J.H. Perepezko

Nucleation in undercooled liquids

During solidification the amount of undercooling is an important factor in determining microstructural development by controlling phase selection during nucleation. Usually crystallization is initiated at a heterogeneous catalytic site at a relatively small undercooling. The use of fine (10–20 μm) droplet samples is an effective method of obtaining a substantial increase in undercooling to the range (0.3−0.4)Tm (where Tm is the melting temperature) during slow cooling. Droplet undercooling behavior can be controlled by size variation and coating treatments, but the undercooling limit often appears to be set by a surface catalysis consistent with classical nucleation theory. The operative solidification reaction in droplets is determined by a competition between the equilibrium and possible metastable phase products which can be modified by the effect of cooling rate and undercooling on nucleation kinetics. With a controlled heterogeneous catalysis, specific nucleation reactions and product structures may be selected and metastable phase equilibria may be determined experimentally.

An experimental evaluation of the phase relationships and solubilities in the NbCr system

The phase relationships and solubilities have been determined experimentally in a revised evaluation of the NbCr alloy system. The results from a series of microstructural investigations on arc-cast and equilibrated samples, together with X-ray diffraction and compositional analyses, provide a basis for modifying the invariant reaction phase compositions and phase boundaries. In addition, new evidence based on X-ray diffraction measurements further establishes the existence of a high temperature C14 Laves polytype as well as an intermediate C36 structure for NbCr2. Moreover, lattice parameters have been determined as a function of composition for the b.c.c. and C15 Laves phases. Within the equilibrium solubility limits of the Nb and Cr terminal b.c.c. solid solutions, the lattice parameters exhibit a slight positive deviation from Vegards rule. For the NbCr2 C15 Laves phase the solubility behavior and the distinct change in the composition dependence of the lattice parameters suggest a different defect structure on either side of the stoichiometric composition in the intermetallic phase field.

Application of ternary phase diagrams to the development of MoSi2-based materials

Abstract A review of the literature reveals ternary phase diagram data for a number of systems involving MoSi2. Although incomplete, this literature provides the initial basis for a rational approach to alloy design. For example, one can assess the high temperature stability of various artificially introduced reinforcements, such as niobium or SiC in an MoSi2 matrix and the possibilities for the development of stable two-phase microstructures in quasibinary alloys in the MoSi2TiSi2 and MoSi2TaSi2 systems. Revised phase diagrams for these latter systems are presented that indicate the absence of the C11b-to-C40 high temperature polymorphic transformation in pure MoSi2.

Analysis of nanocrystal development in Al-Y-Fe and Al-Sm glasses

A high nucleation density is clearly an important prerequisite for the development of nanocrystal dispersions during primary crystallization of metallic glass. A microstructure of 1021 m−3 crystals of 20 nm diameter in an amorphous matrix can be readily detected by TEM, but the net heat generation is too weak for detection by usual DSC methods. This highlights the need for caution in the interpretation of DSC data from amorphous samples. Upon approaching Tg, the enhancement of diffusivity promotes further nucleation and growth. However, the high nanocrystal density (1022–1023 m−3) results in rapid diffusion field impingement which arrests growth. This provides a mechanism to limit nanocrystal growth and maintain the high density of nanocrystals. By modeling the nanocrystal development in terms of diffusional growth including diffusion field impingement, a good accounting is obtained for the DSC exotherm. The characteristic asymmetric exotherm with a relatively sharp onset and a tail at high temperature is an indication of kinetic stabilization of primary nanocrystal development.

Thermodynamic properties of undercooled liquid metals

Abstract In the form of a droplet emulsion, a number of liquid metals and alloys have been undercooled by substantial amounts (0.3–0.4 T m ) before crystallization. For example, pure Hg, In, Sn, and Bi can be undercooled by 90°C, 110°C, 187°C and 227°C respectively. Differential scanning calorimetry has been applied to determine the heat capacity difference, ∪Cp, between the undercooled liquid and crystalline solid. With decreasing temperature an increasing value of ∪Cp is observed which implies a reduction in configurational entropy of the liquid. In addition the heat capacity measurements allow for the determination of the free energy and the onset of the hypercooled regime which are useful in the evaluation of the crystallization kinetics.

A geometric analysis of solubility ranges in Laves phases

Abstract Laves phases nominally occur at the AB 2 stoichiometry but can exhibit a range of solubility involving non-stoichiometric compositions in binary alloys. The solubility trends in the reported binary C14, C15 and C36 structures have been analyzed in terms of the atom size requirements that are known to stabilize the Laves phases. For example, Laves phases exist at metallic diameter ratios ( D A / D B ) between ∼1.05 and 1.68 with the ideal diameter ratio existing at ∼ 1.225. Although less than 25% of the Laves phases within the D A / D B ratios of 1.05–1.68 have defined ranges of homogeneity, the frequency of the number of intermetallic phases exhibiting any solubility range is increased by a factor of approximately two to three within specific D A / D B ratios of 1.12–1.26 (C14 and C36 phases) and 1.1–1.35 (C15 phases). The upper and lower bounds for the C15 structures can be physically defined as the limits at which the A-B atom distance contractions are greater than the A-A atom distance and B-B atom distance contractions, respectively. For all three main polytypes the occurrence of solubility corresponds to a lattice-adjusted contraction between 0–15%. The contraction size rule is a geometric argument based upon the contraction of the atoms forming the intermetallic structure and appears to be an important relationship in describing ranges of homogeneity in Laves phases. The relationships developed are applied to interpret potential defect mechanisms and alloying behavior in binary and ternary Laves phases. In addition, extended ternary solubility ranges normal to a pseudobinary direction can be predicted with suitable solute additions having a metallic diameter between that of the A and B atoms.

