将等离子旋转电极法所制Zr55Cu30Al10Ni5 (Zr55)粉末在1000 K退火处理后作为沉积材料,应用激光立体成形技术沉积Zr55块体非晶合金,考察工艺参数及退火态粉末尺寸对熔覆层晶化行为的影响。结果表明,不同尺寸退火态粉末组织均由Al5Ni3Zr2、CuZr2和Al2Zr3相组成。以不同激光线能量熔覆后,试样的熔池区主要为非晶,晶化区由熔池底部到热影响区依次分布NiZr2纳米晶、CuZr2+ZrCu枝状共晶和CuZr2+ZrCu球粒状共晶,共晶尺寸随着距熔池区距离的增加而减小。当激光线能量较低时,熔覆层均保持较高含量的非晶相。随着激光线能量的增大,尺寸为75~106 μm的退火态粉末所制试样的晶化程度无明显加强,而尺寸为106~150 μm的退火态粉末所制试样的晶化程度显著加剧。Zr55合金熔覆沉积层的晶化差异受粉末本身相结构影响较小,主要由熔覆不同尺寸粉末时熔池及热影响区的热历史决定。
Laser solid forming (LSF) provides an innovative way in building the bulk metallic glasses (BMGs) due to its inherently rapid heating and cooling process and point by point additive manufacturing process, which can eliminate the limitation of critical casting size of BMGs. The annealed powder has been demonstrated to be applicable to the preparation of BMGs with high content of amorphous phase using LSF. In this work, the plasma rotating electrode processed (PREPed) Zr55Cu30Al10Ni5 (Zr55) powders annealed at 1000 K are used for LSF of Zr55 BMGs. The influences of powder size and laser processing parameter on the crystallization characteristic of the deposit are investigated, and the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) is analyzed. It is found that the microstructures of the pre-annealed Zr55 powders are composed of the Al5Ni3Zr2, CuZr2 and Al2Zr3 phases. As the heat input increases from 7.0 J/mm to 15.7 J/mm, the every deposited layer presents a periodic repeating gradient microstructure (amorphous, NiZr2 nanocrystal, CuZr2+ZrCu dendrite-like eutectic, CuZr2+ZrCu spherulite-like eutectic) from the molten pool to the HAZ. The size of the eutectic phase in the HAZ decreases as the increase of distance from the featureless amorphous zone. On condition that the laser heat input is less than 7.0 J/mm, the deposits contain a high content of amorphous phase. As the increase of laser heat input, the crystallization degree of HAZ does not increase obviously for the deposit prepared by the powder with size range of 75~106 μm. However, the crystallization degree of HAZ increases significantly for the deposit prepared by the powder with size range of 106~150 μm. That is because the lower overheating temperature and shorter existing time of the molten pool enhances the heredity of Al5Ni3Zr2 clusters and other intermetallic clusters in remelted alloy melt during LSF of coarser powder, which decreases the thermal stability of the already-deposited layer and induces the severe crystallization. It is deduced that the raw state of annealed powders has a minimal impact on the crystallization behavior of the Zr55 deposited layers when the content of Al5Ni3Zr2 phase is same in different sizes of annealed powders. The thermal history of RZ and HAZ during deposition is the primary factor to affect the crystallization behavior in the Zr55 deposits fabricated by different powder sizes.
