Quenched-in Nuclei Research Articles

Evolution of an industrial-grade Zr-based bulk metallic glass during multiple laser beam melting

Selective laser melting (SLM), taking advantage of its inherent rapid cooling rates and near-net-shape forming ability, has been employed to fabricate bulk metallic glasses (BMGs). However, crystallization is frequently triggered during the SLM process, which results in the loss of advantageous properties of BMGs, such as extremely high hardness and near-theoretical yield strength. Although many studies have been conducted to investigate SLM of BMGs, there is still a lack of knowledge about the microstructural and compositional evolution during the laser beam processing, particularly the micromechanical property response upon crystallization.In the present work, a systematic investigation is performed to gain a much better understanding about the evolution of microstructure and composition as well as the corresponding micromechanical property change during multiple laser beam melting. The material used in this study is an industrial-grade Zr-based BMG Zr59.3Cu28.8Al10.4Nb1.5 (AMZ4) with two different oxygen levels. AMZ4 demonstrates its good thermal stability by the fact that observable crystalline structure appears around the melt pool only after more than once laser beam treatment. The compositional stability of AMZ4 is manifested by the homogeneous elemental distribution on the melt pool area after even twenty-five laser beam remelting. The laser-metal interaction, melting and subsequent solidification are not effectively influenced by the emerging and expanding of crystallization zone (or heat affected zone, HAZ). Higher oxygen content results in not only a larger HAZ but also more quenched-in nuclei at the melt pool bottom. The HAZ does not exhibit a fully crystallized structure, but rather has a mixture of amorphous and crystalline phases. Crystallization of AMZ4 leads to an increase in hardness and Young’s modulus of the material.

Structural and mechanical properties of Hf55Cu28Ni5Al12bulk metallic glass alloy prepared at different cooling rates

• In this report, the effect of cooling rate in Hf 55 Cu 28 Ni 5 Al 12 alloy prepared by melt spinning and suction casting methods has been studied. • Decreasing cooling rate results in formation of Hf 2 Cu quenched-in nuclei in amorphous matrix in 3 mm cast rod. • Formation of Hf 2 Cu and Al 16 Hf 6 Ni 7 in amorphous matrix is observed by varying the cooling rate in 5 mm cast rod. • Presence of quenched in nuclei in 3 mm cast rod was confirmed through by kinetic calculations using a model considering pre-existing nuclei. • Hardness of 3 mm cast rod was estimated using modified spherical cavity model which was found to be close to experimentally observed value. • Fracture toughness of 3 and 5 mm cast rods was estimated based on vein length and crack length created during indentation. • 5 mm rod showed lower fracture toughness than 3 mm cast rod due to the presence of the secondary phases.

Kinetics of structure formation in the vicinity of the glass transition

The structural stability of a gold-based bulk metallic glass (Au49Ag5.5Pd2.3Cu26.9Si16.3) was investigated by fast differential scanning calorimetry (FDSC). A chemically homogeneous glass (CHG) containing no quenched-in nuclei was produced using sufficiently fast cooling from the melt. To characterize the stability of this glass, the structural variations during annealing in the glassy state were investigated and a sequence of five different events observed. These are structural relaxation, an incubation period, nucleation, crystallization, and finally reorganization. The kinetics of these processes and the stability of the crystalline structures are discussed.

Crystallization mechanism of Zr55Cu30Al10Ni5 metallic glass in an extended range of heating rates

The crystallization mechanisms of Zr55Cu30Al10Ni5 (Zr55) amorphous alloys from slow continuous heating to rapid laser heating are investigated. The multiphase nanocrystalline structures such as CuZr2-type, NiZr2-type and Al2Zr3-type phases with various compositions are observed at heating rates lower than 50 K/s, while the CuZr2-type/ZrCu-type eutectic dendrites are produced at heating rates higher than 50 K/s. A numerical model is established to calculate the onset crystallization temperatures as a function of the heating rates. This model indicates that in addition to rapid dendritic growth from a certain number of quenched-in nuclei, the non-Arrhenius behavior of high diffusion coefficient in the supercooled liquid region contribute to the serious crystallization during rapid heating process. The activation energy of dendritic crystallization is obviously smaller than that of multiphase nanocrystallization, which is due to a much smaller scale of diffusion distance and larger atomic diffusivity during dendritic growth.

