Stages Of Devitrification Research Articles

In Situ Mapping of Phase Evolutions in Rapidly Heated Zr-Based Bulk Metallic Glass with Oxygen Impurities.

Metallic glasses exhibit unique mechanical properties. For metallic glass composites (MGC), composed of dispersed nanocrystalline phases in an amorphous matrix, these properties can be enhanced or deteriorated depending on the volume fraction and size distribution of the crystalline phases. Understanding the evolution of crystalline phases during devitrification of bulk metallic glasses upon heating is key to realizing the production of these composites. Here, results are presented from a combination of in situ small- and wide-angle X-ray scattering (SAXS and WAXS) measurements during heating of Zr-based metallic glass samples at rates ranging from 102 to 104 Ks-1 with a time resolution of 4ms. By combining a detailed analysis of scattering experiments with numerical simulations, for the first time, it is shown how the amount of oxygen impurities in the samples influences the early stages of devitrification and changes the dominant nucleation mechanism from homogeneous to heterogeneous. During melting, the oxygen rich phase becomes the dominant crystalline phase whereas the main phases dissolve. The approach used in this study is well suited for investigation of rapid phase evolution during devitrification, which is important for the development of MGC.

Impact of stress induced anisotropic modifications on magneto-impedance behaviour of rapidly quenched CoFe based microwires

The investigation addresses the role of Al in devitrification stage of a series of (Co94Fe6)72.5Si12.5−xAlxB13Cr2 (X = 0, 2, 4, 6 at%) rapidly quenched microwires. The incorporation of Al displayed reduction in primary onset and activation energy (A.E) while it facilitated widening of span between primary and secondary crystallization onsets. The Giant Magneto-Impedance (GMI) behaviour was influenced by Al concentration as well as applied uniaxial stress. Optimal Al containing alloy (x ∼ 2 at%) displayed signature of asymmetric dual peak at the operating frequency of 1 MHz and its prospective application as a very low magnetic field sensor in pre-stressed condition. However, with increasing stress, the asymmetric character transformed to symmetric dual peak which is attributed to the transition of transverse anisotropy to circular anisotropy via helical anisotropy. High stress sensitivity has been attributed to either dominant transverse or circular anisotropy. The transition of stress induced anisotropies in the microwires has been portrayed schematically to elucidate the transitions.

Structure of Amorphous Phase and Nanocrystalline Phases in the Devitrification of Iron Alloys

The Fe–B–Si–P–C alloys with crystalline phases of different type that are deposited in the initial stages of devitrification are studied by means of X-ray structural analysis. The alloy structure is modeled by an ensemble of small incoherent particles consisting of tetrahedral close-packed (tcp) crystalline phases. The calculation results and experimental data are in good agreement for all types of tcp phases with small particle size. However, X-ray analysis cannot distinguish between the amorphous state and heterogeneous structure with deposits of one of the tcp phases in the early stages of solidification. Such nonuniform structure may change the properties of the alloy, although the diffuse-scattering pattern remains unchanged.

A comparative study of Sm networks in Al-10 at.%Sm glass and associated crystalline phases

The Al–Sm system is selected as a model system to study the transition process from liquid and amorphous to crystalline states. In recent work, we have shown that, in addition to long-range translational periodicity, crystal structures display well-defined short-range local atomic packing motifs that transcends liquid, amorphous and crystalline states. In this paper, we investigate the longer range spatial packing of these short-range motifs by studying the interconnections of Sm–Sm networks in different amorphous and crystalline samples obtained from molecular dynamics simulations. In our analysis, we concentrate on Sm–Sm distances in the range ~5.0–7.2 Å, corresponding to Sm atoms in the second and third shells of Sm-centred clusters. We discover a number of empirical rules characterising the evolution of Sm networks from the liquid and amorphous states to associated metastable crystalline phases experimentally observed in the initial stages of devitrification of different amorphous samples. As direct simulation of glass formation is difficult because of the vast difference between experimental quench rates and what is achievable on the computer, we hope these rules will be helpful in building a better picture of structural evolution during glass formation as well as a more detailed description of phase selection and growth during devitrification.