块体非晶合金因其具有优异的力学、物理和化学性能[1~4],在航空、航天、机械、化工等领域拥有广阔的应用前景。然而采用传统工艺制备大块非晶时,通常存在临界冷速和尺寸的限制。近年来,很多学者利用激光立体成形(laser solid forming,LSF)所具有的逐点逐层三维自由实体快速熔覆沉积成形的特点来制备任意尺寸的大块非晶合金,这为促进块体非晶合金在工程领域的应用和发展创造了一条新的途径。Ye和Shin采用每沉积一层就停留5 s再进行后续沉积的方式来避免热累积效应所产生的晶化,最终激光立体成形制备了晶化较少的铁基大尺寸非晶合金。Yang等对比了激光快速熔凝和激光立体成形锆基非晶合金的晶化行为,指出如果熔覆层的厚度大于热影响区的宽度就有可能实现大块非晶合金的激光立体成形制备。Pauly等采用选区激光熔化技术制备了非晶含量较高的铁基大尺寸非晶合金。由于激光立体成形过程是激光、粉末和基材的快速相互作用过程,基于合金熔体结构的遗传性,合金粉末的初始晶化状态将对熔覆沉积层的晶化特征产生影响。Balla和Bandyopadhyay采用晶态和非晶态相混合的粉末激光立体成形制备直径10 mm、高15 mm的铁基非晶合金时,发现熔覆层的晶化相可能来源于原始大尺寸粉末中已有的晶化相,认为只有采用完全非晶态粉末才有可能得到非晶态熔覆层。本课题组前期工作采用原始Zr55Cu30Al10Ni5 (Zr55)粉末和高温退火处理后的Zr55粉末熔覆沉积制备非晶合金时,发现未退火的小尺寸粉末所制熔覆层的晶化程度较小,而采用未退火的大尺寸粉末所制熔覆层中出现了许多源于原始粉末的Al5Ni3Zr2晶化相,晶化程度较为严重。为了提高大尺寸粉末的利用率,将大尺寸粉末进行退火处理后再熔覆沉积,反而可以明显减小熔覆层的晶化程度进而获得非晶含量较高的熔覆层。可见,除了采用原始小尺寸Zr55粉末,退火态Zr55粉末也可以用于激光立体成形制备非晶合金。然而,高温退火处理后的Zr55粉末尺寸对熔覆层晶化行为的影响仍不清晰,需要进一步考察以不同工艺参数激光立体成形不同尺寸退火态Zr55粉末的晶化行为。
本工作筛分选取等离子旋转电极雾化法(plasma rotating electrode processing,PREP )所制的不同尺寸Zr55粉末,并将粉末高温退火处理后进行激光立体成形。通过分析不同激光工艺参数下退火态合金粉末所制熔覆层的晶化特征,考察退火态粉末尺寸以及工艺参数对熔覆层熔池区及其热影响区晶化行为的影响机制,以期为激光立体成形大块非晶合金提供实验基础和理论依据。
采用纯度99.99% (质量分数),尺寸为30 mm×10 mm×3 mm的纯Zr板作为基体材料。通过PREP法制备Zr55合金粉末,粉末实测化学成分和名义化学成分如
采用尺寸为75~106 μm和106~150 μm的退火态粉末进行激光立体成形Zr55非晶合金时,发现仅在线能量为7.0 J/mm时,熔覆沉积层中含有较高含量的非晶相。随着激光线能量的增大,熔覆层的晶化程度均有所增加,其中大尺寸粉末所制熔覆层的晶化程度更为明显。为了分析以不同激光线能量熔覆沉积不同尺寸Zr55粉末时的晶化行为,采用Comsol软件对单道熔覆非晶合金所产生的温度场进行模拟。模拟计算的三维数值模型中的基材尺寸为30 mm×10 mm×3 mm,激光能量分布近似为Gauss分布,激光束垂直照射沉积样品的上表面, 且以一定的速率移动。熔覆沉积过程中的热传导行为由瞬态热传导偏微分方程控制。模型采用第三类边界条件,考虑了工件表面的对流换热和辐射换热。粉末材料的密度、定压比热容的计算参考文献。由于预置粉末床由不同尺寸的退火态Zr55粉末铺置而成,粉末材料对激光的吸收率以及粉末床的有效导热系数也有所不同。其中细粉颗粒因其较高的比表面积而具有高的激光吸收率。不同尺寸预置粉末床的有效导热系数参考文献进行计算。激光线能量为7.0 J/mm时,熔覆沉积不同尺寸粉末的温度场模拟结果如
(1) 尺寸为75~106 μm以及106~150 μm的Zr55Cu30Al10Ni5 (Zr55)退火态粉末组织均由Al5Ni3Zr2、CuZr2和Al2Zr3相组成。经不同激光线能量(7.0、10.8和15.7 J/mm)熔覆沉积后,熔覆层的熔池区都能保持非晶态。
(3) 随着激光线能量的增大,采用尺寸为106~150 μm的退火态粉末所制试样的热影响区的晶化程度加重,而尺寸为75~106 μm的退火态粉末所制试样中热影响区的晶化程度无明显变化。理论推导了Zr55熔覆沉积层非晶区的连续加热相变曲线,证明熔覆小尺寸Zr55粉末时熔池区较高的过热度导致已沉积非晶区具有较高的起始晶化温度和热稳定性。
The authors have declared that no competing interests exist.
Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)-based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass-transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF-based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.