Critical crystallization properties of an industrial-grade Zr-based metallic glass used in additive manufacturing

The major challenge to overcome when processing metallic glasses (MGs) is to avoid crystallization. Therefore, time-temperature-transformation (TTT) diagrams are used, but they are often derived from experiments in which heating or cooling rates are limited and do not cover the full range of processing conditions, especially those encountered in additive manufacturing (AM) where the rates are very high. Here, an industrial-grade Zr-based MG (AMZ4) is investigated via fast differential scanning calorimetry (FDSC). The critical cooling and heating rates of AMZ4 are experimentally measured and TTT diagrams are determined upon heating and cooling. The critical heating rate of 45,000 K/s is 18 times the critical cooling rate, which indicates the presence of a self-doped glass (SDG) that includes quenched-in nuclei. The results illustrate that AMZ4 is very sensitive to crystallization, even in laser-based AM conditions, where heating and cooling steps need to be distinguished.

Laser additive manufacturing of metallic glasses: issues in vitrification and mechanical properties

Abstract Bulk metallic glasses (BMGs), on account of their attractive properties, have now begun to witness a few commercial applications, e.g. in coatings and micro-gears. Additive manufacturing (AM) or 3D printing, although established for crystalline alloys, has only recently been used for synthesizing BMG components. The issues arising in 3D printing of BMGs are of current relevance, and this review focuses on the key scientific aspects, namely vitrification (or crystallization) during printing, mechanical properties of printed glassy alloys and the use of AM in identifying newer BMGs. Available data on crystallization during printing of a variety of BMGs are analysed in terms of schematic time–temperature–transformation diagrams and the complex interplay between thermal cycles, the presence of quenched-in nuclei in the glass and oxygen contamination in a way that is hoped to be broadly applicable to most alloy systems. Also reviewed are three key factors influencing mechanical properties of printed BMGs, i.e. porosity, crystallinity and oxygen contamination and thereby potential strategies for improvement are suggested. The review concludes with a discussion on the use of AM for combinatorial alloy development aimed at identifying better glass-forming compositions, which may in turn facilitate greater use of AM in manufacturing glassy components with desired properties.

Fingerprints of homogeneous nucleation and crystal growth in polyamide 66 as studied by combined infrared spectroscopy and fast scanning chip calorimetry

Homogenous crystal nucleation and growth in polyamide 66 (PA66) are followed in situ by means of a combination of FTIR spectroscopy and fast scanning chip calorimetry (FSC). Therefore, a novel setup with a calorimetry chip equipped with an IR-transparent SiN membrane was developed, which enables to examine IR spectroscopic and FSC experiments on the identical specimen. Because of the small amount of sample material (~ 100 ng), it is possible to achieve heating and cooling rates up to 5000 Ks−1, and hence to quench the sample into a fully amorphous state without quenched-in homogeneous crystal nuclei. Annealing the film then allows to determine the onset of homogenous nucleation and crystal growth by means of FSC, whereas molecular interactions are unraveled by FTIR spectroscopy. It is demonstrated that different moieties of PA66 respond distinctly during crystallization; far-reaching interactions such as hydrogen bonding are established prior to onset of short-range steric hindrance.Graphical Crystal nucleation and growth are followed in situ by means of a combination of FTIR spectroscopy and fast scanning chip calorimetry. For this purpose a novel IR-transparent chip enables to examine both measurements on the identical specimen, while heating and cooling rates up to 5000 Ks−1 and hence quenching into a fully amorphous state can be achieved

Nanostructural formation kinetics in an Fe86B14 soft magnetic alloy investigated by in situ transport measurements under isothermal conditions

