Potent Nucleation Sites Research Articles

Mechanisms of grain refinement by intensive shearing of AZ91 alloy melt

It has been demonstrated recently that intensive melt shearing can be an effective approach to the grain refinement of both shape casting and continuous casting of Mg alloys. In the present study, the mechanisms of grain refinement by intensive melt shearing were investigated through a combination of both modelling and experimental approaches. The measurement of the cooling curves during solidification, quantification of grain size of the solidified samples, and image analysis of the MgO particle size and size distribution in the pressurized filtration samples were performed for the AZ91 alloy with and without intensive melt shearing. The experimental results were then used as input parameters for the free growth model to investigate the mechanisms of grain refinement by intensive melt shearing. The experimental results showed that, although intensive melt shearing does not change the nucleation starting temperature, it increases the nucleation finishing temperature, giving rise to a reduced nucleation undercooling. The theoretical modelling using the free growth model revealed quantitatively that intensive melt shearing can effectively disperse MgO particles densely populated in the oxide films into more individual particles in the alloy melt, resulting in an increase in the MgO particle density by three orders of magnitude and the density of active nucleating MgO particles by a factor of 20 compared with those of the non-sheared melt. Therefore, the grain refining effect of intensive melt shearing can be confidently attributed to the significantly increased refining efficiency of the naturally occurring MgO particles in the alloy melt as potent nucleation sites.

The effect of Al 8Mn 5 intermetallic particles on grain size of as-cast Mg–Al–Zn AZ91D alloy

Al 8Mn 5 intermetallic particles are usually present in most of commercial Mg–Al based alloys. In recent years, the ability for the Al 8Mn 5 particles to act as potent nucleation sites for the α-Mg grains during the solidification of the Mg–Al alloys has been the subject of debate. In this work, the effect of Al 8Mn 5 particles on grain size of an Mg–9Al–1Zn (AZ91D) alloy was assessed. The experimental result from two AZ91D alloys with different Mn concentrations, i.e., 0.02 wt.% and 0.22 wt.% respectively, indicated that the grain size did not show any notable change. Further, high resolution transmission electron microscopy (HRTEM) was conducted on samples with concentrated Al 8Mn 5 particles collected by pressurised filtration of the AZ91D alloy melt. The extensive TEM examination on the Al 8Mn 5/α-Mg interfaces revealed no crystallographic orientation relationship (OR) between the Al 8Mn 5 and α-Mg crystals. It is therefore concluded that Al 8Mn 5 particles are unlikely to act as effective nucleation sites for the α-Mg during solidification of Mg–Al based alloys.

Progress in the Development and Use of Grain Refiner Based on Cerium Sulfide or Titanium Compound for Carbon Steel

A fine-grained microstructure yields the optimum combination of strength and toughness of steel. Moreover refinement of the as-cast structure can reduce the tendency for hot-cracking during forging and rolling. This paper describes how small inclusions can be used to control the microstructure of steels. These small inclusions (dispersoids) are oxides, sulfides, nitrides and carbides which are in the 1 μm size range and capable of promoting grain refinement during solidification by a process of epitaxial nucleation or in the solid state through intragranular nucleation of ferrite. Such particles are sufficiently small to be harmless from a toughness point of view, but at the same time large enough to act as potent nucleation sites during phase transformation. The dispersoids can either be created by balanced additions of strong oxide and sulfide forming elements to an impure steel melt or be added directly into the liquid steel through a specially designed master alloy containing the nucleating particles. In both cases it is possible to manipulate the steel microstructure in a positive direction, but the latter method, involving the use of a master alloy, has probably a wider industrial application.

Microstructure Control of Steels through Dispersoid Metallurgy Using Novel Grain Refining Alloys

This paper describes how small inclusions (designated dispersoids) can be used to control the microstructure of steels. The term refers to oxides, sulphides, nitrides and carbides which are in the 1 μm size range and capable of promoting grain refinement during solidification by a process of epitaxial nucleation or in the solid state through intragranular nucleation of ferrite. Such particles are sufficiently small to be harmless from a toughness point of view, but at the same time large enough to act as potent nucleation sites during phase transformation. The dispersoids can either be created by balanced additions of strong oxide and sulphide forming elements to an impure steel melt or be added directly into the liquid steel through a specially designed master alloy containing the nucleating particles. In both cases it is possible to manipulate the steel microstructure in a positive direction, but the latter method, involving the use of a master alloy, has probably a wider industrial application. The direction of the research now being undertaken at NTNU/SINTEF to make the grain refining alloys commercially available is briefly described towards the end of the paper.

