Journal of Alloys and Compounds 567 (2013) 15–20

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Journal of Alloys and Compound s

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Microstructures and mechanical properties of (Ti,Cr)Al/Al2O3 compositesfabricated by reactive hot pressing from Ti, Al and Cr2O3

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.03.066

⇑ Corresponding author. Tel.: +86 029 86168688.E-mail addresses: zhujf@sust.edu.cn (J. Zhu), yangwenwen0909@yahoo.cn

(W. Yang).

Jianfeng Zhu ⇑, Wenwen Yang, Fen Wang Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science & Technology, Xi’an, Shaanxi 710021, China

a r t i c l e i n f o

Article history:Received 19 January 2013 Received in revised form 4 March 2013 Accepted 5 March 2013 Available online 21 March 2013

Keywords:TiAlAl2O3

MicrostructureMechanical properties

a b s t r a c t

Titanium aluminide composites reinforced with in situ formed Al2O3 were fabricated by a reactive hot press method, using Cr2O3, Al and Ti powders as raw materials. The effect of Cr2O3 addition on the micro- structures and mechanical properties of the (Ti,Cr)Al/Al2O3 composite was characterized. The results show that the TiAl matrix is refined gradually with increasing the amount of Cr2O3. The Vickers-hardness and densities of the (Ti,Cr)Al/Al2O3 composite increase gradually with increasing Cr2O3 content. When the Cr2O3 content is 4.47 wt.%, the compressive strength, flexural strength and fracture toughness ofthe composite reach the maximum values of 1032 MPa, 634 MPa and 9.78 MPa m1/2, which are substan- tially higher than those of the monolithic TiAl based alloy by about 18%, 78% and 80.5%, respectively. The increase in mechanical properties for the (Ti,Cr)Al/Al2O3 composite is due to the toughening effect arising from the combination of Cr solid solution in TiAl matrix and in situ formed Al2O3.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

TiAl based intermetalli c compound has been considered to beone of the best candidate materials both for ambient and elevated temperature applicati ons due to its excellent properties such aslow density, high hardness and attractive oxidation resistance athigh temperature s [1–3]. However, such a compound suffers from limited ductility and fracture toughness at room temperature ,which hindered its broader practical applicati ons as a light-weigh tstructural material.

Considerable efforts have been made to improve the low temper- ature ductility and fracture toughness through the methods of com- position optimization and microstructure control [4–9]. Effects ofNb, V, Cr, Si, and B on the microstructur e and properties of TiAl alloys have been studied. Their results show that a significant improve- ment of the room temperature ductility and fracture toughnes s ofTiAl alloy were achieved by microstructur e control and alloying with ternary elements. Moreover, the previous works revealed that some elements such as Cr, Mn and Nb could also improve the oxidation resistance of TiAl [10].

On the other hand, the development of TiAl matrix composites is an alternative promising way to improve the mechanical proper- ties of TiAl intermet allics. Particulates such as TiB, TiC, TiN and Si3N4 reinforced composites have been studied due to their isotro- pic property, amenability of component forming and low cost [11].

Among them, it was also reported that Al2O3 was identified as acompatib le and thermochem ically stable reinforcing phase for TiAl matrix attributed to its excellent chemical stability, high hardness,high modulus and excellent thermo-mec hanical behavior [11–17].These investiga tions reveal that the existence of secondary-p hase Al2O3 particles can hinder the movement of the grain boundari es,restrain the growth of grains, and improve the mechanical proper- ties. Nevertheless, the investiga tions on reinforcing the TiAl inter- metallic by combination of solid solution and incorporation ofsecondar y ceramic phase are scarce.

The present study combined the idea of both solid solution with Cr and composite strengthene d by Al2O3. These TiAl based compos- ites were produced by hot-press-assisted in situ reaction synthesis method with elemental powder mixtures of Ti, Al and Cr2O3 basedon the thermite reactions between Al and Cr2O3. The effect of the Cr2O3 addition on the microstru cture and mechanical propertie sof the resultant products was investiga ted in detail.

2. Experimental

Commercially available Al (280 mesh, 99.3% purity), Ti (280 mesh, 99.3% purity)and Cr2O3 (500 mesh, 99.5% purity) powders were used as the starting materials for ball milling. The following mixtures were prepared according to Table 1. The mix- ture powders were ball milled in alcohol for 1 h and dried less than 40 �C for several hours. Then the as-milled powders were compacted uniaxially under 10 MPa in agraphite mold coated with BN inside. The compacted powders were firstly slowly heated to 900 �C in vacuum (�10�2 Pa), held for 1 h. And then the compact was heated to 1300 �C at a rate of 5 �C/min under pressure of 17 MPa and held for 2 hwith the pressure maintained. Finally, the sample was cooled down to room tem- perature. The surface layer of the samples was machined off to remove contami- nants prior to characterization.

Table 1The compone nt of the samples (wt.%).