Relations between thermal history and secondary structures of ignimbrites exclusive of rheomorphism

Newly calculated model density profiles of ignimbrites also provide model thermal histories, which serve as a framework to help analyze the development of cooling joints, devitrification textures, and lithophysal cavities. Only those parts of sections of Rattlesnake Tuff in central Oregon that resided >600 °C for at least two years show devitrification. Low tensile strength means that joints form by thermoelastic contraction after cooling by as little as 25 °C. This means that columnar joints formed in as little time as a few weeks after deposition of the Aravaipa Tuff in SE Arizona. Compaction is rapid as well, so both columnar jointing and compaction are complete before the onset of devitrification in deposits <40 m thick. A section of Rattlesnake Tuff shows stratabound occurrences of devitrified spots and cavities; devitrification appears to have begun at scattered spots. Most spots are bounded by crescentic cavities in formerly ductile shards. The equation for conductive cooling of a spherical heat source shows that for rock volumes larger than ∼15 cm diameter, the rate of cooling is insufficient to prevent heating by latent heat by as much as 12 °C. Inflation therefore appears to have been caused by vapor released by devitrification and its slight adiabatic expansion. Eventual wholesale devitrification of matrix resulted in more-devitrified spots scattered in less devitrified matrix. Lithophysal cavities in the Rattlesnake Tuff and in the Peach Springs Tuff in NW Arizona are more abundant in lower density horizons, showing that cavity growth is favored in permeable zones between impermeable horizons. The Peach Springs Tuff and other deposits described in the literature have horizontal joints that formed during cooling and whose origin has yet to be deterministically modeled. The longest joints are most closely spaced in the zone of cavities and formed after columnar joints. It is inferred that after formation, each vertical column responds independently to evolving stresses; at this time, asperities together with gas pressures accompanying cavity formation result in horizontal joints. Columnar joints do not completely relieve growing gas pressures during devitrification, likely because of sealing by wholesale inflation of the rock mass and by mineral deposition. Some devitrified horizons of Rattlesnake Tuff and Peach Springs Tuff are densely fractured, resulting in a rubbly surface. These small fractures show zigzag traces, bifurcations, abrupt terminations, and pinch-and-swell walls, similar to ductile fractures described by [Eichhubl (2004)][1]. Their origin is interpreted to be the result of tensile stress due to porosity increase during later stages of devitrification. Closely adjacent sections of Rattlesnake Tuff at one site differ by 18% in thickness, reflecting buried paleotopography. Each section is devitrified. The thicker profile has a zone of lithophysal cavities near its base; the thinner profile has few cavities. The greater thickness translates to only 0.18 MPa additional lithostatic pressure at the base of the thicker profile, which serves to emphasize the subtle differences in initial conditions that can lead to formation of cavities. [1]: #ref-17

Comparison of the Crystallization Behavior of Fe-Si-B-Cu and Fe-Si-B-Cu-Nb-Based Amorphous Soft Magnetic Alloys

The role of the solute elements, copper, and niobium, on the different stages of de-vitrification or crystallization of two amorphous soft magnetic alloys, Fe73.5Si13.5B9Nb3Cu1, also referred to as FINEMET, and a Fe76.5Si13.5B9Cu1 alloy, a model composition without Nb, has been investigated in detail by coupling atom probe tomography and transmission electron microscopy. The effects of copper clustering and niobium pile-up at the propagating interface between the α-Fe3Si nanocrystals and the amorphous matrix, on the nucleation and growth kinetics have been addressed. The results demonstrate that while Cu clustering takes place in both alloys in the early stages, the added presence of Nb in FINEMET severely restricts the diffusivity of solute elements such as Cu, Si, and B. Therefore, the kinetics of solute partitioning and mobility of the nanocrystal/amorphous matrix interface is substantially slower in FINEMET as compared to the Fe76.5Si13.5B9Cu1 alloy. Consequently, the presence of Nb limits the growth rate of the α-Fe3Si nanocrystals in FINEMET and results in the activation of a larger no. of nucleation sites, leading to a substantially more refined microstructure as compared to the Fe76.5Si13.5B9Cu1 alloy.

