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.
•Present features and structure-property relationships of Al-based amorphous alloys.•Different models for the nucleation behaviors are compared and evaluated.•Summarize the effects of minor alloying additions on GFA and crystallization.
For high temperature applications nickel base superalloys with aluminide coatings are reaching the service limit. Alternate materials such as refractory metal silicide alloys can extend high ...temperature capability, but the silica surfaces require coatings for enhanced environmental resistance. We present a review of an effective strategy to achieve the environmental resistance through the use of an integrated Mo-Si-B based coating that is applied by a co-deposition of Si and B by pack cementation. During oxidation of the (Si+B)-pack alloys, the initial MoSi2 outer layer is consumed by formation of the Mo5Si3 (T1) phase and the development of the underlying Mo5SiB2, T2 borosilicide and/or MoB boride phase layer. The T1 phase with B has excellent oxidation resistance and the loss of Si to the substrate is blocked by the underlying diffusion barrier (T2). Any damage to the outer T1 layer can be recovered from the T2+MoB layer to yield a self-healing characteristic. The Mo-Si-B based coating on Mo based alloys exhibits robust performance up to at least 1700°C not only to high temperature oxidation, but also to water vapor and corrosive deposit attack. Some of the applications are discussed for use of the coating strategy on ceramic materials.
•We present a review of an effective strategy to achieve the environmental resistance through the use of an integrated Mo-Si-B based coating that is applied by a co-deposition of Si and B by pack cementation•We present a review of an effective strategy to achieve the environmental resistance through the use of an integrated Mo-Si-B based coating that is applied by a co-deposition of Si and B by pack cementation•Some of the applications are discussed for use of the coating strategy on ceramic materials.
Ultrastable glasses and nanoglasses are two emerging materials with novel properties that have been investigated separately. In order to explore the combined effect of ultrastable character and a ...nanoglass with a nanoglobular microstructure on the kinetic behavior, the glass transition and crystallization behaviors of an ultrastable nanoglass and a melt-spun ribbon of Au-based metallic glass were examined by differential scanning calorimetry at heating rates (ϕ) of up to 40,000Ks−1. The nanoglass shows ultrastable kinetic characters at low ϕ (e.g. 300Ks−1) and similar kinetic behaviors at high ϕ (e.g. 30,000Ks–1) compared to the melt-spun ribbon. The nanoglobular interfaces remain amorphous and appear to act as a kinetic constraint to induce a higher crystallization temperature compared to the melt-spun ribbon. The interface constraint effect disappears at 30,000Ks−1. These results indicate that the nanoglobular microstructure can act to increase metallic glass stability and provide another mechanism for the synthesis of ultrastable glass.
Nucleation of shear bands is of critical importance in the deformation behaviors of metallic glasses, but the detailed examination of nucleation kinetics and the correlation with the structure and ...relaxation kinetics has been few. Here, based upon instrumented nanoindentation with spherical indenter and a great deal of the observed first pop-in events, one unique trimodal distribution of shear band nucleation events in a Gd-based metallic glass was reported. Four different loading rates were employed to study the evolution of the nucleation distribution with external loading. In the analyses of nucleation kinetics, the site density, nucleation rate and activation barrier for three nucleation events were obtained based on the cooperative deformation model, which clearly display the trimodal character. The discovery of a trimodal shear band nucleation distribution provides new insights for understanding the relationship between the heterogeneous structure, relaxation kinetics and deformation mechanism and opportunities for controlling the ductility of amorphous alloys.
•An effective strategy by nanoindentation reveal the multiple distribution of shear band nucleation.•A unique trimodal distribution of shear band nucleation sites in Gd-based MG for the first time.•The diagram of first pop-in load with loading rates for three nucleation sites were plotted.•The nucleation sits density, nucleation rate and activation barrier for three nucleation sites were determined.•The types of shear band nucleation sites are related to the types of characteristic relaxation behavior.
The synthesis of robust coatings that provide protection against environmental attack at ultrahigh temperatures is a difficult challenge. To achieve this goal for Mo-base alloys, the fundamental ...concepts of reactive diffusion pathway analysis and kinetic biasing are used to design a multilayer Mo-Si-B-base coating with a phase sequencing that allows for structural and thermodynamic compatibility and an underlying diffusion barrier to maintain coating integrity. The coating design concepts have a general applicability. The coating structure evolution during high-temperature exposure facilitates a prolonged lifetime as well as self-healing capability. The borosilicide coatings that can be synthesized by a pack cementation process yield superior environmental resistance for Mo-base systems at temperatures up to at least 1,700°C and can be adapted to apply to other refractory metal and ceramic systems.