Intermetallic phase formation during annealing of Al/Ni multilayers

The phase evolution during annealing of Al/Ni multilayer samples prepared by ion‐beam sputtering with composition modulation wavelengths Λ between 10 and 400 nm was determined using x‐ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of Al0.40Ni0.60 and Al0.75Ni0.25 were investigated. For the Al0.40Ni0.60 samples the following results were obtained. A measure of the degree of periodicity and the sharpness of the interfaces in a sample with Λ=80 nm was the large number (over 20) of peaks observed in small‐angle x‐ray scattering measurements. A sample with Λ=10 nm was transformed by heat treatment directly to the AlNi phase. In the Λ=80 nm sample, the first phase formed after annealing was the metastable η phase. The η phase was identified as Al9Ni2. In the 400 nm wavelength sample, both the metastable η phase and the stable Al3Ni formed after the first exothermic reaction. For the Al0.75Ni0.25 samples two results were obtained. A Λ=11.4 nm sample transfor...

DTA AND HEAT-FLUX DSC MEASUREMENTS OF ALLOY MELTING AND FREEZING

Publisher Summary This chapter focuses on differential thermal analysis (DTA) and heat-flux differential scanning calorimetry (HF-DSC) of metals and alloys. A thermal analysis guide focused only on metals and alloys is appropriate because metals and alloys behave quite differently from molecular materials such as polymers and organics. Freezing and melting occur rapidly in response to changes in temperature compared to other materials. Melting and freezing transformations, once initiated, take place within, at most, a degree of local thermodynamic equilibrium. Therefore, the chapter also focuses on melting and solidification behavior because special methods can be employed that are not necessarily useful for a broader class of materials and processes. The chapter intends to provide the thermal analysis user with the considerations that are necessary for proper sample preparation and to illustrate how the sample characteristics influence the proper interpretation and analysis of measurements. The chapter describes different types of information usually sought from DTA/HF-DSC during the melting and freezing of alloys. The details of instruments, operation and calibration are described. The goal is to describe the thermal lags between the sample and sample thermocouple that must be understood to enable good analysis of data. The chapter also details the response of the DTA to binary and ternary alloys, respectively.

Structural evolution and phase formation in cold-rolled aluminum-nickel multilayers

Abstract Multilayer samples of nickel and aluminum with the compositions of Al–20 at% Ni and Al–25 at% Ni were prepared by the repeated folding and cold rolling (F&R) of elemental foils. Characterization by X-ray diffraction, scanning electron microscopy and transmission electron microscopy/selected-area electron diffraction reveals that the rolling procedure results in a decrease in thickness of the elemental layers to below 0.1 μm and a reduction in grain size to below 50 nm, but does not induce formation of an intermediate phase. Differential scanning calorimetry (DSC) measurements on samples subjected to different numbers of F&R cycles show double exotherms related to an initial nucleation and lateral growth and a subsequent thickening of reactively formed Al3Ni. The DSC data agree qualitatively with predictions of an existing kinetic model for multilayer reactions, but a decisively improved quantitative fit is obtained when a distribution of layer spacings is considered, rather than a well-defined modulation wavelength. The modified kinetic model can be inverted to yield information on the evolution of the distribution of layer sizes during deformation from DSC data.

Liquidus projection for the Mo-rich portion of the Mo–Si–B ternary system

The solidification behavior in the Mo-rich portion of the Mo‐Si‐B ternary system has been examined based on the microstructures of arc-cast alloys. Several solidification characteristics in the Mo-rich portion of the system have been identified using XRD, SEM and TEM. The liquidus projection in the Mo-rich portion includes six primary solidification reactions; five reactions originating from the Mo‐Si and Mo‐B binary sections (Mo, Mo2B, bMoB, Mo3Si and Mo5Si3) and one from the ternary-based (Mo5SiB2 )T 2 phase. The liquidus surface in general descends from the high melting temperature Mo‐B binary side to the lower melting temperature Mo‐Si binary side. The solidification path of alloys with compositions in the T2 phase region is always preceded by the peritectic reaction of bMoB+L)T2. In addition, four Class II reactions (four-phase reactions) and one Class I reaction (invariant ternary eutectic reaction) have been identified. The extent of solidification segregation in alloys with compositions in the Mo(ss)+T2 two-phase field is discussed as it pertains to materials processing. # 2000 Elsevier Science Ltd. All rights reserved.