Microstructure Characterization and Mechanical Properties of In Situ Synthesized Ti₂AlN/Ti48Al2Cr2Nb Composites

With moderate content Ti2AlN (3 vol%), Ti2AlN/4822 composite prepared by in situ reactive arc-melting exhibits higher compression strength and better ductility than unreinforced 4822 alloy. The excellent properties are attributed to the introduction of Ti2AlN, which enhances the matrix, significantly refines the grain size, and decreases the dendritic segregation of the composites. Ternary nitride Ti2AlN is introduced into TiAl matrix to fabricate TiAl based composites, which is considered as an effective way to improve the high temperature strength and the room temperature ductility of TiAl based alloys. In this paper, Ti2AlN/Ti48Al2Cr2Nb (4822) (in at%) composites were prepared by in situ method, reactive arc-melting. By comparison with 4822 alloy, the influence of Ti2AlN on the microstructure and mechanical properties of the composites were investigated. The results show that the in situ introduced Ti2AlN greatly refines grains size and reduces microsegregation of the composites. With moderate content Ti2AlN, the composites exhibit excellent comprehensive property. TiAl-based alloys are particularly attractive for aerospace engines and automobile components due to their good combination of high strength-to-density ratio and excellent high-temperature creep and oxidation resistance.1-3 However, low ductility (<2%) and fracture toughness at room temperature, lack of high temperature strength and oxidation resistant over 800 °C are the main obstacles to practical application of γ-TiAl based alloys.4 In order to improve the properties of TiAl alloys, methods such as composition optimization, microstructure control and composite strengthening are considered and applied. Thereinto, composite technology is regarded as an effective method to provide both suitable ductility and toughness at ambient temperatures and excellent high temperature strength.5 In recent years, as the emergence of in situ production applied in intermetallic matrix composites, various reinforcements are introduced into TiAl matrix, such as TiB2, Ti5Si3, Al2O3, Ti2AlC, and Ti2AlN.6-9 Among these, ternary nitride Ti2AlN exhibits both metallic and ceramic properties, such as low density, high elastic modulus, easy machinability and high-temperature oxidation resistance, and a better structure stability than Ti2AlC.10, 11 Moreover, the density and thermal expansion resistance of Ti2AlN (4.31 g cm−3, 8.8 × 10−6 K−1) approach to those of TiAl (3.8 g cm−3, 12 × 10−6 K−1), which make the nitride to be a promising reinforcement candidate for TiAl composites.11, 12 Until now, several in situ techniques have been used to fabricate fully dense Ti2AlN/TiAl composites, such as hot press sintering of amorphous Ti and Al powders mechanical alloyed in N2 atmosphere, combustion reaction of Ti and Al powders in N2 atmosphere.13, 14 However, the products synthesized by these methods are generally porous. Therefore, fully dense synthesis techniques, such as hot isostatic pressing reactive sintering, pressure casting and reactive arc-melting, were applied to produce Ti2AlN/TiAl composites.15, 16 Among these, reactive arc-melting technique provides distinct advantages of large scale production and cost effective. The bending strength and fracture toughness of the in situ Ti2AlN/TiAl composites prepared by reactive arc-melting were superior than that of the non-reinforced TiAl alloy.17 However, these works only involved the mechanical property of the composites and focused on Ti–Al binary matrix, few have concerned about the influence of Ti2AlN on the microstructure and properties of micro-alloyed TiAl composites. Thus, in this work, Ti48Al2Cr2Nb (4822) (in at%) was chose as the matrix and the method of reactive arc-melting was used to fabricate Ti2AlN/4822 composites. The microstructure and mechanical properties of the composites were investigated. Raw materials of Ti sponge (>99.99 wt%), pure Al (>99.99 wt%), master alloy Al–Nb (52.47 wt% Nb) and Ti–Cr (26.58 wt% Cr) were prepared for 4822 matrix, and foils of titanium (99.99 wt% purity), aluminum (99.99 wt% purity), and TiN powder (99.997 wt% purity) were used for Ti2AlN reinforcement. Three theoretical contents of 3, 6, and 9 vol% Ti2AlN were designed. To fabricate the Ti2AlN/4822 composites, firstly TiN powder was wrapped in Ti and Al foils and made into small parcels, and then they were put into the raw materials for matrix dispersedly placed in the crucible of non-consumable vacuum arc-melting (VAR) furnace. 4822 alloy was also prepared for comparison. To ensure sufficient reaction and chemical homogeneity, electromagnetic stirring was conducted and the ingots with the weight of 30 g were remelted for four times. The phase composition was determined by X-ray diffraction (XRD). The microstructure characteristics were characterized by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). The SEM observations were carried out in back-scattered electron (BSE) mode. Samples for SEM were prepared by standard metallographic polishing procedures and etched by Kroll's solution. The compressive test was conducted at room temperature with a nominal strain rate of 1 × 10−4 s−1 and compressive specimens were prepared as a dimension of Ø 4 × 8 mm. Figure 1 shows the XRD patterns of 4822 alloy and Ti2AlN/4822 composites with different content of Ti2AlN. 4822 alloy mainly consists of γ-TiAl and α2-Ti3Al phase. In 3 vol% Ti2AlN/4822 composite, only γ and α2 can be detected. When Ti2AlN content increases to 6 vol%, the diffraction peaks of Ti2AlN appear and their intensities increase with the increase of Ti2AlN content. No traces of starting materials, especially TiN, were detected, which indicated the Ti2AlN/4822 composites could be fabricated in situ by the method of reactive arc-melting successfully. It should be note that Ti2AlN was not found in 3 vol% Ti2AlN/4822 composite, which is related to certain solubility of nitrogen