Metallic glasses are non-equilibrium materials and the glass transition temperature upon heating Tg,h can be used to characterize the kinetic stability of the glass. Annealing below the glass ...transition is well-known to induce relaxation processes that reduce the glass enthalpy. We demonstrate that a liquid-cooled Au-based metallic glass can achieve very high kinetic stability by an optimal annealing treatment to yield a large increase in Tg,h of 28 K; this is 3–5 times larger than the increase usually reported. The measured enthalpy decrease of 1100 J/mol is about 50% of the difference between the as-cooled glass and the equilibrium crystalline state and reaches the extrapolated enthalpy of the supercooled liquid. The optimal annealing conditions can be determined by an enthalpy-temperature-time (ETT) diagram which is proposed for the first time based on the comprehensive examination of relaxation processes. At equilibrium, a direct relation is established between the increase in kinetic stability as measured by the increase in Tgh and the enthalpy decrease.
The 3-dimensional plot (a) of enthalpy decrease (ΔH) versus isothermal annealing time (logarithmic) and annealing temperatures. (b) The 2-dimensional contours of the ΔH. Each contour (from red to blue) represents a 50 J/mol decrease. From lower-left to upper-right, the diagram can be divided into two zones before crystallization: the nonequilibrium glass state and the metastable equilibrium supercooled liquid state. The dashed curve shows the optimal annealing temperatures to achieve the lowest enthalpy for a given annealing time. Display omitted
For high-temperature application beyond the range of Ni-base superalloys, Mo–Si–B alloys with composition that yield the ternary intermetallic, Mo
5SiB
2, T
2, phase as a key microstructure ...constituent, offer an attractive property balance of high-melting temperature, oxidation resistance, and useful high-temperature mechanical properties. With the T
2 ternary phase as the focal point of the microstructure designs, the fundamental basis of the alloying behavior in T
2 including the mutual solid solution with a wide range of transition metals has been established in terms of the governing geometric and electronic factors. For non-stoichiometric compositions, constitutional defects such as vacancies for Mo-rich compositions and anti-site defects for Mo-lean compositions control the homogeneity range. Moreover, the aggregation of constitutional vacancies has been discovered to play a key role in the development of dislocation and precipitation reactions in the T
2 phase that directly impact high-temperature structural performance. The characteristically sluggish diffusion rates within the T
2 phase have also been quantified and applied to the materials processing strategies. The materials design based on the phase stability, diffusion and defect structure analysis in the Mo–Si–B system can also be applied to the design of new multiphase high-temperature alloys with balanced environmental and mechanical properties.
The composition and phase constituency of Mo–Si–B alloys are known to be important parameters in determining the oxidation response. For three phase Mo
+
T
2
+
Mo
3Si alloys with constant composition ...and phase constituency, it is observed that a refined microstructure scale provides superior oxidation resistance. The transient stage of oxidation is shortened and the recession of the alloy is decreased with microstructural refinement. In order to identify the phase interaction during the transient stage, oxidation of each of the three alloy phases, Mo, Mo
3Si (A15) and Mo
5SiB
2 (T
2) has been investigated separately. Quantification of the separate phase size distributions by image analysis was coupled with the individual phase oxidation response to evaluate the overall oxidation behavior and phase interaction effects. A kinetic model for oxidation of Mo–Si–B alloys is proposed that incorporates the key role of microstructure scale on the transient stage and provides guidance for microstructure design.
During Si-pack cementation a MoSi
2 outer layer was synthesized on a Mo–Si–B alloy with a three-phase Mo
+
Mo
5SiB
2
+
Mo
3Si microstructure. Following oxidation exposure, the MoSi
2 phase layer was ...replaced by B containing Mo
5Si
3 (T
1) phase in contact with a Mo
5SiB
2 layer. The synthesized T
1 phase provided an excellent oxidation resistance and stability.
Melt undercooling and nucleation kinetics Perepezko, J.H.; Wilde, G.
Current opinion in solid state & materials science,
02/2016, Letnik:
20, Številka:
1
Journal Article
Recenzirano
Odprti dostop
•Nucleation catalysis by primary phases and incorporated particles.•Nucleation catalysis environment effects on nucleation undercooling.•Nucleation analysis.•Effects of melt superheat on ...nucleation.•Finite-size effects.
While melt undercooling is often observed during solidification the control of undercooling and nucleation is difficult due to the numerous possible heterogeneous sites present in even high purity melts. The identification of active nucleation sites has been a continuing challenge that requires developing well planned experimentation. In samples with well-defined and controlled undercooling, the identification can be established for a number of the active sites and mechanisms that can act to catalyze nucleation. The sites and mechanisms that have been identified involve primary phases developed during cooling of alloy melts, liquid-added particle interfaces being modified (e.g. by adsorption or reaction) creating a particle type independent nucleation potency, dissolved impurities precipitating out of the melt at high undercooling or acting to alter local atomic arrangements in the melt to catalyze nucleation and nucleation sites resulting from residual solid preserved in cavities in inclusions or surface coatings. Recent advances in clarifying the structure of liquids reveal local atomic arrangements such as short range order, medium range order and cluster structures, but the role of these local heterogeneities on the undercooling and crystallization behavior is incompletely understood at present.
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