This paper reviews past developments and present understanding of the glass-forming ability, structure and physical, chemical, mechanical and magnetic properties of bulk glassy alloys (BGA) with the emphasis on recent results obtained since 1990, together with applications of BGA, achieved mainly in Tohoku University. After introducing the fundamental concepts around glassy alloys (GA) in Sections 02and02 describes the progress of the study of structural relaxation leading to the discovery of GA with a large supercooled liquid region. Section 3 reviews the history of BGA development, followed by BGA systems and their features in Section 4, and features of glassy structure in Section 5. Sections , , 02and02 summarize the engineering and standardization of Zr-based BGA, followed by the origins of the development of useful materials on the basis of experimental data on the compositional effect on the fundamental properties of basic ternary and quaternary Zr-based BGA. Sections 02and02 include the glass-forming ability and dynamic mechanical properties of Zr-based hypoeutectic BGA and Cu–Zr–Al–Ag BGA. Mechanical properties of Ni- and Zr-based BGA at low temperatures are shown in Section 12, while Section 13 describes the formation and properties of Ni-free Ti-based BGA. Sections 02and02 deal with porous Zr-based BGA, including spherical pores and commercialized ferromagnetic and high-strength Fe-based GA, respectively, then Section 16 reviews supercooled liquid formation. Applications for Zr-, Ti- and Fe-based GA are described in Section 17. In conclusion, Section 18 attempts to assess the present knowledge of the structure and physical properties and identify some outstanding problems for future work.
AlNiCu amorphous alloy (bulk and powder with different morphologies) under different heating and cooling rate conditions, and has found that the crystallization behavior of ZrAlNiCu under different thermal history conditions was quite different. Under conventional heat treatment condition, the heating and cooling rates of the alloy were lower, the ZrAlNiCu bulk amorphous alloy began to crystallization above the outset crystallization temperature Tx, the CuZr nanocrystalline was formed. With increasing temperature, its grain size became bigger, when exceeding the melting point, the thick CuZr lath+eutectic organizations of lath (CuZr)+(Zr, Cu, Al, Ni) were formed under furnace cooling condition; under laser melting condition, however, the heating and cooling rates of alloy were higher, with increasing number of pulse frequency, the crystallization effect of ZrAlNiCu bulk amorphous alloy accumulated gradually, in heat affected zone spherulites were precipitated and grew up, subsequently became unstable and finally a belt of equiax crystals were formed along the weld pool boundary. For amorphous alloy powder, due to its poorer the cooling performance than bulk amorphous, the cooling rate was lower, causing the powder particles crystallized into equiaxed dendrite during laser melting process, and with increasing laser energy, the equiaxed dendrite size increased.
Bulk metallic materials have ordinarily been produced by melting and solidification processes for the last several thousand years. However, metallic liquid is unstable at temperatures below the melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys are composed of a crystalline structure. Recently, this common concept was exploded by the findings of the stabilization phenomenon of the supercooled liquid for a number of alloys in the Mg-, lanthanide-, Zr-, Ti-, Fe-, Co-, Pd u- and Ni-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multicomponent systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the production of bulk amorphous alloys in the thickness range of 1 100 mm by using various casting processes. Bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good corrosion resistance and have been used for sporting goods materials. The stabilization also leads to the appearance of a large supercooled liquid region before crystallization and enables high-strain rate superplasticity through Newtonian flow. The new Fe- and Co-based amorphous alloys exhibit a large supercooled liquid region and good soft magnetic properties which are characterized by low coercive force and high permeability. Furthermore, homogeneous dispersion of nanoscale particles into Zr-based bulk amorphous alloys was found to cause an improvement of tensile strength without detriment to good ductility. The discovery of the stabilization phenomenon, followed by the clarification of the stabilization criteria of the supercooled liquid, will promise the future definite development of bulk amorphous alloys as new basic science and engineering materials.
The superplasticity and diffusion bonding behavior were examined on the Zr–10Al–10Ni–15Cu metallic glass indicating amorphous structure. The material behaved in a superplastic manner in supercooled liquid region, and successfully diffusion bonded at these conditions.
61Laser deposition was used to synthesize multi-layer Fe-based metallic glass coating.61Microstructural analysis showed the amorphous-crystalline composite structure.61Average hardness is close to the hardness of the amorphous regions.61The resultant amorphous phase significantly improved the wear resistance.