Fine Structure of Shear Bands Formed during Hot Deformation of Two Austenitic Steels

Shear bands formed during both cold and hot plastic deformation have been linked with several proposed mechanisms for the formation of ultrafine grains. The aim of the present work was to undertake a detailed investigation of the microstructural and crystallographic characteristics of the shear bands formed during hot deformation of a 22Cr-19Ni-3Mo (mass%) austenitic stainless steel and a Fe-30 mass%Ni based austenitic model alloy. These alloys were subjected to deformation in torsion and plane strain compression (PSC), respectively, at temperatures of 900 � C and 950 � C and strain rates of 0.7 s � 1 and 10 s � 1 , respectively. Transmission electron microscopy and electron backscatter diffraction in conjunction with scanning electron microscopy were employed in the investigation. It has been observed that shear bands already started to form at moderate strains in a matrix of pre-existing microbands and were composed of fine, slightly elongated subgrains (fragments). These bands propagated along a similar macroscopic path and the subgrains, present within their substructure, were rotated relative to the surrounding matrix about axes approximately parallel to the sample radial and transverse directions for deformation in torsion and PSC, respectively. The subgrain boundaries were largely observed to be non-crystallographic, suggesting that the subgrains generally formed via multiple slip processes. Shear bands appeared to form through a co-operative nucleation of originally isolated subgrains that gradually interconnected with the others to form long, thin bands that subsequently thickened via the formation of new subgrains. The observed small dimensions of the subgrains present within shear bands and their large misorientations clearly indicate that these subgrains can serve as potent nucleation sites for the formation of ultrafine grain structures during both subsequent recrystallisation, as observed during the present PSC experiments, and phase transformation.

Late sulfur inoculation of spheroidal graphite cast irons

The addition of small amounts of sulfur in the form of pyrite granules to post-inoculated ductile irons was investigated. Two types of nodular iron were used in this study: high silicon ductile iron and high carbon equivalent (CE) ductile iron. Due to the low levels of sulfur employed careful statistical analysis of the results was conducted. In high silicon ductile irons, the late addition of sulfur resulted in a larger fraction ferrite as revealed by image analysis. Differential thermal analysis and thermodynamic modeling of comparable ternary Fe-C-Si alloys further verified these results. Study of shape factors of graphite nodules indicated no significant effect of the low levels of sulfur added to the post-inoculated irons. For the high carbon equivalent irons, higher temperature primary graphite arrests were observed, suggesting that sulfur promoted the formation of potent substrates for the formation of graphite nodules at higher temperatures. Concurrently, through microstructural characterization it was corroborated that higher ferrite volume fractions were also attained. Under similar experimental conditions, the sulfur effect differs with different carbon equivalents. In high silicon (2.9 wt.% Si) eutectic irons, low sulfur additions did not noticeably alter the cast iron solidification. Conversely, in high CE (hypereutectic) irons there exists a stronger influence on the primary graphite arrest temperature. In both ductile iron types, image analysis of the resulting microstructures indicated that the late additions of sulfur to the post-inoculated melts produced larger nodule counts. These results reaffirmed the conclusion that sulfur plays a direct role in the formation of graphite, for instance by reacting and forming complex Mg oxysulfide particles that develop into potent nucleation sites for graphite.

Solidification of high-refractory ruthenium-containing superalloys

The effect of alloy chemistry on single crystal solidification has been investigated in a series of model high refractory Ni-base superalloys with variations in ruthenium, rhenium and aluminum. Over the range of composition investigated, the initial phase to form during solidification was either the γ phase (FCC Ni-solid solution) or the δ phase (HCP Re-rich phase). In a quaternary Ni-Al-Ta-Re alloy containing 1.7 at.% (5.6 wt.%) Re, δ-Re nucleated at a temperature well above the liquidus temperature of pure Ni and grew unconstrained into six-fold Re-rich dendrites. These dendrites served as potent nucleation sites for γ grains as temperature decreased during directional solidification. Ruthenium additions in the range of 2.5∽9.0 at.% (4.1∽14.1 wt.%) lowered the temperature at which the δ nucleated and eventually suppressed the formation of this phase during solidification. Ru additions also increased the liquidus temperatures of the multicomponent superalloys. The implications for the design of Ru-containing superalloys are discussed.