Ti Al Cr2O3 Theoretical target Al2O3 content

T0a 64 36 0 0T1 62.47 36.27 1.49 1T3 59.55 36.74 4.47 3T5 56.63 37.21 7.45 5T7 53.71 37.68 10.43 7T12 46.40 38.85 17.89 12T15 42.02 39.56 22.36 15

a TX refers to the sample number, and X is the targeted weight percent of Al2O3

phase.

Table 2The calculated lattice parameters of TiAl phases in the (Ti,Cr)Al/Al2O3 composites.

Specimen Calculated lattice parameters

a (Å) c (Å) c/a

T0 3.9988 4.0794 1.0202 T1 3.9870 4.0720 1.0213 T5 3.9870 4.0698 1.0207 T9 3.9772 4.0668 1.0225 T12 3.9738 4.0616 1.0221 JCPDS (65-8565) reference 3.9760 4.0490 1.0184

16 J. Zhu et al. / Journal of Alloys and Compounds 567 (2013) 15–20

The hardness of the samples was tested by using a Vickers hardness testing ma- chine (HXD-1000, Shanghai second optical Ltd., China) using a load of 9.8 N applied for 15 s. The bulk density of the as-synthesized composites was measured by the Archimedes method. The measurement of the compressive strength, flexuralstrength and fracture toughness were conducted in a universal material testing ma- chine (PT-1036PC, Perfect Instrument Co. Ltd., Taiwan China). The specimens used for compressive strength test were 4 � 4 � 8 mm3 in size, and the crosshead speed was 0.05 mm/min. Three point bending tests were used to evaluate the flexuralstrength and fracture toughness. The specimens for flexural strength testing were 3 � 4 � 30 mm3 in size. The single-edge notched beam method with dimensions of 4 � 4 � 30 mm3 were used for the fracture toughness testing. The measurement of the flexural strength and the fracture toughness were performed at a crosshead speed of 0.5 and 0.05 mm/min with a span of 24 mm, respectively. The flexuralstrength and fracture toughness were determined with the following formulas,respectively.

r ¼ 3pl

2bh2 ð1Þ

K IC ¼ Y � 3PLa1=2=ð2BW2Þ ð2Þ

where P is the breaking load of the specimen, L, b, h, Y, and a are the span, width,height, form factor, and notch depth of the specimen, respectively. The final values of hardness, density, compressing strength, flexural strength and fracture toughness were calculated by averaging five individual measurements.

The phase composition of the TiAl based composites was identified by the X-ray diffraction analysis (XRD, D/max 2200PC, Rigaku, Japan) with Cu Ka radiation oper- ating at 40 kV. The surfaces and fracture surfaces of the samples were examined in ascanning electron microscope SEM (SEM, JSM-6700, JEOL Ltd., Japan) equipped with energy dispersive spectroscopy (EDS).

3. Results and discussion

3.1. Microstructu re

The XRD patterns of TiAl monolith ic and its in situ composites hot pressed at 1300 �C for 2 h with different amounts of in situ

Fig. 1. XRD patterns of (Ti,Cr)Al/Al2O3 composite hot p

formed Al2O3 are given in Fig. 1. As shown in Fig. 1, all the starting materials appear to have undergone complete reactions, and noany initial materials existed in the resultant materials. The single monolith ic TiAl was fabricated with the raw materials of Ti64–Al36 (mole ratio of 1:1), which is consistent with the Ti–Al phase binary diagram, and the reaction could be expresse d as the follow- ing equation:

Tiþ Al! TiAl ð3Þ

When the starting materials contain 1.49 wt.% (the target Al2O3

content is 1.0 wt.%) (Fig. 1b), the product contains both TiAl and Ti3Al, with the former one as the major compound. The previous work reported that the thermit reaction Al and Cr2O3 could pro- duce Cr and Al2O3 [18], and Cr would further replace some Ti toform (Ti,Cr)Al solid solution, which would alter the ratio between the Ti and Al, subsequent ly the excess Ti would form minor Ti3Alphase. However, Al2O3 was not clearly detected due to the fact that its amount might be less than the XRD detecting level. Meanwhi le,it is to be noted that the intensity of TiAl is evidently lowered and broadened , suggesting that the TiAl matrix is refined obviously.

When increasing the Cr2O3 amount to 4.47 wt.% (3.0 wt.% Al2O3)(Fig. 1c), the (Ti,Cr)Al/Al2O3 composite consists of the resemble phases as the product contains 1.0 wt.% Al2O3 phase, which further testify the occurrence of the thermit reaction between Al and Cr2O3.When increasing the Cr2O3 amount from 4.47 to 10.43 wt.%, the intensity of the Al2O3 diffraction peaks increases gradually, while the diffraction peaks of TiAl broaden and their intensities decrease obviously, suggesting that the average crystallite size of the matrix TiAl is further reduced. It can also be found from the right of Fig. 1that the reflection peaks of TiAl shift to large angles with the incre- ment of Cr2O3 in the initial constituents, which is attributed to the decrease of lattice parameters. As presented in Table 2, lattice con- stant a changes from 3.9938 to 3.9738 Å and lattice constant c

ressed at 1300 �C with different contents of Cr2O3.