Occurrence of well-crystallized Nontronite in the Gas Vesicles of a Megaporphyritic Basalt Flow near Nighoj, Ahmednagar district, Maharashtra

Abstract Occurrence of well crystallized nontronite in a megaporphyritic basalt flow from Nighoj area, Maharashtra, India is reported. Thin section studies of the vesicles from periphery to the centre reveal the presence of greenish palagonite, probably indicating the stages of devitrification of glass and the weathering of palagonite to the formation of nontronite. The centre of the vesicles is marked by the presence of dark greenish-brown, radiating and fibrous sheets of nontronite. X-ray diffraction (XRD) analysis reveals the dominant presence of smectite (nontronite) as the major phase with traces of pyroxene. Scanning Electron Microscopy (SEM) shows well developed sheets of nontronite within the gas vesicles of the megaporphyritic basalt flows. In the present investigation, the nontronite within the gas vesicles of the megaporphyritic basalts at Nighoj was formed in-situ due to the devitrification of glass containing palagonite. Palagonite is an intermediate step in the alteration that may be accomplished by the steam generated in quenching of the lava. Weathering of palagonite under conditions of poor drainage is essential to the genesis of nontronite as has been suggested by Victor and Vernon (1946).

Primary precipitation phase with β-Mn structure in FeSiBP base multicomponent metallic glasses

Abstract Rapidly quenched amorphous soft magnetic FeSiPB alloys were studied at the initial stages of devitrification by means of transmission electron microscopy and X-ray diffraction. Metastable π-phase with the β-Mn type structure is formed as a primary phase in alloys with 8–9 at.%B, 7–10%Si, 3–5%P. Transition from amorphous to π-phase proceeds via nearly polymorphous transformation, composition of the phase is close to that of amorphous matrix, it contains metalloid atoms at interstitial positions. Small alloying additions are prone to change the type of the primary phase to α-Mn type and related structures.

Metastable primary precipitation phases in multicomponent glass forming Fe-base alloys with metalloids

Structure transformations at the initial stages of devitrification have been investigated for alloys with good glass forming ability on the base of Fe–B–Si, Fe–P–C, Fe–P–B–C systems with various additions. Systematization of the metastable phases formed in these alloys showed that: (a) metastable phases at the initial stages of crystallization are tetrahedrally close packed (TCP) or aperiodic structures and possess compositions close to the mean composition of the alloy; (b) the observed TCP phases belong to one family of structures related to α-Mn for essentially different types of alloying systems, having in common a high glass forming ability; (с) for alloys based on the same system with different small alloying additions, structure of the metastable phase can change within this family of related phases. The obtained results show that Fe-based metallic glasses with metalloids are not essentially different from metal–metal type glasses.

Structure Transformations in Multicomponent Amorphous Alloys in Connection with Grain-Boundary Segregations and Temper Embrittlement of Steels

Abstract Investigation of glass-forming alloys on the base of Fe–P system with additions of C, Mn, Cr, Si, and Ni with compositions modeling segregations on grain boundaries in steels revealed the existence of a metastable phase that precipitates at the initial stages of devitrification. This crystalline tetrahedrally close-packed phase of α–Mn structure type (χ-phase) and its aperiodic modification (cubic quasicrystal) contain less phosphorus than the known stable and metastable phosphides. Positions of P and C atoms in the unit cell of the χ-phase lattice have been established. The necessary condition for precipitation of the quasicrystalline phase is the presence of carbon. Additions of Mn + Si, Mn, or Cr promote both formation of the χ-phase and formation of the quasicrystal, their influence decreasing in this order. Composition dependences of liability to formation of the quasicrystalline structures in the model alloys agree with the data on the impact of alloying elements on the temper embrittlement of steels.