Moderation of the recrystallization texture by nucleation at copper-type shear bands in Al-1Mg

The critical dependency of copper type shear band formation on deformation variables in Al-1Mg has been described (Acta mater., 49 (2001) 2739). The influence of copper type shear bands on recrystallization behaviour during post-deformation annealing in Al-1Mg is now discussed. Local orientation measurements and orientation images of grains nucleating at shear bands have been made using the electron back-scatter diffraction (EBSD) technique. Bulk recrystallization texture is investigated from X-ray diffraction (XRD) measurements. Shear bands are shown to be potent nucleation sites during annealing and a weak, almost random, texture associated with nucleation at shear bands has been inferred. Most significantly, shear band formation is found to have a pronounced moderating influence on the strength of the potentially dominant cube component of the recrystallization texture.

Crystallisation and Phase Decomposition Processes in Pr<SUB>4.5</SUB>Fe<SUB>77</SUB>B<SUB>18.5</SUB> Amorphous Ribbon

The isothermal crystallisation processes and the decomposition behaviours of the crystallised phases in Pr 4.5 Fe 77 B 18.5 amorphous ribbons prepared by a melt quench technique have been investigated by means of differential thermal analysis, X-ray diffraction, and high resolution transmission electron microscopy, and the following results were obtained. The phase initially crystallised is Fe 3 B and the crystallisation of Pr 2 Fe 23 B 3 particles occurs immediately after the Fe 3 B formation. This is probably due to the fact that Fe 3 B/amorphous interphase boundaries provide potent nucleation sites for Pr 2 Fe 23 B 3 particles in view of chemistry in the vicinity of Fe 3 B particles and the structure of the interfaces. Then, fine hard magnetic Pr 2 Fe 14 B particles nucleate at Fe 3 B interfaces directly. In the following processes, Pr 2 Fe 14 B and Pr 2 Fe 23 B 3 particles grow by the coalescence of fine particles. In the later annealing stages, Pr 2 Fe 23 B 3 particles decompose into Pr 2 Fe 14 B networks involving spherical α-Fe particles in a eutectoid-like mode. At the final stage of annealing these Pr 2 Fe 14 B networks transform in-situ into those of PrFe 4 B 4 .

Effect of Inclusion Size on the Nucleation of Acicular Ferrite in Welds.

Low-carbon steel weld with a high density of oxide inclusions prepared using a experimental metal-cored wire has been examined to study the effect of inclusion size on the formation of acicular ferrite, and to understand the role of inclusion in the nucleation of ferrite lath. Depending on the ferrite morphology associated with inclusions, a total of 282 inclusions observed under TEM could be classified into two groups, i.e. the non-nucleant and the nucleant. Experimental results showed that the group of inclusions acted as nucleant were appreciably larger in size compared with those of non-nucleant resulting in the increased probability of nucleation with the increase of inclusion size, even though the chemical and structural natures appeared to be the same. The group of nucleant-inclusion was further divided into two types depending on the degree of nucleation, which was evaluated by the number of ferrite lath nucleated. Statistical analysis performed on inclusion size indicated that the larger the inclusion size is the more ferrite laths could be nucleated. Those laths nucleated from a large single inclusion have grown in many different radial directions and mostly had a different crystallographic orientation from those of adjacent ferrite laths. As a result of this study, it is demonstrated that larger inclusions are indeed more potent nucleation sites when compared with those of smaller size. Thus it could be concluded that the provision of the inclusion surface as for the inert surface for the heterogeneous nucleation of acicular ferrite lath would be the principal role of inclusions playing in the weld metal of low alloy steels. Other possible mechanisms were also considered, but they were unlikely to be operated in the present weld metal system.

