Effect of Plasma Spheroidization Process on the Microstructure and Crystallographic Phases of Silica, Alumina and Nickel Particles

During the plasma spheroidization process powders undergo different changes in their microstructures and crystal phases. In this paper, simple calculation of heat transfer between the plasma and a suspended particle was performed based on three hypotheses for the purpose of guiding experiments. Experimental investigation of the crystal phases and microstructural changes during the plasma processing was made using silica, alumina and nickel powders as starting materials. It has been revealed from the experimental results that these materials undergo different changes in crystal phases and microstructures, and these changes are essentially determined by the structures, properties and aggregate states of the starting materials.

Changes in lamellar microstructure by parallel twinning during creep in soft PST crystal of TiAl alloy

Compression creep tests of a Ti-48%Al (mole fraction) alloy were carried out at 1?150?K with soft-orientated PST crystal. Parallel twinning took place during the creep. Changes in lamellar microstructure caused by the parallel twinning were investigated, and their effects on creep deformation behavior were discussed. The results show that the parallel twinning occurs in an early stage of creep, and makes significant contribution to creep strain in the domains favorably oriented for the twinning. The nucleation of parallel twins finishes at a strain of about 3%. There is a critical resolved shear stress for parallel twinning, and it is about 50?MPa in the Ti 48%Al PST crystals at 1?150?K. The activity of parallel twinning increases with increasing applied stress or in a coarse lamellar material. The addition of parallel twins reduces the average value of lamellar spacing. In general, the refinement of lamellar structure should improve creep resistance. However the strengthening by parallel twinning is not evident in creep of the soft PST crystals because the soft deformation modes are the dominant deformation mode in the crystals.

Effects of γ/γ interfacial structures on continuous coarsening of lamellar microstructure in TiAl alloy during aging

The effects of γ/γ interfacial structures on continuous coarsening of the fully lamellar microstructure in Ti 48Al alloy aged at 1 150 ℃ were investigated by using transmission electron microscopy(TEM). Continuous lamellar coarsening can be achieved not only by migration of interface faults(such as ledges, edges and curved interfaces) but also by migration and decomposition of perfect γ/γ lamellar interfaces. Thermal grooves, initiative positions of interfacial dissociation, can frequently form at the triple point junctions between the 120° rotational ordered γ domain boundaries within γ lamellae and the lamellar interfaces. During the early stage of aging at 1 150 ℃, the interface migration and dissociation took place preferentially at the 120° rotational ordered lamellar interfaces. Comparing the relative thermal stability of the true twin, pseudo twin and 120° rotational ordered γ/γ lamellar interfaces shows that the 120° rotational ordered lamellar interface is the most unstable. The reason of this phenomenon was analyzed through the comparisons of the interfacial energies and atomic arrangements of the three types of γ/γ lamellar interfaces.

Transformation mechanism of lamellar microstructure of AZ80 wrought Mg alloy during warm deformation

The microstructure especially the lamellar second phase evolution by a combination of deformation and heat treatment for AZ80 alloy was investigated.The results show that there are finer lamellar Mg_(17)Al_(12) phases after hot compression with the increasing strain,while there are coarse lamellar discontinuous precipitation cells ofβ-Mg_(17)Al_(12) phase spreading from the grain boundaries into the grains after T6 treatment of the compressed samples.The lamellar morphologies especially the lamellar distance ofβ-Mg_(17)Al_(12) phase precipitation of the T6 treated deformation specimen at different strains differ from each other as there are different grain boundaries in the corresponding compressed specimens.

Kink-band formation in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic alloys with Mg/Laves-phase lamellar microstructure

For the development of high-strength Mg alloys,active use of Laves phases such as C14-type Mg_(2)Yb and Mg_(2)Ca is strongly expected.However,the brittleness of the Laves phases is the biggest obstacle to it.We first found that kink-band formation can be induced in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic lamellar alloys when a stress is applied parallel to the lamellar interface,leading to a high yield stress accompanied with ductility.That is,microstructural control can induce a new deformation mode that is not activated in the constituent phases,thereby inducing ductility.It was clarified that the geometric relationship between the operative slip plane in the constituent phases and the lamellar interface,and the microstructural features that provide kink-band nucleation sites are important factors for controlling kink-band formation.The obtained results show a possibility to open the new door for the development of novel high-strength structural material using the kink bands.

