Differential scanning calorimetry and reaction kinetics studies of γ + α2 Ti aluminide

doi:10.1016/j.matchemphys.2012.09.039

Materials Chemistry and Physics

Volume 137, Issue 2, 14 December 2012, Pages 483-492

https://doi.org/10.1016/j.matchemphys.2012.09.039 Get rights and content

Abstract

Reaction synthesis method for titanium aluminide processing consists of an exothermic reaction among alloying elements present and primarily between titanium and aluminium particles at specific temperature range. Study of this reaction helps in understanding the process of aluminide formation. Differential scanning calorimetry (DSC) study is the suitable method to study such reactions. In the present work, five different alloy mixtures based on Ti48Al2Cr2Nb0.1B are prepared and DSC study is carried out. Onset temperature, peak temperature and completion temperature of the major exothermic reaction is analyzed at different heating rates. Further, kinetics of the reaction is studied using Johnson–Mehl–Avrami equation. Activation energy and Avrami parameter are calculated and compared with the reported works on binary alloy. It has been observed that exothermic reaction is triggered by melting of aluminium. Boron assists in increasing the enthalpy of reaction by boride formation. Primary reaction product is found to be TiAl₃. Activation energy as well as Avrami parameter is found to have marginal variation due to small change in alloying elements in different alloys and due to heating rates in the same alloy.

Highlights

► Reaction kinetics studies of Ti–aluminide alloy powder mixtures carried out. ► Five compositions studied through non-isothermal differential scanning calorimetry. ► Effect of minor boron addition and role of Ti particle size is noted. ► Activation energies using JMA equations are between 169.5 and 192.49 kJ mol⁻¹.

Introduction

As an alternate to ingot metallurgy, aluminides are synthesized through reaction synthesis (RS) route wherein the exothermic heat of reaction of elemental powders is utilized to achieve the compound formation. This powder metallurgy (PM) route is getting a lot of attention globally due to its attractive characteristics. It includes (i) absence of elemental loss/cracking/elemental segregation as prevalent in ingot metallurgy/pre-alloyed powder metallurgy (PM) route, (ii) use of cheaper elemental powders as compared to pre-alloyed powder used in other PM route, (iii) use of desired fine grained powders (iv) simple process and with the use of moderate pressures, it is possible to achieve high density near-net-shape (NNS) products.

The exothermic reaction during RS process starts at specific temperature, which has an important role in obtaining the desired phase and density in the final product. Reaction temperature and phase formation also vary with heating rates. It can be determined through differential scanning calorimetry experiments of powder samples. It has been reported that the trialuminide of Ti, i.e. TiAl₃ is the first phase to form and this phase governs the transformation of other Ti rich aluminides [1]. It is also reported that TiAl₃ forms at the expense of the pre-existing Ti₃Al or TiAl phases in the presence of excess aluminium. It indicates its kinetic and thermodynamic stability [2], [3].

Other investigations also revealed that the desired phase of γ-TiAl was formed in the Ti/Al multilayer after the TiAl₃ phase reached a certain thickness and if all the aluminium was consumed [1], [2], [3], indicating that γ-TiAl was the second phase to form in the Ti aluminide system followed by the formation of α₂-Ti₃Al and TiAl₂, which grew together with TiAl₃ and γ-TiAl. In general, it can be inferred that all these sequences of intermetallic phase formation are governed not only by the thermodynamics of the alloy system but also by the kinetics of the reaction process, especially in the early stage of the phase reaction, which results in phase selection in the intermetallic formation process [4]. Therefore, analysis of kinetics of aluminide formation is very important for selection of optimum reaction conditions during sintering of powder compacts such that the synthesis process can be made to go to its completion/partial completion depending on the requirement. This study also helps in deciding on the required heating temperature and heating rate for the reaction.

It has been reported that kinetic steps involved in the formation of the aluminide include two important factors (a) exothermic chemical reaction at the Ti/liquid Al interface leading to the formation of TiAl₃ and (b) flow of fragmented particles away from the reaction interface due to thermal currents inside liquid Al. In step (a), for continuous chemical reaction to occur, supply of aluminium to the reaction site by diffusion through the already formed TiAl₃ is essential. This is possible only by diffusion of aluminium through TiAl₃ to the Ti/TiAl interface. In view of this, activation energy for the formation of TiAl₃ is equal to the sum of activation energies for chemical reaction between Ti and Al and for diffusion of Al/Ti through the TiAl₃ layer [1]. In the present work, activation energy of TiAl₃ formation has been discussed in detail, which provides adequate indication of reaction.