Effect of notch toughness and hardness on sliding wear of Cu 50Hf 41.5Al 8.5 bulk metallic glass

Dry sliding wear experiments with a Cu50Hf41.5Al8.5 bulk metallic glass were conducted following different annealing times of the glass at 510 °C. For as-cast and structurally relaxed samples and for the initial devitrification stage, wear is hardness controlled and improves with annealing. Upon longer annealing at 510 °C, the decrease in toughness increasingly controls the wear behavior. For prolonged annealing times, wear deteriorates and is toughness controlled. The wear stages can be quantified based on the theory of brittle wear.

Sliding wear behavior of Cu50Hf41.5Al8.5 bulk metallic glass

Abstract Sliding wear behavior of a copper-based bulk metallic glass (Cu 50 Hf 41.5 Al 8.5 ) was investigated for both as-cast and annealed samples. The wear resistance increased during isothermal annealing near the glass transition temperature. Nanocrystals developed during the annealing for annealing times up to 300 min. A linear relation between hardness and wear resistance was observed during the early stages of devitrification, but at longer annealing times the wear resistance increased less than the hardness. For the as-cast sample and a sample that was structurally relaxed, nanocrystallites developed during sliding and the wear resistance improved for longer sliding distances when the nanocrystals developed. The transmission electron microscopy analysis of the annealed samples suggests that the deviation from the linear hardness vs. wear resistance relation occurs at a critical volume fraction of the nanocrystals. The improvement in wear resistance with sliding appears to result from an increased surface temperature rather than directly from the sliding-induced nanocrystal formation.

Electrochemical response of amorphous and devitrified Al‐Ni‐La‐X (X = Ag, Cu) alloys

AbstractThe electrochemical response of melt‐spun Al‐Ni‐La alloys with partial substitution of Ni after different stages of devitrification was studied. The base alloy was found to have the best corrosion resistance. It was observed that primary crystallization caused minimal deterioration in the corrosion resistance of the base alloy as compared to its amorphous state. The substitute alloys had different corrosion resistance dependent on the substituting element with the Ag containing alloy having the least resistance. This could be attributed to the operation of local galvanic cells, enhanced by chemical heterogeneities in the alloys. Secondary crystallization caused a reduction in the corrosion resistance of all the alloys due to the creation of intermetallic phases that increased the galvanic activity.

Correlation between microstructure and temperature dependence of magnetic properties in Fe60Co18(Nb,Zr)6B15Cu1 alloy series

Temperature dependence of magnetic properties of nanocrystalline Fe60Co18Cu1B15Nb6−xZrx (x=0, 3, 6) alloys has been studied at different stages of devitrification. Transmission electron microscopy shows nanocrystals of the size ∼5 nm, which remains almost constant along the nanocrystallization process. Curie temperature of the residual amorphous phase decreases as nanocrystallization progresses for all the studied alloys. Thermal dependence of the exchange stiffness constant is obtained from the measurement of specific magnetization and coercivity as a function of crystalline fraction and temperature for the three studied alloys.

Segregation-controlled nanocrystallization in an Al–Ni–La metallic glass

The early stages of devitrification of Al89Ni6La5 metallic glass were investigated using three-dimensional atom probe. Even in the as-quenched amorphous state, nanometer-sized fluctuations of Ni and Al were observed. The Al-rich fluctuations act as nucleation sites for the crystallization at 443K in which α-Al clusters with a number density up to 5×1023m−3 nucleate and grow to nonspherical, 12nm large crystals. α-Al particles are almost pure in Al and are surrounded by a La-rich shell. It is concluded that La segregation controls the growth and limits the size of the α-Al nanocrystals.