The crystallization characteristics of Zr 55Al 10Ni 5Cu 30 bulk metallic glasses BMGs during pulsed laser surface melting (PLSM) were examined, and the crystallization behavior during Laser solid forming (LSF) of Zr 55Al 10Ni 5Cu 30 BMGs with the pre-laid powder method on the amorphous substrates was further investigated. It was found that the BMG could keep the amorphous state after PLSM with six pulses and crystallization began to occur in heat-affected zone (HAZ) after PLSM with twelve pulses. There was no crystallization occurred in the deposit with one and two layers during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers. The crystallization mainly occurred in HAZ during PLSM and LSF. A physical model was proposed to describe laser solid forming of BMGs, which explained the formation mechanism of BMGs during laser solid forming. It is shown that the crystallization during the PLSM and LSF process was mainly caused by the accumulation of structural relaxation in the HAZ. The size of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. Based on the present model and experiment results, we can reckon that bulk metallic glasses could be achieved by LSF without size limitation.
Metallic glasses and their descendants, the so-called bulk metallic glasses (BMGs), can be regarded as frozen liquids with a high resistance to crystallization. The lack of a conventional structure turns them into a material exhibiting near-theoretical strength, low Young's modulus and large elasticity. These unique mechanical properties can be only obtained when the metallic melts are rapidly cooled to bypass the nucleation and growth of crystals. Most of the commonly known and used processing routes, such as casting, melt spinning or gas atomization, have intrinsic limitations regarding the complexity and dimensions of the geometries. Here, it is shown that selective laser melting (SLM), which is usually used to process conventional metallic alloys and polymers, can be applied to implement complex geometries and components from an Fe-base metallic glass. This approach is in principle viable for a large variety of metallic alloys and paves the way for the novel synthesis of materials and the development of parts with advanced functional and structural properties without limitations in size and intricacy.
A medium-range order (MRO) structure characterized by the pre-peaks in the structure factorS(Q)curves of typical glass-forming Au55Cu25Si20melt has been detected using the high temperature x-ray diffractometer. Combining theab initiomolecular dynamics simulations and the experiments, we explore and discuss the structure of the MRO cluster. During the rapid solidification, some structural information carried by the MRO structure is inherited from the melt to the amorphous solid, which promotes the glass formation. Through the comparison of microstructures between amorphous and crystal phases, we also discuss the heredity mechanism.
78 LENS64 was used to fabricate bulk Fe-based amorphous alloy components. 78 Estimated cooling rates were much higher than required to achieve amorphization. 78 Crystalline phases in the laser processed alloy originated from feedstock powder. 78 Finer feedstock powder can achieve full amorphization.
61The Zr55BMGs prepared by LSF with finer powder contains 90.802vol% amorphous phase.61The crystallization behavior during LSF of powders with different sizes is analyzed.61The crystallization extent of the deposit increases with the powder size.61The microhardness of various microstructure zones in the deposit is characterized.
The crystallization behavior of Zr 55 Cu 30 Al 10 Ni 5 bulk amorphous alloy during laser solid forming (LSF) was analyzed. Since laser surface remelting (LSM) is a key process for the LSF, the crystallization behavior of as-cast Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glasses (BMGs) during LSM was also investigated. It was found that the amorphous state of the as-cast BMGs was maintained when they were repeatedly remelted four times in a single-trace LSM, and as for the LSF of Zr 55 Cu 30 Al 10 Ni 5 bulk amorphous alloy, the crystallization primarily occurred in the HAZ between the adjacent traces and layers after the two layers were deposited. The as-deposited microstructure exhibited a series of phase evolutions from the molten pool to the HAZ as follows: the amorphous → NiZr 2 –type nanocrystal+amorphous → NiZr 2 –type equiaxed dendrite+amorphous → Cu 10 Zr 7 –type dendrite+NiZr 2 –type nanocrystal. Among these microstructural patterns, the NiZr 2 –type nanocrystals and equiaxed dendrites primarily formed from the rapid solidification of the remelted liquid in the laser processing process, and the Cu 10 Zr 7 –type dendrites in the HAZ primarily formed by the crystallization of pre-existed nuclei in the already-deposited amorphous substrate.