Reaction synthesis process can be pressure assisted as well as pressureless. For the pressure assisted RS, reaction kinetics is expected to be accelerated due to closer contact of reacting powders/phases and so reaction kinetic study of pressureless system shall provide the input for both types of processes. Kinetics study of Ti aluminide system has been reported [5], [6], [7]. However it is either limited to binary system or carried out with solid state system [1]. Therefore, study of reaction kinetics of alloys under study is found to be essential, which shall provide guideline for selection of reaction synthesis parameters and also confirm the phase formation sequence.

Considering this, kinetic studies under pressureless condition have been selected for various combinations of alloy powder mixture around the composition of Ti48Al. Thorough analysis of exothermic reaction with addition of specific alloying elements to binary alloy (Ti48Al, at%) has been carried out. Attempt has been made to understand the mechanism of aluminide formation, where exothermic reaction is the basis for aluminide formation.

Several methods have been attempted to study the reaction kinetics and mechanisms of self-propagating high temperature synthesis (SHS) processes [8], [9], [10], [11], [12], [13], [14], [15]. High temperature thermoanalytical methods (such as DTA, DSC, etc.) have been preferred due to their advantages on experimental easiness with large data generation ability. In thermoanalytical methods, either temperature of the system is maintained constant (isothermal) or heating rate of sample is kept constant where temperature of the system varies (non-isothermal). Among these two, non-isothermal method is preferred for systems like Ti–aluminide where heat is generated due to exothermic reaction and where maintenance of isothermal condition is difficult.

In the present work, kinetics of reaction has been studied through analysis of non-isothermal DSC at various heating rates. Activation energy and kinetic parameter ‘n’ for aluminide formation are obtained using the Johnson–Mehl–Avrami (JMA) equation.

Section snippets

Experimental

The reaction synthesis of aluminides of titanium–aluminium system is exothermic in nature and hence DSC is a suitable technique to study the reaction and kinetics of the process. For this reason, all the alloy compositions are subjected to non-isothermal DSC analysis. Perkin–Elmer make heat flux differential scanning calorimeter was used, which gives output in microwatts. The DSC signal is derived from the temperature difference between the sample and an inert reference material by a

Reaction characteristics

The exothermic reaction between aluminium and titanium results in the formation of various phases like TiAl₃, TiAl₂, TiAl and Ti₃Al starting from aluminium rich side of the Ti–Al phase diagram [15], where primary phase is TiAl₃. This has been further confirmed by XRD. Since TiAl₃ is the major phase, the reaction kinetics is assumed to be controlled by this reaction and therefore, kinetic parameters have been obtained for this phase and the same has been discussed in the present paper.

Five

Conclusions

1.
Reaction kinetics studies of Ti–aluminide alloy powder mixture has been carried out through non-isothermal differential scanning calorimetry (DSC) experiments at four heating rates i.e. 10, 20, 30 and 40 K min⁻¹. Formation of TiAl₃ phase along with minor phases like, TiAl₂ and TiAl is noted through X-ray diffraction (XRD).
2.
Change in reaction enthalpy of alloys is observed due to addition B and also with change in heating rates.
3.
Onset temperature of reaction for all the alloys is found to be just

Acknowledgements

Authors are thankful to IIC, IIT Roorkee for characterization support, and Director, VSSC for granting permission to publish this work.

References (45)

D. Yang et al.
Intermetallics
(2009)
X. Wang et al.
Mater. Sci. Eng. A
(1994)
J.B. Holt et al.
Mater. Sci. Eng.
(1985)
F. Stein et al.
Acta Mater.
(2001)
M. Nassik et al.
J. Alloy Compd.
(2003)
O. Kubaschewski et al.
Acta Met.
(1955)
A.S. Ramos et al.
Surf. Coat. Technol.
(2005)
T.J. Jewett et al.
Mater. Sci. Eng. A
(1997)
S. Divinski et al.
Intermetallics
(2006)
Chr Herzig et al.
Intermetallics
(2001)

M. Huang et al.

J. Alloy Compd.

(2005)

J.M. Criado et al.

Acta Metall.

(1987)

A.A. Joraid

Thermochim. Acta

(2005)

M. Lamirand et al.

Scr. Mater.

