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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.30 No.2 pp.57-60
DOI : https://doi.org/10.3740/MRSK.2020.30.2.57

# High-Temperature Deformation Behavior of Ti3Al Prepared by Mechanical Alloying and Hot Pressing

Chang-Suk Han1, Sung-Yooun Jin1, Hyuk-Ku Kwon2
1Dept. of ICT Automotive Engineering, Hoseo University, 201, Sandan7-ro, Seongmun-myeon, Dangjin City, Chungnam 31702, Republic of Korea
2Dept. of Environmental Engineering, Hoseo University, 20, Hoseo-ro 79beon-gil, Baebang-eup, Asan City, Chungnam 31499, Republic of Korea
Corresponding author E-Mail : hancs@hoseo.edu (C. -S. Han, Hoseo Univ.)
November 15, 2019 November 5, 2019 January 22, 2020

## Abstract

Titanium aluminides have attracted special interest as light-weight/high-temperature materials for structural applications. The major problem limiting practical use of these compounds is their poor ductility and formability. The powder metallurgy processing route has been an attractive alternative for such materials. A mixture of Ti and Al elemental powders was fabricated to a mechanical alloying process. The processed powder was hot pressed in a vacuum, and a fully densified compact with ultra-fine grain structure consisting of Ti3Al intermetallic compound was obtained. During the compressive deformation of the compact at 1173 K, typical dynamic recrystallization (DR), which introduces a certain extent of grain refinement, was observed. The compact had high density and consisted of an ultra-fine equiaxial grain structure. Average grain diameter was 1.5 μm. Typical TEM micrographs depicting the internal structure of the specimen deformed to 0.09 true strain are provided, in which it can be seen that many small recrystallized grains having no apparent dislocation structure are generated at grain boundaries where well-developed dislocations with high density are observed in the neighboring grains. The compact showed a large m-value such as 0.44 at 1173 K. Moreover, the grain structure remained equiaxed during deformation at this temperature. Therefore, the compressive deformation of the compact was presumed to progress by superplastic flow, primarily controlled by DR.

## 초록

© Materials Research Society of Korea. All rights reserved.

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Titanium aluminides (TiAl and Ti3Al) have attracted special interest as a light-weigh/high-temperature material for structural applications. The major problem limiting the practical use of these compounds is their poor ductility and formability. The powder metallurgy processing route has been attractive alternative for such materials. Mechanical alloying (MA) was proposed about scores of years ago as a technique for combining alloys.1) Many researchers have applied this technique for producing a compact of TiAl (γ) intermetallic compound, and have reported its characteristics in the structure and the mechanical properties.2-5) The purpose of the present study is to make a compact of Ti3Al (α2) by similar procedure, and examine the structure and the high-temperature deformation behavior of the compact.

## 2. Experimental Procedure

Elemental titanium and aluminum powders having an average particle size of 31 μm and 26 μm in diameter respectively, were mixed to give the starting composition of Ti-30 at.%Al which approximately corresponds to the center of the composition range of α2 phase in the Al- Ti equilibrium diagram.6) The MA by ball-milling was performed in a stainless steel chamber on argon atmosphere. The ratio of the ball (stainless steel) to powder weight was 60 to 1. The process of MA was followed by scanning electron microscopy (SEM) and X-ray diffractometer (XRD).

The compact (approximately 12 mm φ × 6 mm) was produced by vacuum hot pressing (VHP) of the MAprocessed powder at 1173 K for 1 hr under 100 MPa. Transmission electron microscopy (TEM) of the compact was performed with a JEM-2000FX equipment combined with an energy-dispersive X-ray (EDX) analyzer. Thin foils used for the TEM observations were prepared by ion milling.

Compression tests of the specimens (3 mm × 3 mm × 6 mm) machined from the compact was carried out at 1173 K. The specimens quenched during and after compressive deformation were offered to TEM observation. Compressive strain-rate-change test at l173 K was also performed for the specimen, in order to examine the high temperature deformation behavior of the compact.

## 3. Results and Discussion

### 3.1 MA process

The changes in the XRD patterns of the mixture of Ti- 30 at.%Al powder with MA processing time are shown in Fig. 1. In the as-mixed state, all the expected lines of Ti and Al elemental powders are observed. After 720 ks of MA processing, these sharp peaks have almost disappeared and a broad maximum near 2θ = 40° has appeared.

Superimposed on this broad maximum are relatively sharper Bragg peaks characteristic of the microcrystallized elemental phases. No notable changes in the XRD patterns with further increase in MA processing time up to 1800 ks were observed. The morphology and the microstructure of the powder MA-processed for 1800 ks are shown in Fig. 2. In this stage, the shape of the particles of the MAprocessed powder become nearly spherical (average particle size: 25 μm in diameter) as seen in Fig. 2(a). The microstructure of the powder particles typically shown in Fig. 2(b) presumably consists of a mixture of amorphous and microcrystallized elemental phases coincident with the results of the XRD.