Microstructure and Mechanical Properties of Two-phaseTiAl Alloys in Lamellar Form

In two-phase TiAl alloys, the lamellar structures are of special interest and importance since they are so common and persistent. not only under as-cast conditions but also after thermal treatment. However. the lamellar structures are still poor in ductility,although they are beneficial for toughness and high temperature strength. This article will review the recent progress made in understanding the basic mechanical properties of the γ and α2 phases which comprise the two-phase alloys in Iamellar form, and discuss how an improved balance of strength and ductillty in the lamellar form may be achieved

Numerical modeling of damping capacity of Zn-Al alloys with fully lamellar microstructures

The damping behaviors of Zn-Al alloys with fully lamellar microstructures were simulated with the cell method. The influences of the grain boundary condition, the strain amplitude, the number of the lamellae in the grain (N) and the content ratio of Zn and Al in Zn-Al alloys on the damping capacity were investigated. The results indicate that the grain boundary condition has great influence on the damping capacity of Zn-Al alloys, and also affects the relationship between the damping capacity and the number of lamellae (N). The variation of damping capacity with the strain amplitude is increasing exponentially with the strain amplitude and the damping capacity increases with the increasing of content of Zn.

Effect of Rare Earth on Microstructure of γ-TiAl Intermetallics

The rare earth (RE) elements (Ce, La) were added to binary Ti 47% Al alloys (atomic fraction) by Induction Skull Melting. The element Ce of 1.0 atomic percent was added individually, and La of 0.2 atomic percent was added individually. This article studied the influences of rare earth metal (Ce, La) on microstructure of as cast TiAl based alloy by XRD, SEM, EMPA and TEM measurement methodology. The results show that most of rare earth rich phases (AlCe, AlLa) are uniformly distributed in grain boundary in the shape of discontinuous network, and some particles of rare earth rich phases within the grains are mainly ellipsoids. In addition, rare earth element can obviously refine the grain size and the lamellar thickness of as cast TiAl based alloy samples. The grain size of Ti 47Al 1.0Ce 0.2La alloy reaches about 30～80 μm, and the lamellar thickness of its γ phase and α 2 phase are less than 200 and 20 nm, respectively.

MICROSTRUCTURAL STABILITY OF LAMELLAR TiAl BASED ALLOYS AT HIGH TEMPERATURES

The microstructural stability of lamellar TiAl base alloys at high temperatures was studied by conventional and high resolution transmission electron microscopy. The influence of substructures on the thermal stability of lamellar structure was emphasized. These substructures produced by thermal mechanical treatments include the interfacial dislocations and ledges, the subgrain boundaries, the impinged T(Q) twins and misorientated lamellar interfaces. The microstructural change of three kinds of lamellar TiAl base alloys containing differents type and densities of substructures were compared during exposure at 800~1 000 ℃. It was found that the existence of such substructures could accelerate the degeneration of lamellar structure, leading to the rapid necking and break up of α 2 plates, the coarsening of γ plates, and the formation of new γ grains. As a result, the lamellar structure with substructures started to degenerate after thermal exposure at 800℃ for 4.5 h. While only slight coarsening was observed at the colony boundaries in the lamellar structure without substructures even after exposure at 900 ℃ for 7 d.

Effect of annealing on the microstructures and Vickers hardness at room temperature of intermetallics in Mo-Si system

The microstructures and Vickers hardness at room temperature of arc-meltingprocessed intermetallics of Mo_5Si_3-MoSi_2 hypoeutectic alloy and hypereutectic alloy annealed at1200℃ for different time were investigated. Lamellar structure consisted of Mo_5Si_3 (D8m) phaseand MoSi_2 (C11_b) phase was observed in all the alloys. For Mo_5Si_3-MoSi_2 hypoeutectic alloy, thelamellar structure was found only after annealing and developed well with fine spacing on the orderof hundred nanometers after annealing at 1200℃ for 48 h. But when the annealing time was up to 96h, the well-developed lamellar structure was destroyed. For Mo_5Si_3-MoSi_2 hypereutectic alloy, thelamellar structure was found both before and after annealing. However the volume fraction andspacing of the lamellar structure did not change significantly before and after annealing. Theeffects of the formation, development and destruction of lamellar structure on Vickers hardness ofalloys were also investigated. When Mo_5Si_3-MoSi_2 hypoeutectic alloy annealed at 1200℃ for 48 h,the Vickers hardness was improved about 19% compared with that without annealing and formation oflamellar structure. The highest Vickers hardness of Mo5Si3-MoSi_2 hypereutectic was increasing about18% when annealing at 1200℃ for 48 h.