(2007)

Y. Mishin et al.

Acta Mater.

(2000)

M. Sujata et al.

J. Mater. Sci. Lett.

(2001)

G. Lucadamo et al.

J. Appl. Phys.

(2002)

R. Bormann

Mater. Res. Soc. Symp. Proc.

(1994)

H.Y. Sohn et al.

J. Mater. Sci.

(1996)

Bhanu Pant et al.

Trans. IIM

(2007)

N. Akhtar et al.

J. Mater. Sci.

(1996)

V.A. Knyazik et al.

J. Therm. Anal.

(1993)

Cited by (7)

Pore formation mechanism and intermetallic phase transformation in Ti-Al alloy during reactive sintering
2023, Journal of Materials Research and Technology
The mechanism of pore formation associated with intermetallic reaction is studied in sintered Ti₅₀Al₅₀ alloy with two different heating rates i.e. 1 °C/min and 10 °C/min. The heating rate significantly influences the pore parameters and the intermetallic reactions. The pore size and porosity increase with the extent of exothermic reactions during sintering. The intermetallic Ti₂Al₅ phase forms by a low-temperature solid-state diffusion of Al at the surface of Ti skeletons, while TiAl₃ and TiAl₂ intermetallic phases form at a high temperature owing to Ti₂Al₅ phase reaction with Al and Ti, respectively. A considerable change in thermal and volume expansion take place in the alloy heated with 10 °C/min compared to 1 °C/min. Therefore, it is found that the heating rate during sintering greatly influences the porous properties of Ti₅₀Al₅₀ by controlling the degree of exothermic reactions.
Microstructure and tribological properties of laser in-situ synthesized Ti<inf>3</inf>Al composite coating on Ti-6Al-4V
2020, Surface and Coatings Technology
Citation Excerpt :
With the increasing of temperature, Al powders were melted and fully cover the surface of Ti powders. When the temperature reaches to 762 °C, the molten Al reacted with Ti powders to form the γ (TiAl) phase [25], which corresponded to the first exothermic peak of 762 °C in DSC curve. The second endothermic peak at 1376 °C corresponded to the formation of α2 phase by a peritectic reaction [26].
Titanium intermetallics are highly significant to protect surfaces of lightweight structures against corrosion and wear. In this study, Ti and Al powders were uniformly mixed with an atomic ratio 65:35 and in-situ synthesized on Ti-6Al-4V substrate by laser cladding with Ar cooling. The microstructure and high temperature tribological behavior were studied. The results show that Ti₃Al (α₂) phase was first precipitated in the form of coarse primary dendrites during cladding solidification, and subsequently α-Ti (α) phase of needle-shaped martensite was formed by fast cooling. The 6:4 ratio of phase α₂ to α of the composite coating provides optimal toughness and high hardness (680 HV). Under the testing up to 500 °C, the average friction coefficient of the composite coating is 0.22 ± 0.011 and a 35% lower wear rate than that of Ti-6Al-4V substrate.
Development of ductile γ+α<inf>2</inf> titanium aluminide through ingot metallurgy route, thermomechanical processing and characterization
2017, Materials Science and Engineering: A
Citation Excerpt :
Unlike reaction synthesized alloy samples [30], DSC result (Fig. 3) does not show any specific peak pertaining to melting reaction confirming absence of segregation of low melting compound in the alloy. It indicates further thermomechanical processing, can be carried out at desirable temperature [30,31] without long time homogenization normally carried out to homogenize the segregated/ heavily alloyed alloys having low melting compounds. Further CALPHAD data on similar alloy was used to finalize the hot working temperature which was selected in the range of 1200–1100 °C considering the better workability of alloy near to the single phase α to α-γ region (Fig. 3b) [32].
γ+α₂ Titanium aluminide (Ti-28Al-10Nb-3Cr-0.02B wt%) was processed through vacuum arc remelting (VAR) route using Ti sponge and pure alloying elements. Electrode for VAR melting which was designed to obtain uniform chemistry across the cross-section and throughout the height of the billet was realized through cold compaction. The alloy was hot worked in the temperature range of 1200–1100 °C. Tensile properties of the alloy at ambient temperature were evaluated at different strain rates ranging from 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹. The alloy exhibited ultimate tensile strength in the range of (386–553 MPa) when tested at ambient temperature and strain rates of 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹, whereas it varied from 630–673 MPa at intermediate strain rates of 10⁻³ s⁻¹ and 10⁻² s⁻¹. Percentage elongation is found to be 4.87–5.49% at strain rates of 10⁻⁴ s⁻¹ and at 10⁻¹ s⁻¹ whereas it showed a relatively higher % elongation (5.42–7.19%) at intermediate strain rates of 10⁻³ s⁻¹ and 10⁻² s⁻¹, indicating intermediate strain rates are better for optimum strength as well as for ductility.