Magnetic permeability of (FeCoGe)88Zr6B5Cu1 alloys: Thermal stability in a wide temperature range

Temperature dependence, from room temperature up to 1000K, of the initial permeability (μ) of Fe83−xCoxGe5Zr6B5Cu1 (x=5 and 20) alloys at different stages of devitrification is reported. As nanocrystallization progresses, room temperature μ decreases but high temperature one increases, leading to an improvement of its thermal stability extended from room temperature up to 915K, characterized by a temperature coefficient of permeability ∼0.1%∕K (μ∼4000) and 0.04%/K (μ∼800) for 5 and 20at.% Co containing alloys, respectively.

Quasiperiodic and Frank-Kasper phases formation during primary crystallization of Fe-base amorphous alloys

Initial stages of devitrification were investigated for iron based amorphous alloys: Fe-P-C-(Cr, Mn, Mo); Fe-B-Si-Nb with 67 - 80 at.% of Fe and 15-25 at.% of P+B+Si+C. Primary precipitated phases of α-Mn or related tetrahedrally close packed structure types, in particular aperiodic structures, were observed. They are characterized by fast nucleation and formation of nanostructures. In some cases specific precrystallization state is formed for which diffraction patterns are consistent with the existence of nano-clusters with local TCP structure. Structure of the liquid or amorphous state can be presented as composition of the same clusters but with the different, noncrystallographic long range order.

Early Stages of Crystallization in Phase-Separated Amorphous Copper-Niobium Alloy Thin Films

The binary Cu-Nb system is rather interesting, because Cu and Nb are virtually immiscible in the solid state, exhibit a relatively strong clustering (phase separating) tendency in the liquid state, and therefore can only be alloyed in a highly metastable form, for example, by vapor quenching techniques such as sputtering. In this article, metastable amorphous Cu-Nb alloy thin films with nominal compositions of Cu-45 at. pct Nb have been deposited by magnetron cosputtering. The amorphous phase in these thin films exhibits nanoscale phase separation in the as-deposited condition. The early stages of devitrification of this amorphous phase have been investigated using three-dimensional atom probe tomography coupled with transmission electron microscopy. The results indicate that devitrification of this phase-separated amorphous phase is initiated by the formation of nanoscale fcc Cu-rich grains uniformly distributed in the amorphous matrix. These nanoscale fcc Cu-rich grains are often faulted and appear to retain some Nb supersaturation. The role of the nanoscale phase separation in the as-deposited amorphous film on the subsequent devitrification is discussed in the context of free energy–composition plots for this system.

The role of Y/lanthanides on the glass forming ability of amorphous steel

The effect of yttrium on improving the amorphous steel glass formability (GFA) is investigated through chemical analysis during the early devitrification stage by employing energy dispersive x-ray spectroscopy and three-dimensional atom probe techniques. Atomic segregation and partitioning are found to occur in association with the formation of the principal competing crystalline M23(CB)6 phase. The partitioning of Y from crystallite particles is confirmed, in agreement with first principles calculations. Additionally, the GFA improvement is also attributed to the strong interatomic bonding between lanthanide and metalloids. The role of Y on improving GFA is verified with growth kinetic measurements.

On the isothermal kinetics analysis of transformations in metastable systems: combined use of isothermal and non-isothermal calorimetry

A procedure to optimize the isothermal calorimetric data of very slow transformation processes of metastable systems is proposed. The method uses an experimental baseline to identify the transitory effects due to the equipment. Moreover, the combined use of isothermal and non-isothermal results is shown to be effective in overcoming the intrinsic problems of low signal and signal drift for such processes. The procedure has been applied to the analysis of the nanocrystallization kinetics of the Fe60Co18Nb6B16 alloy at different devitrification stages. Based on microstructural observations, an instantaneous growth approach was assumed and a phenomenological expression of the dependence of the nucleation frequency with both the transformed fraction and the temperature was obtained.