Spheroidization behavior of dendritic b.c.c. phase in Zr-based β-phase composite

The spheroidization behavior of the dendritic b.c.c. phase dispersed in a bulk metallic glass (BMG) matrix was investigated through applying semi-solid isothermal processing and a subsequent rapid quenching procedure to a Zr-based β-phase composite. The Zr-based composite with the composition of Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 was prefabricated by a water-cooled copper mold-casting method and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the composite consists of a glassy matrix and uniformly distributed fine dendrites of the β-Zr solid solution with the body-centered-cubic (b.c.c.) structure. Based on the differential scanning calorimeter (DSC) examination results, and in view of the b.c.c. β-Zr to h.c.p. α-Zr phase transition temperature, a semi-solid holding temperature of 900℃ was determined. After reheating the prefabricated composite to the semi-solid temperature, followed by an isothermal holding process at this temperature for 5 min, and then quenching the semi-solid mixture into iced-water; the two-phase microstructure composed of a BMG matrix and uniformly dispersed spherical b.c.c. β-Zr particles with a high degree of sphericity was achieved. The present spheroidization transition is a thermodynamically autonomic behavior, and essentially a diffusion process controlled by kinetic factors; and the formation of the BMG matrix should be attributed to the rapid quenching of the semi-solid mixture as well as the large glass-forming ability of the remaining melt in the semi-solid mixture.

Cavity nucleation during hot forging of Ti-6Al-2Zr-1Mo-1V alloy with colony alpha microstructure

The initiation sites and influencing factors of cavity nucleation were investigated for a Ti-6Al-2Zr-1Mo-1V alloy with lamellar starting structure,using the isothermal hot compression test.All samples were deformed to a true strain of 0.70 in the temperature range of 750-950°C and strain rate range of 0.001-10 s-1.The corresponding microstructures were observed by means of the metallurgical microscopy and scanning electron microscopy(SEM).It was found that all cavities occurred at the bulge regions of the compression specimens.Most of cavities nucleated along prior beta boundaries oriented 45°to the compression axis,while others nucleated at the interfaces of lamellar alpha colonies.Cavity nucleation was inhibited with increasing the volume fraction of beta phase and the volume fraction spheroidized of lamellar alpha phase.

Microstructure and microhardness of Ti-48Al alloy prepared by rapid solidification

To improve the microstructure and microhardness,Ti-48Al(at.%)alloy was rapidly solidified by melt spinning under different cooling rates.The microstructure and microhardness of rapidly solidified Ti-48Al alloy were systematically investigated.Results show that the average lamellar colony size of the alloy reduces from 60.6μm to 11μm as the cooling rate increases from 2.3×105 to 5.1×105 K·s-1,caused by the increase of nucleation rate at a higher cooling rate.At the high cooling rate of(4.3-5.1)×105 K·s-1,theαphase is the primary phase,and a few metastableαphases are reserved,which then transform intoα2 phase and subsequently lead to the formation ofα2 equiaxed grain.The lamellar spacing also decreases with the increase of cooling rate.The relationship between lamellar spacing(d)and cooling rate(v)is d=33.6v-1.34.The microhardness increases with the increase of cooling rate because the refined lamellar spacing and grain size can improve the microhardness.

Investigation into microstructure of spray formed V4 cold working mould steel and its roiling and annealing

The material selected for this work was the spray formed Vanadis4 high alloy cold working mould steel （abbreviated to V4 steel）. Its microstructure, hot rolling process, and annealing treatment have been investigated. Observed from the optical and electron microscopes, the as-sprayed V4 steel had the finer microstructure of uniform and equiaxial grains ,while after hot rolling for densification and spheroidized annealing, the V4 steel obtained an excellent spheroidized structure that is favorable to subsequent quenching and tempering treatment. The spheroidized structure and level of annealed hardness of the V4 steel are almost the same as expensive imported powder metallurgy the V4 steel. It is difficult to produce V4 steel with the conventional ingot metallurgical technique, so the multi-step and high-cost powder metallurgy method is generally used at present. Compared to the powder metallurgy technique, using the spray forming technique to produce the V4 steel has obvious advantages and potential market competitiveness in reducing production costs, simplifying working process, and shortening the production cycle.

Effect of microstructure on impact toughness of TC21 alloy

The impact toughness of TC21 alloy after different types of forging and heat treatments was studied. The results show that heat treatment at 915 ℃ for 1 h followed by air-cooling can achieve the highest impact toughness. The crack propagation path of bimodal microstructure is different from that of lamellar microstructure. Boundaries of primary α grain are observed to be preferential sites for microcrack nucleation. With the increase of heat treatment temperature,the volume fraction of primary α phase decreases and the nucleation sites of microcrack at the primary α phase boundaries also decrease,the impact toughness value is effectively improved. The microcracks of lamellar microstructure are located on α/β interface,or the boundary of colony,and/or grain boundary α phase. The crack propagates cross the colony,or along the colony boundary,and/or along β grain boundary. The crack propagation path of lamellar microstructure is dependent on the size,direction of colony. The crack path deflects at grain boundaries,colony boundaries,or arrests and deviates at α/β interface because of crisscross α lamellar. Therefore the impact toughness value of basket microstructure is higher than that of Widmanstatten microstructure.