The alloy is found to have predominantly two phases γ (TiAl) and α₂ (Ti₃Al) with moderately random texture and presence of small volume fraction of β-phase. Microstructure showed presence of varying width of α₂+γ lamellar structure. Transmission electron microscopy (TEM) confirmed presence of extensive twinning, dislocations and stacking faults with two phase structure indicating twinning has played a significant role in deformation. Presence of minor amount of β phase has been found to be contributing factor for improvement in ductility. Fracture analysis shows failure was mainly by rupture of lamellar interfaces. Occasional deformation of lamellae is also noticed in the uniaxial tensile tested samples at intermediate strain rates of 10⁻³ s⁻¹ and 10⁻² s⁻¹ wherever small increase in ductility is observed. Marginally lower ductility at lower strain rate is due to growth of microcracks at interlamellar sites of dissimilar orientation and at higher strain rate due to faster growth of microcracks at lamellae of similar orientation.
Effect of Cr and Nb on the phase transformation and pore formation of Ti-Al base alloys
2017, Journal of Alloys and Compounds
Citation Excerpt :
That is consistent with the previously obtained values by Wang et al. [11] (149 kJ/mol) and Sina et al. [12] (114 kJ/mol) for TiAl3 formation. Gupta et al. [13] have proposed that the diffusion of Al has the prime role in the TiAl3 forming reaction, based on the consideration of the activation energies. A large amount of heat, caused by the exothermic reaction, increases local temperature and eventually induces local melting of Al.
The phase transformation and its relation to the porosity of Ti-Al base alloys are investigated in order to understand the effect of composition on the pore formation during sintering at low and high temperatures. Ti-Al base alloys with nominal compositions of Ti₅₂Al₄₈, Ti₅₀Al₄₈Cr₂, Ti₅₀Al₄₈Nb₂ and Ti₄₈Al₄₈Cr₂Nb₂ are prepared by the conventional elemental powder metallurgy route. The sintering of mixed powders was carried out at 600° C (low-temperature sintering below the Al melting point) and 1200° C (high-temperature sintering). Al-rich intermetallic compound TiAl₃ was primarily formed during low-temperature sintering while Ti-rich intermetallic compound Ti₃Al was formed during high-temperature sintering. The phase transformations increase the overall porosity and pore size of alloys to a similar extent regardless of the alloy composition during the low-temperature sintering. However, large differences in the pore size and pore volume distribution occur depending on the composition of Ti-Al alloys during high-temperature sintering. Chromium and niobium do not affect the formation of Kirkendall voids and thus show a similar void formation behavior at 600° C, but affect the void agglomeration behavior to a different extent due to the different diffusion activities at 1200° C.
Inorganic Salt Catalysed Hydrothermal Carbonisation (HTC) of Cellulose
2022, Catalysts
Precipitation of Carbides and Dissolution of Widmanstätten Structure for Enhanced Hardness in Ti <inf>2</inf> AlNb-Based Alloys
2019, Journal of Materials Engineering and Performance

View all citing articles on Scopus

View full text

Differential scanning calorimetry and reaction kinetics studies of γ + α2 Ti aluminide

Abstract

Highlights

Introduction

Section snippets

Experimental

Reaction characteristics

Conclusions

Acknowledgements

Intermetallics

Mater. Sci. Eng. A

Mater. Sci. Eng.

Acta Mater.

J. Alloy Compd.

Acta Met.

Surf. Coat. Technol.

Mater. Sci. Eng. A

Intermetallics

Intermetallics

J. Alloy Compd.

Acta Metall.

Thermochim. Acta

Scr. Mater.

Acta Mater.

J. Mater. Sci. Lett.

J. Appl. Phys.

Mater. Res. Soc. Symp. Proc.

J. Mater. Sci.

Trans. IIM

J. Mater. Sci.

J. Therm. Anal.

Differential scanning calorimetry and reaction kinetics studies of γ + α₂ Ti aluminide