L. Battezzati

Solid state reactions in Al/Ni alternate foils induced by cold rolling and annealing

Al/Ni composites made of alternate foils having overall composition Al50Ni50 and Al66Ni34 were rolled up to 75 times folding them after every rolling pass to restore approximately the original thickness. It was found that the deformation of the composite is sustained by the Ni with Al acting as transmitting medium. The logarithmic reduction of foil thickness scales with the number of rolling passes. A nanocrystalline state of the elements, particularly Ni, is progressively reached. No detectable reaction is caused by repeated co-deformation. Reactions in the composites occur on annealing. The sequence of phases obtained at Al/Ni interfaces via nucleation and growth, and identified by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, reproduces that found on annealing deposited multilayers and ball-milled powders: Al3Ni, Al3Ni2, NiAl. All reactions are strongly activated by deformation, i.e. they occur at lower temperature as revealed by continuous heating experiments in a differential scanning calorimeter. The overall set of experimental results is consistent with reaction mechanisms of nucleation and growth with grain-boundary interdiffusion as the rate-determining step. This view is supported by comparison with a collection of data for the activation energy of diffusion, grain growth, and ordering in Al–Ni phases.

Undercooling of NiB and FeB alloys and their metastable phase diagrams

Abstract NiB and Fe Balloy have been undercooled in the cell of a high temperature differential scanning calorimeter (DSC) obtaining thermal data for their melting and solidification. Primary solidification was suppressed up to an undercooling ΔT larger than 200 K: eutectic solidification was suppressed up to ΔT larger than 100 K. It is demonstrated, using the Lipton-Kurz-Trivedi model, that at low undercooling the dendrite growth process is described by the calorimeter trace owing to limited recalescence, whereas at high undercooling the signal is dominated by recalescence effects. The solidification microstructures are discussed in relation to the dendrite growth rate and the mechanism of eutectic freezing. For both NiB and FeB a metastable phase was found to solidify preferentially with respect to the stable eutectic. The metastable phase is Ni2Bh in NiB and very likely Fe3B in FeB. For each phase the enthalpy and entropy of formation are estimated and metastable phase diagrams are drawn. An evaluation of the excess specific heat of the liquid eutectics is made and the results conform to the value expected for glass-forming alloys.

Mechanical alloying of the Al–Ti system

Abstract We have investigated the possibility of an amorphization reaction by mechanical alloying for two compositions of the Al–Ti binary system. While the Al-rich composition Al75Ti25 appears to give, after milling for 21 h, an Al(Ti) highly cubic phase, the Ti-rich composition Al25Ti75 does amorphize using various milling conditions. The progress of the amorphization as a function of time of milling was monitored by X-ray diffraction. At first, Al atoms diffuse into the host lattice of hexagonal Ti; subsequently, the milling accumulates a critical density of disorder that causes the Ti(Al) crystalline phase to collapse into an amorphous phase. The formation of amorphous alloys is discussed on the basis of thermodynamic models. The Miedema model is compared with a calculation of phase diagrams approach which has been modified to account for dependence of thermodynamic properties of the liquid upon temperature. T 0 curves are presented, showing a glass-forming range in agreement with experiments.

X-ray diffraction study of the amorphization process by mechanical alloying of the NiTi system☆

Abstract X-ray Fourier methods were used to follow fragmentation phenomena and structural defects during mechanical alloying processes in the NiTi system. Furthermore, a methodological X-ray diffraction approach was developed to study the kinetics of the process up to complete amorphization of the powders. Results show that the effective crystallite size and defect concentration of the unreacted powders achieve asymptotic values after mechanical alloying for about 5 h, whereas the amorphization process is accomplished after 30 h. The limiting value of the effective sizes, calculated after correction for the strain contribution, is around 30 nm, which is one order of magnitude larger than the values reported in these materials from a line-broadening analysis using the Scherrer equation. As for the defect concentration, elemental titanium and nickel powders milled separately show approximately the same strain content as the NiTi mixture treated under the same experimental conditions. However, the pre-induced disorder does not speed up the solid state reaction; in fact, when the single elements milled separately are reacted, the amorphization process is also completed in 30 h.

Thermal properties of mechanically alloyed Ni50Ti50 powders

Abstract Ni 50 Ti 50 amorphous powders were prepared by the novel technique of ball milling pure elements. Amorphization was complete after milling for about 35 h. X-ray diffraction was used to determine the amount of amorphous phase produced. Differential scanning calorimetry (DSC) was employed for thermal analysis. At various stages of alloying, two phenomena were detected. In the temperature range 450–650 K, the amorphization of part of the alloy occurs, giving a large exothermal signal. Peak shape analysis was used to estimate the coefficient of interdiffusion in the amorphous alloy. In the temperature range 680–780 K, crystallization occurs. The DSC peak changes its shape and area with milling time. Increases in the temperature of the maximum and of the heat crystallization were observed. The role of the specific heat difference between liquid and crystal phases for solid state amorphization is outlined.