### 3.2 Structure of the compact produced by VHP

A compact having relative density of 99.8 % was obtained. The results of the XRD measurement shown in Fig. 3 indicate that the compact consists of α2 single phase. This fact was also supported by these results of the TEM observation combined with the EDX analysis, as seen in Fig. 4. At the same time, the TEM micrograph shown in Fig. 4(a) indicates that the compact has an ultra-fine α2 grain structure. The average grain diameter is approximately 1.5 μm.

### 3.3 High-temperature deformation behavior of the compact

True stress-true strain curve obtained from the results of the compression test of the compact at 1173 K and initial strain rate of 2.8 × 10−3 s−1 was shown in Fig. 5. During the initial stages of the compressive deformation, a sharp stress peak which indicates the occurrence of a typical dynamic recrystallization (DR) is observed.7) In order to confirm such a fact, we carried out TEM observation of the specimens immediately quenched during and after the compression test.

Typical TEM micrographs depicting the internal structure of the specimen deformed to 0.09 true strain are shown in Fig. 6, where (b) is the high magnification photograph of the white square line area of (a). Many small recrystallized grains having no apparent dislocation structure are generated at grain boundaries where the well developed dislocations with high density are observed in the neighboring grains.

Fig, 7 shows typical TEM micrographs observed in the specimen deformed to 0.28 true strain. In this figure, (b) is the high magnification photograph of the white square line area of (a). A little amount of recrystallized small grains which presumably generated in the later stages of the deformation is similarly observed.

These results of the TEM observation evidently support the occurrence of DR during the deformation. Such a DR introduces a certain extent of gain refinement, which can be recognized by comparison between Fig. 4(a) and Fig. 7(a) (average grain diameter, 1.0 μm). In addition to these results, the grain structure of the specimen remains approximately equiaxed during the deformation [Fig. 7(a)]. This fact coincides with the general characteristic of the superplastically deformed alloys. The specimen of the compact was offered to the compressive strain-ratechange test at 1173 K. The relationship between true stress-true strain rate obtained from the results of the test was shown in Fig. 8. The results give the value of strain- rate sensitivity exponent, m, for the compact to be 0.44 according to the general equation:

$σ = C ε ′ m$
(1)

where σ, ε and C are true stress, true strain-rate and a constant, respectively. The relationship obtained from the similar experimental results of the specimen of TiAl compact, which showed no evidence of DR, and was confirmed to be deformed by superplastic flow controlled by grain boundary sliding,3) is also shown in Fig. 8.

The value of m for the compact (α2) of the present study is considerably larger than that of TiAl, and sufficiently satisfies a general requirement for the superplastic material, namely m > 0.3.8) Kim et al. have reported that an ultrafine grained (grain size = 1.6 μm) boron-doped Ni3Al showed m > 0.3 and exhibits superplasticity accompanied by DR during tensile deformation at 973 K.9) From these results, they have proposed that DR plays an important role for relieving a stress concentration arising from grain boundary sliding, and suppressing cavitation during the superplastic deformation.9) The similar interpretation seems to be adaptable to the deformation behavior of Ti3Al compact in the present study. Namely its compressive deformation process presumably progressed by superplastic flow primary controlled by DR.

## 4. Conclusions

A Ti3Al compact was produced by hot pressing of the mechanically alloyed powder, and compressive hightemperature deformation behavior of the compact was investigated. The results obtained can be summarized as follows;

• (1) The compact had high density and was consisted of ultra-fine equiaxial grain (average grain diameter =1.5 μm) structure.

• (2) During compressive deformation of the compact at 1173 K, a typical dynamic recrystallization (DR) which introduces a certain extent of grain refinement was observed.

• (3) The compact showed a large m-value such as 0.44 at 1173 K. Moreover, the grain structure remained equiaxed during deformation at this temperature. Therefore, the compressive deformation of the compact presumed to be progressed by superplastic flow primary controlled by DR.

## Figure

XRD patterns of Ti-30 at%Al powder ball-milled for various times.

SEM micrographs of Ti-30 at.%Al powder ball-milled for 1800 ks. (a): Shape of the powder particles. (b): Microstructure of a sectioned particle.

XRD pattern of the compact.

TEM micrograph of the compact (a) and the results of EDX analysis of the typical grains from 1 to 10 (b): Ti and Al composition of corresponding circled areas.

True stress-true strain curves at l173 K and initial strain rate of 2.8 × 10−3 s−1 by compression test.

(a) TEM micrographs of the specimen water quenched after compressive deformation to the true strain, έ = 0.09. (b) is the high magnification micrograph of the white square line area.

(a) TEM micrographs of the specimen water quenched after compressive deformation to the true strain, έ = 0.28. (b) is the high magnification micrograph of the white square line area.

True stress-true strain rate curve obtained from the results of compressive strain rate change tests of the compact at 1173 K.

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