The soft magnetic properties in iron-based nanocrystalline alloys have generally been achieved through the addition of nonmagnetic additives, but this has the unwanted effect of reducing the saturation magnetization. In a recent advance, it was shown that magnetically soft nanostructured alloys could be prepared from a simple, binary Fe-B amorphous precursor by ultrarapid annealing (URA). These materials had a saturation magnetization comparable to that of silicon steel, while a refined grain structure resulted in considerably lower core losses through the exchange averaging effect of the magnetic anisotropy. As yet, the formation mechanism of the soft magnetic nanostructure in this simple binary system has not been identified. We investigated the crystallization process of amorphous ${\mathrm{Fe}}_{86}{\mathrm{B}}_{14}$ using in situ transport measurements. The relationship between the resistivity and the volume fraction of bcc-Fe crystallites was calibrated by $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy. In this paper, we show that the isothermal crystallization behavior of these Fe-B amorphous alloys is well described by the Johnson-Mehl-Avrami-Kolmogorov kinetic model and that the the Avrami exponent ($n$) shows a tendency to increase with increasing annealing temperature (${T}_{\mathrm{a}}$); the exponent $n$ is approximately 1.5 at ${T}_{\mathrm{a}}<700\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ while $n>2$ at ${T}_{\mathrm{a}}=729\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. This suggests that below 700 K the nucleation rate is governed solely by a limited number of quenched-in nuclei whereas at higher temperatures, homogeneous nucleation starts to dominate. Since the onset of crystallization is increased dramatically by rapid heating, the formation of the refined nanostructure induced by URA can be well understood by an increase in the number density of nuclei due to homogeneous nucleation. The increased homogeneous nucleation at higher annealing temperatures can be linked to enhanced atomic transport due to viscous flow.

Existence of multiple critical cooling rates which generate different types of monolithic metallic glass

Via fast differential scanning calorimetry using an Au-based glass as an example, we show that metallic glasses should be classified into two types of amorphous/monolithic glass. The first type, termed self-doped glass (SDG), forms quenched-in nuclei or nucleation precursors upon cooling, whereas in the so-called chemically homogeneous glass (CHG) no quenched-in structures are found. For the Au-based glass investigated, the critical cooling and heating rates for the SDG are 500 K s−1 and 20,000 K s−1, respectively; for the CHG they are 4000 K s−1 and 6000 K s−1. The similarity in the critical rates for CHG, so far not reported in literature, and CHG’s tendency towards stochastic nucleation underline the novelty of this glass state. Identifying different types of metallic glass, as is possible by advanced chip calorimetry, and comparing them with molecular and polymeric systems may help to elaborate a more generalized glass theory and improve metallic glass processing.

Effect of quenching temperature on local ordering in Al85Ni5Co2Y8 metallic glass

The effect of quenching temperature on local atomic structure of Al85Ni5Co2Y8 metallic glass has been investigated in the present study. With increasing quenching temperature from 1273 K to 1573 K, the amount of medium range order (MRO) tends to decrease, while the structural unit size remains almost constant. The higher amount of MRO leads to movement of the crystallization nose of time-temperature-transformation (TTT) curve to a longer time scale by raising the viscosity of the liquid. On the other hand, quenched-in nuclei are present in the samples with higher quenching temperature, since the probability of intersecting the crystallization nose during cooling becomes higher with increasing quenching temperature. As a result, typical two types of as-quenched microstructure can be obtained, i.e. fully amorphous structure with higher amount of MRO and that with embedded quenched-in nuclei as well as relatively lower amount of MRO. The sample quenched at 1273 K without quenched-in nuclei exhibits the most fragile behavior, indicating that the fragility can be affected by the quenching temperature as well as the alloy composition in Al-based metallic glass.

Modeling heterogeneous melting in phase change memory devices

We present thermodynamic crystallization and melting models and calculate phase change velocities in Ge2Sb2Te5 based on kinetic and thermodynamic parameters with a focus on the impacts of grain boundary melting. The calculated phase change velocities are strong functions of grain size, with smaller grains beginning to melt at lower temperatures. Phase change velocities are continuous functions of temperature which determine crystallization and melting rates. Hence, set and reset times as well as power and peak current requirements for switching are strong functions of grain size. Grain boundary amorphization can lead to a sufficient increase in cell resistance for small-grain phase change materials even if the whole active region does not completely amorphize. Isolated grains left in the amorphous regions, the quenched-in nuclei, facilitate templated crystal growth and significantly reduce set times for phase change memory cells. We demonstrate the significance of heterogeneous melting through 2-D electrothermal simulations coupled with a dynamic material phase change model. Our results show reset and set times on the order of ∼1 ns for 30 nm wide confined nanocrystalline (7.5 nm–25 nm radius crystals) phase change memory cells.