TiVNbTa高熵合金球形粉末制备及选区激光熔化成形

利用氢化脱氢、机械球磨、等离子体球化技术成功制备了TiVNbTa难熔高熵合金球形粉末,并采用选区激光熔化(SLM)技术将此球形粉末制备成TiVNbTa难熔高熵合金块体试样,研究不同SLM工艺参数对合金显微组织和力学性能的影响。结果表明:所制备的粉末球形度高、分散良好,平均粒度为48μm;SLM成形的TiVNbTa难熔高熵合金具有超细晶组织,其熔池心部为胞状晶区,熔池边缘为柱状晶区。随着激光功率的增加,合金的压缩屈服强度整体变化不大,但塑性应变呈下降趋势;然而,随着扫描速率的增加,合金的压缩屈服强度和塑性应变持续下降。当激光功率为175 W、扫描速率为400 mm/s时,沉积态合金具有最低的孔隙率,XOY面和YOZ面的孔隙率分别降低至0.26%和1.42%,压缩屈服强度σ_(0.2)为1001 MPa,最大抗压强度σ_(bc)为1741 MPa,塑性应变ε_(p)为10.0%。

新型高镍无钴正极材料LiNi_(0.94)Mn_(0.04)Al_(0.02)O_(2)的合成研究

采用固相烧结工艺合成了层状高镍无钴正极材料LiNi_(0.94)Mn_(0.04)Al_(0.02)O_(2)(NMA),并研究了不同烧结温度对NMA正极材料的晶体结构、微观形貌和电化学性能的影响。结果表明,当烧结温度过低时,NMA正极材料的结晶度偏低,并在表面形成残锂。烧结温度过高则会导致层状结构变差和电极表面有害副反应增多。在最佳烧结温度750℃下合成的NMA-750材料具有良好的颗粒形貌、最少的锂镍混排和最完整的层状结构,同时具有最佳的电化学性能:首圈放电比容量(3.0~4.5 V,1 C)为199.5 mA·h/g,循环100圈后容量保持率可达79.04%;在5 C下仍具有147.6 mA·h/g的放电比容量,倍率性能优良。

罩式炉GCr15钢丝球化退火工艺研究

对比分析了GCr15轴承钢普通球化退火工艺和改进球化退火工艺处理后的金相组织和硬度。结果表明,GCr15轴承钢采用改进球化退火工艺处理后比普通球化退火处理后硬度值低6 HBW,金相组织级别波动小,经0.2~0.3 mm拉拔减径后,成品材料硬度满足要求。改进球化退火工艺采用氢气作为保护气氛,材料无附加脱碳;采用快速加热方式,工艺周期缩短2 h,材料金相组织和硬度均匀性较改进前明显提高;生产效率提高,经济效益显著。

不同冶炼方式下的H13的锻造生产及组织性能对比

随着新能源汽车的快速发展,急需对高热疲劳性、高使用寿命的铝压铸模具钢材进行研发应用。而H13属于热作模具钢,普遍应用于铝挤压、铝压铸、热模锻等行业,市场需求量较大。主要根据原材料冶炼方式的不同,对H13锻圆的锻造生产以及组织性能进行对比分析。

Superior strength-plasticity synergy in a heterogeneous lamellar Ti_(2)AlC/TiAl composite with unique interfacial microstructure

Improving the plasticity of TiAl alloys at room temperature has been a longstanding challenge for the de-velopment of next-generation aerospace engines.By adopting the nacre-like architecture design strategy,we have obtained a novel heterogeneous lamellar Ti_(2)AlC/TiAl composite with superior strength-plasticity synergy,i.e.,compressive strength of∼2065 MPa and fracture strain of∼27%.A combination of micropil-lar compression and large-scale atomistic simulation has revealed that the superior strength-plasticity synergy is attributed to the collaboration of Ti_(2)AlC reinforcement,lamellar architecture and heteroge-neous interface.More specifically,multiple deformation modes in Ti_(2)AlC,i.e.,basal-plane dislocations,atomic-scale ripples and kink bands,could be activated during the compression,thus promoting the plas-tic deformation capability of composite.Meanwhile,the lamellar architecture could not only induce sig-nificant stress redistribution and crack deflection between Ti_(2)AlC and TiAl,but also generate high-density SFs and DTs interactions in TiAl,leading to an improved strength and strain hardening ability.In addi-tion,profuse unique Ti_(2)AlC(1¯10¯3)/TiAl(111)interfaces in the composite could dramatically contribute to the strength and plasticity due to the interface-mediated dislocation nucleation and obstruction mecha-nisms.These findings offer a promising paradigm for tailoring microstructure of TiAl matrix composites with extraordinary strength and plasticity at ambient temperature.