucleation rate and grain refinement.     

镁合金铸态晶粒细化技术的研究进展

综述了熔体过热法、氯化铁法、碳质材料变质处理法、熔体搅拌法以及加Zr、Ca或Sr等镁合金铸态晶粒细化技术的研究现状和最新进展,重点分析了不同细化方法的工艺特点和细化机理,指出了镁合金铸态晶粒细化研究还存在的问题,并对今后的发展方向进行了展望.     

Effects of rare-earth elements on the glass-forming ability and mechanical properties of Cu46Zr47-xAl7Mx (M = Ce, Pr, Tb, and Gd) bulk metallic glasses

Cu46Zr47-xAl7Mx(M = Ce,Pr,Tb,and Gd) bulk metallic glassy(BMG) alloys were prepared by copper-mold vacuum suction casting.The effects of rare-earth elements on the glass-forming ability(GFA),thermal stability,and mechanical properties of Cu46Zr47-xAl7Mx were investigated.The GFA of Cu46Zr47-xAl7Mx(M = Ce,Pr) alloys is dependent on the content of Ce and Pr,and the optimal content is 4.at %.Cu46Zr47-xAl7Tbx(x = 2,4,and 5) amorphous alloys with a diameter of 5 mm can be prepared.The GFA of Cu46Zr47-xAl7Gdx(x = 2,4,and 5) increases with increasing Gd.Tx and Tp of all decrease.Tg is dependent on the rare-earth element and its content.△Tx for most of these alloys decreases except the Cu46Zr42Al7Gd5 alloy.The activation energies △Eg,△Ex,and △Ep for the Cu46Zr42Al7Gd5 BMG alloy with Kissinger equations are 340.7,211.3,and 211.3 kJ/mol,respectively.These values with Ozawa equations are 334.8,210.3,and 210.3 kJ/mol,respectively.The Cu46Zr45Al7Tb2 alloy presents the highest microhardness,Hv 590,while the Cu46Zr43Al7Pr4 alloy presents the least,Hv 479.The compressive strength(σc.f.) of the Cu46Zr43Al7Gd4 BMG alloy is higher than that of the Cu46Zr43Al7Tb4 BMG alloy.     

On dendritic solidification of multicomponent alloys with unequal liquid diffusion coefficients

A model has been developed previously by Rappaz and Thévoz (Acta metall., 1987, 35, 1478; 1987, 35, 2929) for the solidification of binary alloys having an equiaxed dendritic morphology. The extension of this model to multicomponent alloys is straightforward if the diffusion coefficients in the liquid of the various solute elements are equal. When they are different however, it becomes necessary to distinguish the concentrations at the dendrite tip position, governing the growth kinetics, from the average concentrations within the interdendritic liquid region. A model is presented which couples the dendrite growth kinetics with an overall solute balance performed at the scale of the grain. The diffusion profile outside the grain envelope is calculated using a Landau transformation and an implicit scheme. The calculated solidification paths during and after recalescence are compared with the Scheil approximation for various conditions. Additionally, a model predicting the secondary dendrite arm spacing in multicomponent alloys is briefly described.     

基于基团贡献法估算锂离子动力电池正极材料LiNixCoyMnzO2的ΔHf,298θ和ΔGf,298θ

基于基团贡献法对裡离子动力电池正极材料LiNi0.6Co0.2Mn0.2O2、LiNi0.5Co0.2Mn0.3O2、LiNi0.8Co0.1Mn0.1O2和LiNi1/3Co1/3Mn1/3O2的△Hf,298θ和△Gf,298θ进行估算。首先采用基团贡献法对56种固体无机化合物的△Hf,298θ和△Gf,298θ进行估算,估算值与文献值相比,相对误差绝对值都在4%之内。基于基团贡献法首次构建了估算锂离子动力电池正极材料LiNixCOyMnzO2的△Hf,298θ和△Gf,298θ的数学模型,结合XPS实验数据分析结果,对LiNi0.6Co0.2Mn0.2O2、LiNi0.5Co0.2Mn0.3O2、LiNi0.8Co0.1Mn0.1O2和LiNi1/3Co1/3Mn1/3O2正极材料的Hθf,298和ΔGθf,298进行估算,对应正极材料的△Hf,298θ和△Gf,298θ估算值分别为-705.39,-703.90,-695.67,-705.17 kJ·mol^-1和-647.98,-640.04,-631.10,-642.41 kJ·mol^-1。     

铝含量对应变诱发法半固态镁合金的影响

结合半固态合金成分设计原则,通过热力学计算,研究了3种成分的半固态镁合金。通过组织观察和流变学参数分析表明,半固态组织的形成速度随铝含量增加而加快,固相粒子变得细小、圆整。分析得出铝元素主要通过两个方面影响半固态组织,一个是液相形核,另一个是再结晶晶粒尺寸。随着铝含量增加,液相形核数量增多,再结晶晶粒尺寸减小,这些都促进了半固态组织的形成和演变,有利于镁合金半固态加工。     

GRAIN REFINEMENT IN BULK UNDERCOOLED Ni-BASED ALLOY

Fluxing of 5 g bulk melt Ni77Si13B10 permits high undercoolings to be attained prior to nu-cleation onset.Investigations of grain refinement in the bulk undercooled alloy as a function ofundercooling,recalescence behavior and cooling rate have been reported.A significant inhomo-geneity of reduction in grain size of a bulk sample is observed,which is caused by the different so-lidification conditions:(1)recalescence process,and(2)the followed plateau in which the heatrelease and extraction rates are equal.It is concluded that the homogeneous refined microstructurecan be achieved if the initial undercooling prior to nucleation,or cooling rate after recalescence isfurther increased.     