Al-based amorphous alloys: Glass-forming ability, crystallization behavior and effects of minor alloying additions

Abstract The main recent advances in Al-based amorphous alloys are discussed in three areas: crystallization behavior, glass-forming ability and minor alloying effects. First, as a brief introduction, the special features and structure-property relationships are presented for the Al-based amorphous alloys. The different models for the nucleation behaviors, such as phase separation model and the quenched-in nuclei model, are compared and evaluated. In addition, an overview is given for the effects of minor alloying additions on glass-forming ability (GFA) and the crystallization behavior in terms of the reported structural, thermodynamic, kinetic factors.

Modification of the Ti40Cu36Zr10Pd14 BMG Crystallization Mechanism with Heating Rates 10-140 K/min

The article presents investigations of Ti40Cu36Zr10Pd14 bulk metallic glass crystallization process heated with the rates of 10, 60, 100 and 140 K/min. High heating rates experiments were performed in a new type of differential scanning calorimeter equipped with a fast responding thermal sensor. Phase composition and microstructure were studied with x-ray diffraction and transmission electron microscopy. The observed crystallization proceeded in two separate steps. Applied high rates of heating/cooling resulted in the crystallization of only one CuTi phase, replacing typical multi-phase crystallization. The microstructure after crystallization was polycrystalline with some amount of amorphous phase retained. Kinetic parameters were determined with the use of the Kissinger and Friedman iso-conversional analysis and Matusita–Sakka iso-kinetic model. The kinetic analysis supplies results concerning autocatalytically activated mechanism of primary crystallization with decreasing activation energy and small density of quenched-in nuclei, in good agreement with previous structural investigations. The mechanism of secondary crystallization required dense nuclei site, increasing activation energy and large nucleation frequency. The amorphous phase of Ti40Cu36Zr10Pd14 BMG revealed high thermal stability against crystallization. Application of high heating rates in DSC experiments might be useful for the determination of mechanism and kinetic parameters in investigations of metallic glasses crystallization, giving reasonable results.

Crystal growth limitation as a critical factor for formation of Fe-based bulk metallic glasses

In the present work we study the formation mechanism and crystallization behavior of Fe-based bulk metallic glasses influenced by the addition of rare-earth elements. Samples are characterized by conventional X-ray diffractometry, optical microscopy, and high-resolution field-emission-gun scanning and transmission electron microscopy. In the rare-earth-containing alloys, bulk metallic glasses are formed with populations of quenched-in nuclei. Consequently, crystallization on isothermal annealing occurs without any incubation period. This behavior, not typical of bulk metallic glasses in general, implies that in the rare-earth-containing alloys glass formation is possible only because of restricted crystal growth: both on cooling from the molten and on heating from the glassy state, growth of the primary χ-Fe36Cr12Mo10 crystals is too slow to be significant on a reasonable timescale. The low growth rate is connected with large inhomogeneous strain in the growing nanoparticles, while nucleation of eutectic colonies is hampered by slow diffusion of a rare-earth alloying element.

Nanoscale nuclei in phase change materials: Origin of different crystallization mechanisms of Ge2Sb2Te5 and AgInSbTe

Phase change memory devices are based on the rapid and reversible amorphous-to-crystalline transformations of phase change materials, such as Ge2Sb2Te5 and AgInSbTe. Since the maximum switching speed of these devices is typically limited by crystallization speed, understanding the crystallization process is of crucial importance. While Ge2Sb2Te5 and AgInSbTe show very different crystallization mechanisms from their melt-quenched states, the nanostructural origin of this difference has not been clearly demonstrated. Here, we show that an amorphous state includes different sizes and number of nanoscale nuclei, after thermal treatment such as melt-quenching or furnace annealing is performed. We employ fluctuation transmission electron microscopy to detect nanoscale nuclei embedded in amorphous materials, and use a pump-probe laser technique and atomic force microscopy to study the kinetics of nucleation and growth. We confirm that melt-quenched amorphous Ge2Sb2Te5 includes considerably larger and more quenched-in nuclei than its as-deposited state, while melt-quenched AgInSbTe does not, and explain this contrast by the different ratio between quenching time and nucleation time in these materials. In addition to providing insights to the crystallization process in these technologically important devices, this study presents experimental illustrations of temperature-dependence of nucleation rate and growth speed, which was predicted by theory of phase transformation but rarely demonstrated.

