Synthesis of Transition Metal Aluminides from Elemental Powder Mixtures

Hossein Sina

Research output: ThesisDoctoral Thesis (compilation)

662 Downloads (Pure)

Abstract

Structural components such as those used in gas turbines for energy conversion applications, are subjected to high loads at elevated temperatures. In order to ensure that the components survive under these conditions for a sufficiently long time, it is imperative to select materials with good high temperature properties like strength and corrosion resistance. Such materials are usually alloys with a microstructure consisting of several phases where one or more phases have a strengthening effect. These phases are generally hard and are compounds formed from two or more metals. Current trends show an increasing interest in the use of such intermetallic compounds. The vast potential of intermetallic compounds like aluminides emerges from a combination of their attractive characteristics such as high melting point, high-temperature strength and excellent oxidation resistance. Transition metal aluminides fall into this category and the formation of these compounds from elemental powder mixtures is the focus of this thesis.
This work presents differential scanning calorimeter (DSC) studies on the formation of aluminides in the Ti-Al, Fe-Al, Nb-Al, Ta-Al binary systems and extended to the ternary Al-Ni-Ti system. The synthesis of various aluminides was followed by heating the well-mixed powder mixtures to temperatures at which aluminide formation is initiated, accompanied by heat evolution. The effect of initial composition, particle size and heating rate on the onset temperatures was also studied. The products obtained during various stages of reaction were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD).
The results show that aluminide formation could be initiated at temperatures below the melting point of aluminum, only in the case of relatively low melting metals like iron and titanium. Onset temperatures for reactions involving niobium and tantalum were much higher and showed the importance of wetting the particle surfaces of niobium and tantalum by molten aluminum. In the Fe-Al system, with coarse iron particles, an exothermic peak was observed before the melting of aluminum, followed by a stronger peak at much higher temperatures.
In all cases, irrespective of the initial composition of the powder mixture, aluminides rich in aluminum were the first compounds to form at the onset of reaction. These compounds had the general formula MAl3 where M represents Ti, Nb and Ta. However, the reaction between iron and aluminum particles led to the initial formation of Fe2Al5. In general, obtaining single phase, homogeneous products from Al-rich samples was relatively easier and did not require heating much beyond the combustion peak. A single phase product was also yielded in Fe-40 at.%Al powder compacts with fine-sized iron particles after heating to 1000°C.
A sharper reaction peak and lower onset temperatures were obtained on decreasing the aluminum content in the mixture or the particle size of the transitional element. The onset temperatures for reaction showed a tendency to increase with increasing heating rates. Multiphase, heterogeneous products were obtained at 1000°C in samples containing lower aluminum contents or those with coarse iron particles. It was relatively more difficult to homogenize multiphase products in the Ta-Al system than the rest and the easiest in the Fe-Al system. The evolution of phases during the heat treatment of the samples was followed using microstructural and X-ray diffraction studies.
Extending the studies to a ternary system, the progress of reaction in a compacted powder mixture containing two aluminide forming metals like nickel and titanium was followed using DSC. The effect of composition was studied by varying the aluminum addition (0 to 40 at.%) to nickel and titanium powders in equal proportions. The powder compacts were heated at 20°C min-1 up to 1200°C, in a continuous stream of pure and dry argon gas. Two main exothermic peaks were observed and for all the samples studied, the first exothermic peak was in the interval 595°-625°C. The heat release increased with increase in the aluminum content of the samples. At this stage, microstructural studies showed that Al3Ni and Al3Ni2 were the major constituent phases and only a thin layer of Al3Ti was observed around the titanium powder particles, indicating a diffusional barrier. The second exothermic peak was observed in the interval 938°-946°C which corresponds to the reaction between nickel and titanium. Titanium-rich and nickel-rich ternary compounds, in addition to some binary compounds, have been observed after this reaction in all aluminum containing samples. The formation of AlNi2Ti (t4) phase together with some new ternary compounds was observed in most of the Al-Ni-Ti samples after heating to 1200°C. The compositions of two of these phases, unidentified and not reported in literature so far, are close to Al2Ni3Ti5 and Al36Ni28Ti36.
The present work has shown that while transition metal aluminides can be produced through the powder metallurgy route, the synthesis of pure compounds requires careful control over critical process parameters like particle sizes of the reactants, initial composition of the powder mixture, heating temperature and time. The reactive sintering process can be optimized using the knowledge obtained from studies on the evolution of phases in various powder mixtures.
Original languageEnglish
QualificationDoctor
Awarding Institution
  • Materials Engineering
Supervisors/Advisors
  • Iyengar, Srinivasan, Supervisor
Award date2015 Oct 30
Publisher
ISBN (Print)978-91-7623-471-6
Publication statusPublished - 2015

Bibliographical note

Defence details

Date: 2015-10-30
Time: 13:00
Place: Hall M:E at M-Building, LTH

External reviewer(s)

Name: Gasik, Michael
Title: Professor
Affiliation: Aalto University, Finland

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Subject classification (UKÄ)

  • Materials Engineering

Free keywords

  • Aluminides
  • Intermetallics
  • Powder Metallurgy
  • Reactive Sintering
  • Combustion Synthesis
  • Thermal Analysis
  • DSC
  • Onset temperature
  • Phase evolution
  • SEM
  • EDS
  • XRD

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