Evaluation of glass forming ability of alloys

Key step of exploiting a new type BMG (Bulk Metallic Glass) is quickly judging GFA (Glass Forming Ability) of the alloys. The mole melting heats of BMGs are calculated using the weighted averages principle. The reliability and limitation of Trg criterion for CFA are discussed. The reason why Trg of BMGs is larger than 0.5 is discussed. Two new criteria for GFA, △Hmg and △Gg, are proposed. GFA sequence of BMGs is calculated using the △Hmg criterion, the result agrees with that of A. Inoue and the Rc criterion. Furthermore, as an example, the Rc of the alloys developed by Chuang DONG et al is calculated using the △Hmg and △Gg. The ascending sequence of these alloys calculated with the △Hmg criterion agrees with that of Chuang DONG et al. On the contrary, the result by the △Gg criterion is in contrary with Chuang DONG et al, indicating that the △Hmg criterion is better and more convenient than the △Gg criterion. Calculation showed that the optimum △Hmg iS -15.16 k J/mol.     

Effect of Al5Ti1B master alloy on microstructures and properties of AZ61 alloys

AZ61 alloys with different levels of Al5Ti1B master alloy additions were prepared by conventional casting method.The effects of Al5Ti1B contents and holding time on microstructures and microhardness of AZ61 alloys were studied by XRD,OM and microhardness testing techniques.The results show that when the addition level of Al5Ti1B master alloy is less than 0.5%(mass fraction),the average grain size of the alloys decreases with the increase of Al5Ti1B content at the same holding time.But the grain size increases somewhat with further addition of Al5Ti1B.The average grain size of the alloys decreases with the increase of the holding time as it is less than 30 min at the same addition level of Al5Ti1B.It is considered that TiB2 particles can serve as the heterogeneous nucleation sites ofα-Mg during solidification,and heterogeneous nucleation is the main reason for the grain refinement of AZ61 alloys.The microhardness of the refined AZ61 alloys with 1.0%Al5Ti1B addition is increased by about 8%.     

Inoculated Titanium Aluminide Alloys and Their Predicted Propensity for Grain Refinement

This work centers on grain-size model calculations of titanium diboride-inoculated titanium aluminide-based alloys. The model uses the free-growth criterion of grain initiation after heterogeneous nucleation, which implies that grains initiate once their interfacial curvature reaches a maximum. The presented calculations identify quantitatively the influence of inoculation process parameters on the amount of achievable grain refinement. A benchmark test against experimental results allows a discussion of the grain size reduction efficiency of titanium-diboride inoculants.     

含碳、硼变质的Mg-Al合金的显微组织与力学性能

通过光学显微金相定量分析、SEM扫描电镜及EDX分析、XRD分析以及抗拉强度测试等实验手段,研究了含碳、硼元素复合添加剂的Mg-(6%￣9%)Al合金的显微组织和力学性能的影响。试验结果表明:随着合金中Al含量的增加,在伸长率基本不变的情况下,抗拉强度显著提高。合金的抗拉强度明显增加与铝的固溶强化、第二相强化以及碳、硼的晶粒细化作用有关;而合金的伸长率随着Al的增加而基本不变的主要原因是碳、硼的晶粒细化作用。     

Recrystallization developed in the largely undercooled Ni54.6Pd45.4 alloy

The structure evolution driven by recrystallization in the bulk undercooled Ni_54.6Pd_45.4 alloy was investigated by quenching the sample at different cooling stages. It is found that recrystallization of the solid beyond the critical undercooling for grain refinement starts at the end of rapid solidification, and develops during the slow solidification and the subsequent cooling process. At the initial stage of recrystallization, nuclei successively form in the sample, leading to a decrease in grain size. At the later stage, annexation of grains dominates the recrystallization, due to which the grain size increases. As undercooling prior to solidification increases, the deformation degree in the rapidly solidified dendrite net rises, and the recalescence temperature as well as the duration at high temperature decreases. Consequently, the grain size of the recrystallization structure decreases.     

Effects of Y on mechanical properties and damping capacity of ZK60 magnesium alloys

The microstructure,mechanical properties and damping capacity of ZK60-xY(x=0,1.5%,2.5%,4.0%,mass fraction) magnesium alloys were investigated by using the optical microscope(OM),X-ray diffractometer(XRD),universal tensile testing machine and dynamic mechanical analyzer(DMA).The mechanisms for damping capacity of referred alloys were discussed by Granato-Lücke theory.The results show that Y additions remarkably reduce grain size(the average grain size is 21.6,13.0,8.6 and 4.0μm,respectively),and the tensile properties are enhanced with grain refining(the yield tensile strength increases to 292 MPa from 210 MPa and ultimate tensile strength increases to 330 MPa from 315 MPa).For the ZK60-xY(x=0,1.5%,4.0%)alloys,the damping capacity decreases with the increase of Y content.However,for the ZK60-xY(x=2.5%)alloy,the damping capacity improves abnormally,which is possibly related to the formation of Mg3Y2Zn3(W)FCC phase in this alloy.