High velocity oxy-fuel (HVOF) thermal spraying has been used to produce coatings of an Fe–18.9%Cr–16.1%B–4.0%C–2.8%Si–2.4%Mo–1.9%Mn–1.7%W (in at.%) alloy from a commercially available powder (Nanosteel SHS7170). X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were employed to investigate the powder, as-sprayed coatings and annealed coatings which had been heated to temperatures in the range of 550–92502°C for times ranging from 60 to 390002min. Microhardness changes of the coatings were also measured as a function of annealing time and temperature. The powder was found to comprise amorphous and crystalline particles; the former had a maximum diameter of around 2202μm. The coating was composed of splat like regions, arising from rapid solidification of fully molten powder, and near-spherical regions from partially melted powder which had a largely retained its microstructure. The amorphous fraction of the coating was around 50% compared with 18% for the powder. The enthalpies and activation energies for crystallization of the amorphous phase were determined. Crystallization occurred in a two stage process leading to the formation of α-Fe (bcc), Fe 1.1 Cr 0.9 B 0.9 and M 23 C 6 phases. DSC measurements showed that the first stage occurred at 65002°C. Annealing the coating gave a hardening response which depended on temperature and time. The as-sprayed coating had a hardness of 9.202GPa and peak hardnesses of 12.5 and 11.802GPa were obtained at 650 and 75002°C, respectively. With longer annealing times hardness decreased rapidly from the peak.
Fe-based bulk metallic glasses exhibit very high hardness, elastic modulus/limit and wear/corrosion resistance. In the present investigation, an attempt has been made to develop an amorphous coating with Fe 48Cr 15Mo 14Y 2C 15B 6 bulk metallic glass on AISI 4140 substrate by laser surface processing. Following coating, the microstructure and phase aggregate were analyzed by scanning electron microscope and X-ray diffraction, respectively. Microhardness and wear resistance were assessed using Vickers microhardness tester and ball-on-plate wear testing machine, respectively. The coating thickness varied directly with incident laser power and interaction time. Despite trials with wide range of process parameters, the present experiments were unable to retain complete amorphous surface microstructure after laser surface coating. Numerical prediction of the thermal profile and related parameters suggest that the cooling rate and thermal gradient experienced by the coated zone were fairly high. Yet failure to retain amorphous/glassy microstructure of an otherwise bulk metallic glassy alloy suggests that compositional changes (solute redistribution) within the coated zone and across the coating ubstrate interface are responsible for nucleation and growth of crystalline phases from the melt. However, correlation between coating parameters and surface microstructure and properties allowed determination of the optimum conditions that ensured fine grained uniform microstructure with a significant improvement in hardness and wear resistance.
A 3D thermomechanical finite element model including the effect of the powder-to-solid transition has been developed to investigate the transient temperature, transient stresses, residual stresses and warpage of the component made of multiple materials produced using a laser-assisted layer-by-layer fabrication approach. The model encompasses the effects of the temperature- and porosity-dependent thermal conduction and radiation as well as the temperature-dependent natural convection. It is found that changing from the initial solid elements to powder elements results in higher temperature gradients, larger transient and residual stresses, and increased warpage. Mechanisms responsible for these phenomena are discussed based on heat transfer capabilities and constraints resulting from different initial materials.
Influences of casting temperature on the thermal stability of Zr-based metallic glasses (MGs) were analyzed based on continuous heating transformation (CHT) diagrams built up by Kissinger analysis. The results indicate that increasing the casting temperature can enhance the thermal stability of MGs, which attributed to the dissipation of pre-existing metastable local-ordering clusters in the melt resulted from the elevating casting temperature.
A structural model is described for metallic glasses based on a new sphere packing scheme the efficient filling of space by solute-centered clusters. This model combines random positioning of solvent atoms with atomic order of solutes. It shows that metallic glasses contain ?4 topologically distinct species and that solutes possess specific sizes relative to solvent atoms to produce efficient atomic packing. Validation is achieved by quantitative predictions of nearest-neighbor partial coordination numbers, medium-range solute ordering, density and metallic glass topologies. Good agreement is achieved in each of these areas. This model is able to reproduce compositions for a broad range of metallic glasses, provides specific guidance for the exploration of new bulk metallic glasses and may give new insights into other metallic glass studies. The new scheme introduced here for the efficient filling of space in extended systems of unequal spheres may have relevance to other fields.