Evaluating the non-isothermal crystallization behavior of organic molecules from the undercooled melt state using rapid heat/cool calorimetry

Understanding the inherent crystallization tendency of organic molecules from the undercooled melt state is an important criterion to assess the glass-forming ability of a compound, and hence the ability to produce and maintain amorphous materials, an important area for drug delivery of poorly water-soluble therapeutics. The main objective of this study was to determine the critical cooling (Rcrit,cool) and/or critical heating (Rcrit,heat) rates to inhibit crystallization for a group of compounds with diverse chemistries and different crystallization tendencies. Compounds were cooled at various cooling/heating rates using a prototype rapid heat/cool (RHC) calorimeter, and crystallization (or lack thereof) from the undercooled melt was recorded. Results showed some compounds have very high crystallization tendencies [class (I–A)] from the undercooled melt state, with Rcrit,cool rates above 1000 °C min−1, while some compounds with low crystallization tendencies upon cooling [class (II) compounds] exhibited very high crystallization tendencies (Rcrit,heat > 1000 °C min−1) upon subsequent reheating from the amorphous state. The pronounced asymmetry observed between Rcrit,cool and Rcrit,heat rates for class (II) compounds was greatly reduced by rapidly quenching the compound from the melt state, preventing the formation of quenched-in nuclei during cooling and initiating crystallization upon reheating. The ability to apply controlled rapid heating and cooling rates to organic compounds thus provides useful insight into the crystallization tendency of organic compounds.

Spherulitic crystallization behavior of a metallic glass at high heating rates

Understanding crystallization in metallic glasses is an important consideration in the development of processing and manufacturing techniques which will utilize these unique alloys to full advantage. In the present work, the crystalline morphology and crystallization mechanisms of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass are examined at heating rates spanning 4 orders of magnitude. In contrast to the multi-phase nanocrystalline structure formed at low heating rates, high heating rates (>2.5 K/s) result in the formation of a non-equilibrium crystalline phase with spherulitic microstructure and uniform composition. Heating rates as fast as 103 K/s are found to be insufficient to avoid the formation of spherulites. The microstructure suggests that rapid heating suppresses phase separation and nucleation processes at the initial stage of crystallization, resulting in a growth-dominated crystallization behavior. The activation energy for spherulite formation is found to be less than that for nanocrystallization, which is attributed to the lack of phase separation and nucleation steps. Calculation of the Avrami exponent also indicates that spherulites form via the growth of quenched-in nuclei. The number density of quenched-in nuclei is estimated to be 1014 m−3.

Complex crystallization mode of amorphous/nanocrystalline composite Al 86Ni 2Co 5.8Gd 5.7Si 0.5

Kinetics of the first crystallization stage of Al 86Ni 2Co 5.8Gd 5.7Si 0.5 amorphous alloy and structure of the partially crystallized specimens were investigated by measurements of electrical resistance, X-ray diffraction and transmission electron microscopy. The presence of Al nanocrystals and eutectic colonies consisted of mutually oriented crystals of Al and metastable phase was found. The transient nucleation and slowing-down diffusion-limited growth and the interface-controlled growth of the quenched-in nuclei were identified as mechanisms of formation of the Al nanocrystals and eutectic colonies, respectively.

Anomalous glass transition behavior in Cu–Zr–Sn alloy system

We discuss the effect of Sn addition on anomalous glass transition behavior in Cu–Zr bulk-forming metallic glasses. We found that an unusual endothermic reaction in Cu 55Zr 40Sn 5 ribbon can originate from the growth reaction of quenched-in nuclei from 0.4 nm to 3.7 nm in the supercooled liquid region. This anomalous devitrification may prove useful for the synthesis of a novel composite with uniform atomic/nanometer scale heterogeneity modulated by controlling cooling rate as well as by tailoring alloy composition.

Nanocrystallization-induced large room-temperature compressive plastic strain of Ti 40Zr 25Ni 8Cu 9Be 18 BMG

By combining neutron diffraction, high-resolution transmission electron microscopy and thermal analysis measurements, the microstructure of as-cast Ti 40Zr 25Ni 8Cu 9Be 18 BMG with diameter of 2 mm was identified to be quenched-in nuclei/amorphous matrix. The quenched-in nuclei transform to (Ti,Zr)Be 2 nanocrystals with the average size of 4 nm homogeneously distributed into the amorphous matrix during deformation, which could be responsible for the large room-temperature compressive plastic strain, up to 8.0% at a constant strain rate of 5 × 10 −4 s −1.