in α2 phase in TiAl alloys and too little Ti2AlN to be detected by XRD with limited precision. Figure 2 is the optical micrographs of 4822 alloy and Ti2AlN/4822 composites. The microstructure of 4822 alloy shows coarse columnar dendrites with a width of ∼0.3 mm and a length about 2∼5 mm (Figure 2a) while Ti2AlN/4822 composites exhibit fine grain microstructure (Figure 2b–d). The microstructure characteristics of 4822 alloy and Ti2AlN/4822 composites were studies in detail by SEM in BSE mode, as shown in Figure 3. The microstructure of 4822 alloy consists of bright α2 + γ dendritic arm, dark interdendritic γ phase and some white networks distributing in dendrite caused by segregation of Nb and Cr during solidification (Figure 3a). The microsegregation is manifested as Al content with 45.5% in α2 + γ dendrites and 49.15% in interdendritic γ. In Ti2AlN/4822 composites, the matrix exhibits the characteristics of near lamellar microstructure and the reinforcement particles with rodlike or irregular shape are dispersed in the matrix without any orientation relationship with matrix (Figure 3b–d). Compared with the 4822 alloy, the average grain size of the matrix reduced to 31.7 μm for 3% Ti2AlN/4822 composite, and slowly decreased from 30.8 to 26.8 μm as the Ti2AlN content increased from 6 to 9%. In the matrix of Ti2AlN/4822 composites, as shown in Figure 3b, two different regions can be found: dark regions (A) and gray regions (B). The EDS analysis was conducted to reveal composition of the two regions and the reinforcement particles. The results are listed in Table 1. According to the result of EDS, it can be inferred that the dark regions with high Al content are interdendritic γ and the bright regions with low Al content are dendrite arms. Dendritic segregation showing up as aluminum contents for the matrix of Ti2AlN/4822 composites is less significantly than that for the 4822 alloy. This means that the degree of dendritic segregation reduced with the addition of Ti2AlN in the matrix of Ti2AlN/4822 composites. In the Ti2AlN/4822 composites, Ti2AlN reinforcements mainly distributed in the regions of bright dendrite arm, which indicated that the nitrides would first precipitated from the melt and be nucleation cores during solidification of the composites. According to the previous studies, the solidification and solid state transformation in 4822 alloy can be summarized as: L → β + L → β + L + α → L + α → α → α + γ → α2 + γ.18, 19 When the peritectic transformation finishes, the interdendritic Al-enriched liquid phase caused by dendritic segregation reacts with α to produce γ phase. These γ phases will remain without solid transformation during the following cooling process. In the composites, Ti2AlN would be first synthesized and precipitate from melt prior to α phase, and be the nucleation cores of β phase primary dendrite during melting and solidification, which can be manifested in Figure 3b–d, Ti2AlN mainly distributes in the bright dendrite arm. Thus, the dispersed Ti2AlN refines the primary dendrite structure remarkably, and then hinders α growing up and restricts the coarsening of subsequent α2 + γ colony grains. Therefore, grain refinement can be achieved in dendrite arm regions in the Ti2AlN/4822 composites. Figure 4 shows the compression stress–strain curves of 4822 alloy and Ti2AlN/4822 composites. Both stress–strain curves of 4822 alloy and composites present similar characteristics: at the early stage of deformation, the flow stress increased and strain hardening was exhibited, then the stress gradually increased steadily until fracture occurred. Although with similar characteristics, the 4822 alloy and the composites display different compression properties, in Table 2. It can be found that the ultimate compression strength (σUCS) of the Ti2AlN/4822 composites is higher than that of the 4822 alloy, and 3% Ti2AlN/4822 composite possesses the highest σUCS of 2090 MPa. As for the fracture strain (ϵf), 3% Ti2AlN/4822 composite still shows the maximum value with 24.00%, while 6 and 9% Ti2AlN/4822 composites present a little lower level than 4822 alloy. The improvement of compression strength for the composites can be attributed to introduction of Ti2AlN reinforcement and refinement of colony grains of the matrix, which can be explained by the Hall–Petch equation.20 Moderate amount of Ti2AlN also has toughening effect on TiAl composites, such as crack bridging, crack deflection and particle pullout.5 Nevertheless, the composites will be embrittled as the increase of Ti2AlN content due to inherent brittleness of Ti2AlN and innerstress caused by mismatch of thermal expansion coefficient between 4822 matrix and Ti2AlN. It is known that grain refinement often improves ductility. As the increase of Ti2AlN content, the colony grains were refined greatly which would beneficial for the ductility of the composites. However, the brittle effect of Ti2AlN will play crucial role and cause poor ductility when Ti2AlN content exceeds a critical value. Combination of effect of the two aspects, 3% Ti2AlN/4822 composite possesses the best comprehensive compression properties. Microstructures and mechanical properties of Ti2AlN/4822 composites prepared by an in situ method, reactive arc-melting, were investigated. The results show that the composites exhibit a microstructure of near lamellar matrix and homogeneously distributed Ti2AlN particles. The grain size of Ti2AlN/4822 composites is significantly refined due to the introduction of Ti2AlN by refining β phase primary dendrite then hindering α growing up and restricting coarsening of subsequent α2 + γ colony grains. Dendritic segregation in the composites is less serious than that in 4822 alloy. The composites exhibit higher compression strength than 4822 alloy. With moderate content Ti2AlN, the ductility of the composites is improved obviously. 3 vol% Ti2AlN/4822 composite possesses the best comprehensive compression properties with the highest σUCS (2090 MPa) and the largest ϵf (24.00%).