The microstructures of Fe78Si9B13 ingots including the primary master alloy (PMA) and secondary melted alloy (SMA), as well as the precipitates in the annealed amorphous ribbons spun from PMA and SMA ingots have been investigated. The presented data show that SMA ingot performances a better glass forming ability (GFA) than PMA ingot, indicating that remelting treatment can improve the GFA of Fe78Si9B13 alloy. The metastable Fe23B6 phase forms in SMA ribbons after annealing at 600°C and 700°C. The lattice parameter a0 of α-Fe in the annealed SMA ribbons is higher than that in the annealed PMA ribbons, similar to the higher a0 of α-Fe in SMA ingot than that in PMA ingot. The variation of lattice parameter a0 of α-Fe in the present ingots and annealed ribbons shows a heredity phenomenon, which is closely associated with the Fe23B6 structural units (distorted tri-capped trigonal prism clusters) in the melts.
The theory of dendritic growth into undercooled alloy melts is extended to the case of large undercoolings, i.e. to high growth rates. This is done by applying the results of the complete stability analysis of a plane interface to the tip of an Ivantsov dendrite. For small Péclet numbers this model corresponds to a model published previously. For large Péclet numbers i.e. large undercoolings, however, the stability parameters become functions of Péclet numbers and cause drastic changes in the growth behaviour of the dendrite. Furthermore the limit of absolute stability is predicted when the undercooling is equal to the sum of the thermal unit undercooling and the equilibrium freezing range of the alloy.
Das Modell von Jackson-Hunt über eutektisches Wachstum bei kleinen Unterkühlungen wurde verallgemeinert, sodass es sich auch bei den bei rascher Erstarrung typischen grossen Unterkühlungen anwenden l01sst. Die Parameter λ 2 V und λΔT sind bei hohen Geschwindigkeiten nicht mehr konstant. Diese 02nderung wird von der Art des metastabilen Phasendiagramms beeinflusst. Es wird eine Grenzgeschwindigkeit vorausgesagt, bis zu der regul01res gekoppeltes eutektisches Wachstum stattfinden kann. Die Ursache für diese Grenzgeschwindigkeit ist entweder im temperaturabh01ngigen Diffusionskoeffizienten oder in der maximalen Unterkühlung zu finden.
The glass transition in metallic alloy systems can be modelled thermodynamically, using the Calphad approach, as a second-order transition from the supercooled liquid phase, giving good predictability for glass transition temperatures and the thermodynamic stability of the amorphous phase in intermetallic alloy systems. The resultant thermodynamic database can also be used for calculating crystallisation temperatures for the glass devitrification, and the Calphad approach is a powerful tool for designing bulk metallic glass alloys.
The crystallization ofZr41Ti14Cu12Ni10Be23(Vit 1) melts during constant heating is investigated. (Vit 1) melts are cooled with different rates into the amorphous state and the crystallization temperature upon subsequent heating is studied. In addition, Vit 1 melts are cooled using a constant rate to different temperatures and subsequently heated from this temperature with a constant rate. We investigate the influence of the temperature to which the melt was cooled on the crystallization temperature measured upon heating. In both cases the onset temperature of crystallization shows strong history dependence. This can be explained by an accumulating process during cooling and heating. An attempt is made to consider this process in a simple model by steady state nucleation and subsequent growth of the nuclei which results in different crystallization kinetics during cooling or heating. Calculations show qualitative agreement with the experimental results. However, calculated and experimental results differ quantitatively. This difference can be explained by a decomposition process leading to a nonsteady nucleation rate which continuously increases with decreasing temperature.
The present study focuses on synthesizing composite coatings for corrosion resistance using laser surface alloying (LSA). Amorphous powder with nominal composition (Fe48Cr15Mo14Y2C15B6) is used as the precursor powder on AISI 4130 steel substrate and processed with a continuous wave ytterbium Nd-YAG fiber laser. A multi-physics based heat transfer model was developed to evaluate the thermal histories experienced during processing. The thermodynamic parameters like peak temperatures and cooling rates are evaluated using the computational model and correlated to the evolution of microstructure. Phase and microstructural characterization of the coatings was conducted using XRD, SEM and TEM. Anodic polarization tests conducted in HCl medium indicated the enhancement in corrosion resistance of the laser processed samples. The laser processed samples showed better corrosion resistance than the substrate and among the processed samples, the corrosion resistance decreased with increasing laser energy density. The reduction in the corrosion resistance can be attributed to the formation of Cr23C6 nano crystals in the amorphous phase. The operating corrosion mechanisms are discussed with the aid of the thermal modeling results.