Review of Trace Element Effects on High-Temperature Fracture of Fe- and Ni-Base Alloys

Trace elements can have harmful or beneficial effects on elevated-temperature mechanical properties. In iron and low-alloy steels, S at high concentrations forms sulfides which are potent nucleation sites for intergranular creep cavities. As a result, ductility decreases and fracture becomes increasingly intergranular with increasing S concentration. Oxides and nitrides nucleate cavities only in the presence of segregated S; by themselves they are relatively benign. The harmful effects of S can be ameliorated by the addition of carbon and phosphorus. It is unclear whether C and P have beneficial effects of their own, or whether they act merely by counteracting the negative effects of sulfur. Carbides almost never nucleate cavities. In Ni-base alloys, too, S is deleterious, and alloying with strong sulfide getters increases ductility and rupture life. Boron and P interact synergistically and increase stress rupture lives in some alloys but not in others. The detailed mechanism of this interaction is not well understood.

Thermodynamic and mechanical properties of Zr-based bulk metallic glasses

Glasses are generally produced from the highly undercooled liquid state by rapid quenching methods or quasistatically at slow cooling by the effective control of potent heterogeneous nucleation sites. For metallic systems the latter method recently has led to the development of bulk metallic glasses, so-called supermetals, with a complex multicomponent chemistry and advanced engineering properties. As long as crystallization can be prevented, the relevant thermodynamic properties of the metastable glassy and undercooled liquid phases can be measured below and above the glass transition temperature respectively. The data suggest that the glass transition corresponds to kinetic freezing triggered by an underlying entropic instability. Further measurements of the mechanical and wear properties of these new metastable materials point to unique engineering properties for technological applications.

Nucleation of recrystallised grains from cube bands in hot deformed commercial purity aluminium

The present work was initiated in order to obtain a better understanding of the nucleation mechanism of cube oriented grains during recrystallisation of hot deformed aluminium. Investigations included: the stability aspect of cube oriented grains during deformation, a characterisation of cube oriented areas in the as deformed condition, and partly recrystallised states. Commercial purity aluminium, plane strain compressed at various deformation conditions, was investigated mainly using the electron backscattering pattern (EBSP) technique in a scanning electron microscope (SEM). The following deformation and nucleation pattern was observed: cube oriented grains present in the starting material remain orientation metastable during deformation and are smeared out to bands. The cube bands have a unique subgrain size distribution with a long tail towards large subgrains, making them potent nucleation sites. The observed preferred nucleation from cube bands surrounded by the S deformation texture component, is discussed both in terms of a microgrowth selection mechanism and differences in stored energy between the cube and S oriented regions.MST/3247

Metastability and Properties of Metallic Bulk Glass Forming Alloys

ABSTRACTGlasses are generally produced from the highly undercooled liquid state by rapid quenching methods or quasi-statically at slow cooling by the effective control of potent heterogeneous nucleation sites. For metallic systems the latter method recently has led to the development of bulk metallic glass with a complex multicomponent chemistry and advanced engineering properties. With these alloys crystallization can be avoided over a broad temperature / time window. As such, the relevant thermodynamic properties of the metastable glassy and undercooled liquid states can be directly measured below and above the glass transition temperature, respectively. The obtained data give new insight into the nature of the glass transition suggesting that it is not a phase transition in the classical sense but kinetic freezing triggered by an underlying entropie instability to avoid crystallization. Further measurements of the mechanical and wear properties point to the unique engineering properties of these highly disordered materials for technological applications.

Undercooling and crystallization behaviour of antimony droplets

The level of liquid undercooling below the equilibrium melting point experienced typically in a bulk material is limited by the presence of potent heterogeneous nucleation sites. By subdividing the material into a collection of fine (10 to 150μm) droplets the most potent nucleants are isolated into a small fraction of the droplet population, allowing the remainder to reach deep levels of undercooling prior to the onset of solidification. Droplet surface coating characteristics influence both the level and uniformity of the undercooling and, in addition, may alter the nucleation kinetics thereby encouraging the formation of a metastable crystalline phase from the undercooled liquid. An effective means of both creating a stable dispersion of fine droplets and influencing the surface coating is through the droplet emulsion technique. This approach has allowed the level of undercooling for pure antimony to be extended from 0.08 to 0.237Tm through proper control of droplet size, surface coating and applied cooling rate. The formation of a metastable simple cubic phase from the melt has been confirmed using both differential thermal analysis and X-ray diffraction techniques. The simple cubic phase is retained in droplets to temperatures approaching the melting point at a heating rate of 20° C min−1. Drop tube treatment of emulsified samples has also been successful at producing the metastable simple cubic structure by containerless processing.