The Influence of Cu on Combustion Synthesis Mechanism of Ti–Al Intermetallic Compounds Produced From Ti and Al Elemental Powders

ORIGINAL RESEARCH ARTICLE The Influence of Cu on Combustion Synthesis Mechanism of Ti–Al Intermetallic Compounds Produced From Ti and Al Elemental Powders MOHAMMAD JAFARZADEH, MANSOUR SOLTANIEH, and RAZIEH KHOSHHAL The influence of slight content of Cu as an additive on the reaction mechanism and microstructure of Ti–Al intermetallic compounds, during sintering process of Ti and Al elemental powders, was investigated. High-purity powders of Al, Ti, and Cu were used. The molar ratio of Ti/Al was equal to one. Considerable changes in reaction mechanism and heat of formation for different samples with different heating rates and different additive contents during combustion synthesis process were observed. The temperature was monitored by a data acquisition system or DAS. The samples were analyzed by XRD and SEM techniques. According to the experimental findings, complete formation of products was achieved at distinct additive content and heating rate. The initial reaction temperature and the samples’ porosities were lowered by the addition of Cu. The sample hardness varied depending on the heating rate and Cu content. https://doi.org/10.1007/s11661-023-07111-y  The Minerals, Metals & Materials Society and ASM International 2023 I. INTRODUCTION SINCE roughly 60 years ago,[1] Ti–Al intermetallic compounds have been promoted as one of the lightweight, high-temperature structural alloys. These alloys are excellent for a wide range of applications, including turbine power generating materials, aircraft, and automobiles, owing to their diverse physical and mechanical properties, such as low density, high specific strength, and superior creep resistance.[2,3] Low-pressure turbine blades for aircraft engines are, made of TiAl-based alloys, which are also utilized in the automobile sector for exhaust valves and turbocharger turbine wheels.[4] Combustion synthesis, also known as self-propagating high-temperature synthesis (SHS), is a straightforward, alluring, and practical process for producing inorganic materials, including advanced ceramics and intermetallic compounds.[5,6] SHS is an alternative approach to creating advanced materials compared to conventional (casting, welding, and other ceramics, MOHAMMAD JAFARZADEH is with the School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran 1684613114, Iran. MANSOUR SOLTANIEH is with the Iran University of Science and Technology (IUST), Tehran 1684613114, Iran. Contact e-mail: RAZIEH KHOSHHAL is with the Birjand University of Technology, Birjand 9717434765, Iran. Manuscript submitted November 23, 2022; accepted June 8, 2023. Article published online June 26, 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A cermets, and composites’ production) processes.[7] With this technique, the goal is to reach high-temperature materials without melting or casting the ingredients by using the heat of formation from the reactants. In comparison to the traditional approach, the SHS requires fewer equipment and does not require heat treatment.[8] However, combustion synthesis (CS) includes a sintering process in which high-porosity production ensued from high synthesis temperature, restricted probability of reaction control, and kirkendall effect because high diffusion rate of Al in the reaction with M elements (M = Ti, Fe, Ni, and Nb) is inevitable. There are two main types of combustion synthesis: simultaneous combustion synthesis, sometimes known as a ‘‘thermal explosion (TE),’’ and ‘‘self-propagating high-temperature synthesis (SHS).’’ The first approach involves heating the green mixture continuously until it reaches the ignition temperature. The whole sample then reacts synchronously at ignition temperature. This method is suitable for almost all intermetallic compounds owing to external heat applied to whole sample to initiate and continue reaction simultaneously. The second technique begins by continuously heating the sample from one side in order to start and continue reaction layer by layer and by the help of reaction heat in previous layer. It is appropriate for ceramics and intermetallic compounds with a high exothermic enthalpy because the heat of formation form the reaction generated in the externally heated layer in one side of the sample should provide enough heat to initiate reaction on the next layer. Hence, this method could not be applied on the TiAl intermetallic compounds due to VOLUME 54A, SEPTEMBER 2023—3537 low heat of formation, generated during Ti and Al reaction.[9,10] Particle size of green powders, green compact density, furnace atmosphere, heating rate, additives, etc. are some of the variables that play a role in the SHS mechanism. Fine reactant powders may be able to expand the contact surface and, as a result, close the pores.[11] The most common atmospheres for SHS reactions are vacuum, inert gas (such as nitrogen, argon, etc.), and hydrogen (to react with the oxygen in the atmosphere and lower the oxygen partial pressure). The compact density, which is affected by the compacting force, the particle size, and distribution, has a strong impact on the SHS procedure. The precombustion duration and combustion temperature are reduced when the heating rate is increased, and more complete intermediate phases are formed.[12,13] The addition of co-alloying ingredient could also expedite the SHS process and reduce the melting point of aluminum.[14] According to the Al–Cu binary phase diagram (Figure 1), 32.5 wt pct Cu in Al will cause the melting point of Al to drop from 667 C to around 548 C. Due to this, Cu could react with solid Ti particles by supplying a local molten eutectic phase (in high-additive concentration zones). The initial step of synthesis was started at a lower temperature because the reaction was generated in a liquid–solid state, which led to the ignition temperature.[15] Furthermore, additional parameters such as switching the reaction mechanism from a solid–solid to a liquid–solid state and adjusting the diffusion rate by employing the available pre-melt composition had an impact on the synthesis process.[10,16] In addition, the presence of low-temperature melt, which has started the reaction process, may cause porosity to start off small and be less distributed in the subsequent stages. Only a modest heating rate and adequate melt provided at a lower temperature might regulate the phenomenon of porosity.[17,18] The predicted phases from the thermodynamical method are depicted in Figure 2 in accordance with Fig. 2—The Ti–Al–Cu ternary system at 700 C with isothermal sections. the Ti–Al–Cu ternary system and the low additive concentration of the mixture. However, the composition of the reaction’s products is often determined by its kinetics. In this study, the impact of the Cu additive and heating rates on the following evolution of the microstructure and the change in combustion temperature was both examined concurrently. In addition, the impact of an ad (...truncated)