Static mechanical characterization of a bulk amorphous and nanocrystalline Zr40Ti14Ni11Cu10Be25 alloy

Abstract Bulk amorphous samples of a Zr40Ti14Ni11Cu10Be25 alloy were tested in compression at displacements rates from 10−4 s−1 to 10−1 s−1. No dependence of Youngs modulus on strain rate was found. There was, however, a change in modulus on repeated cycling the load because of relaxation effects, up to a steady value of 102 GPa. Ductility was very limited since a maximum plastic strain at fracture of 1% was reached. The tensile strength was 2 GPa. Vein patterns due to strain localization were observed on fractured surfaces. Partially crystallized samples contained a nanocrystalline phase. The hardness increased with an increasing amount of nanocrystalline phase. The toughness decreased to values of 1–2 MPa√m as in brittle ceramics due to loss of intrinsic ductility of the amorphous matrix. Fracture surfaces displayed typical brittle behavior. The possibility of avoiding strain localization in the amorphous phase is discussed.

Liquid-liquid phase separation and remixing in the Cu-Co system

This article deals with the metastable liquid-liquid separation in the Cu-Co system. Several samples with different compositions were investigated by differential scanning calorimetry. High undercooling with respect to the liquidus was reached by means of the glass fluxing technique. The alloys were cycled with several heating and cooling runs in order to determine the temperature of the liquid-liquid separation and of the remixing. For each composition, demixing and remixing temperatures were found to be equal. Nucleation rate calculations of the liquid phase separation were carried out to explain the experimental results. The liquid-liquid separation in the Cu-Co system was found to be a nucleation process occurring with no detectable undercooling below the binodal line.

Microstructure study of a γ-TiAl based alloy containing W and Si

The phase transformations and modulated microstructures in the cast alloy Ti-47Al-2W-0.5Si(at.%) have been studied. It has been shown that microstructure is sensitive to heat treatment and composition. The structure in the alloy heat treated at 1300 degrees C mainly consists of coarse lamellar, fine lamellar, equiaxed gamma grains and beta phase. Homogenizing treatment at 1400 degrees C results in nearly fully-lamellar structure which shows colony-boundary gamma phase grains. The observations showed that small increases in W content result in changes in the amount and morphology of beta phase. For the alloy containing 2.1 at.% W, the beta phase has been observed in three types of morphology: blocky beta phase containing gamma needles and fine silicide particles inside; rod-like beta phase and fine needle-like beta phase. For the alloy variant containing 1.5 at.% W, only a very small amount of rod-like beta phase appeared. Different mechanisms are proposed to explain the occurrence of three types of beta phase, in that W may stabilize it by extending the primary beta phase to lower temperature as well as by yielding new phase zones. A few primary silicide particles have been found in the interdendrite region in the alloys containing more than 0.5 at.% Si, while coherent secondary silicides generally nucleated at the interfaces

Phase selection in Al–TM–RE alloys: nanocrystalline Al versus intermetallics

Abstract Al–TM–RE (TM: transition metal, RE: rare earth metal) and Al–RE alloys were analysed with the aim of studying phase selection under various processing conditions. The metastable phases formed in binary Al-rich systems (notably Al–Sm) are revised for a unified interpretation of the literature. Two groups of alloys had different behaviour as for phase selection. In Al 88 Fe 9 Nd 3 and Al 87 Ni 10 Ce 3 stable intermetallic compounds form as primary phases during solidification. Nanocrystalline Al is formed at high undercooling. This may occur directly in rapid solidification or when fully amorphous materials are suitably annealed. In Al 90 Sm 8 Ni 2 and Al 90 Sm 8 Fe 2 metastable intermetallics form during rapid solidification together with a fraction of amorphous phase, whereas nanocrystalline Al forms on annealing the amorphous phase.

Kinetics of abnormal grain growth in pure iron

Grain growth after primary recrystallization in unstrained and strained specimens of pure iron is examined during isothermal anneals at three different temperatures (664, 680, 690° C). Undeformed specimens undergo continuous grain growth, while the deformed ones show a stage of rapid discontinuous growth. The peculiar characteristics of the abnormal growth kinetics, and in particular the presence of three well-defined stages of growth, are brought out by plotting, versus annealing time, the ratio between the mean grain diameters of deformed and undeformed samples. The parametersn (exponent of the kinetic equation) andQ (apparent activation energy) are determined.