Fig. 2. SEM-EDS spectra of the (Ti,Cr)Al/Al2O3 composite (with Cr2O3 of 4.47 wt.%) hot pressed at 1300 oC.

J. Zhu et al. / Journal of Alloys and Compounds 567 (2013) 15–20 17

decreases from 4.0794 to 4.0616 Å. The decrease of the lattice parameters is ascribed to the smaller size of 0.0615 nm for Cr3+ than0.067 nm for Ti3+ as formatio n of (Ti,Cr)Al solid solution. However ,when the Cr2O3 amount was further increased to 22.36 wt.%(15 wt.% Al2O3) (Fig. 1f), the Ti3Al disappea red accompanyi ng with the appearance of Cr-containin g phases (Cr2Al and Cr5Al8). It is rea- sonable for the formation of the Cr-containin g phases with the increasing of Cr amount to over its solid solution level in TiAl, and subsequent ly to the disappearan ce of Ti3Al phase.

Fig. 2 presents the polished surface morphology and the SEM–EDS analysis of the (Ti,Cr)Al/Al2O3 composite with the Cr2O3

amount of 4.47 wt.% (T3 sample). It can be seen that the product possesses a dark matrix and white fine particles. The results ofEDS chemical analysis combined with the XRD results (Fig. 1)prove that the dark phase is TiAl solid solution. Meanwhile, the bright phase contains only Al and O with an atomic composition of 66.02:33.98. Combined with above XRD results (Fig. 1), it can conclude that they are Al2O3 particles . The mole ratio of Al and Ois discrepancy from the stoichiometric ratio 2:3 for Al2O3 becausethe O element is too light to be analyzed . The Al2O3 particles dis- perse randomly throughout the material.

Fig. 3 shows the SEM surface and fracture micrographs of the TiAl monolithic and (Ti,Cr)Al/Al2O3 composite with different Al2O3 contents . It can be seen that the main phases in the compos- ites consist of primary blocky TiAl matrix and granular Al2O3, and the microstru ctures of composites are affected greatly by the amount of the in situ formed Al2O3. It is apparent that grain size of TiAl based composite with 3 wt.% Al2O3 (Fig. 3c) is obviously smaller than that of TiAl monolithic phase. The fine Al2O3 particlesdisperse uniformly with a size less than 2 lm mainly located at the TiAl grain interfaces (Fig. 3c1), which hinder the growth of the TiAl matrix grains and could improve the mechanical properties of the composites. But when the Al2O3 content further increases, i.e. TiAl/ 12 wt.% Al2O3 composite, agglomerati on of the Al2O3 particles isevidently observed, as shown in Fig. 3d and d1.

3.2. Mechanical properties

Fig. 4 shows the density and Vickers hardness of (Ti,Cr)Al/Al2O3

composite specimens measured at room temperature as a function of Al2O3 amount. The density of the (Ti,Cr)Al/Al2O3 composite

measure d by the Archimedes method increases from 4.21 g/cm 3

to 4.86 g/cm 3 as the Cr2O3 amount increases from 0 to 15 wt.%.These are likely due to the fact that the amount of Cr containing phases increases with the increase of Cr2O3 in the initial materials,which have much higher densities (5.592 g/cm 3 for Cr2Al and 4.208 g/cm 3 for Cr5Al8) than TiAl (3.86 g/cm 3) and Al2O3 (3.97 g/cm3). The increase of hardness is mainly attributed to the higher hardness of the in situ formed Al2O3. In addition, the formation of the Cr-containing phase closed a part of pores, which also leads to an increase in the density and hardness.

Fig. 5 shows the compression strength–displacement curves ofthe (Ti,Cr)Al/Al2O3 composite with different Cr2O3 amounts. It can be seen that the compression strength, ultimate compress ion strength and fracture strain of the (Ti,Cr)Al/Al2O3 composite are all obviously higher than those of the monolithic TiAl. And the com- pression strength peaks at 4.47 wt.% Cr2O3 addition (3 wt.% Al2O3)with the maximum value of 1032 MPa. However, the compress ive strength decreases as the Al2O3 content further increases to7 wt.% Al2O3 content. It is interesting to note that in the compres- sive tests, the failure of the all composites is noncatastroph ic, and exhibits obvious yield points. The transition from elastic to plastic deformat ion indicates that dislocations played an important role during the deformation [19].

The flexural strength and fracture toughness of (Ti,Cr)Al/Al2O3

composite hot pressed at 1300 �C for 2 h with different Al2O3 con-tents are plotted in Fig. 6. It can be seen that the flexural strength and fracture toughness both increase first and then decrease with increasing Cr2O3 amount of the initial materials. When the Al2O3

amount is 3 wt.%, the flexural strength and fracture toughness reach the highest values of 634 MPa and 9.78 MPa m1/2, which are improved by 78% and 80.5%, compared with those of mono- lithic TiAl, respectively .

The mechanical propertie s of the (Ti,Cr)Al/Al2O3 composite fab- ricated in the present work are higher than those reported previ- ously [20]. Typical properties of (Ti,Cr)Al/Al2O3 composite are summari zed in Table 3 for comparison, the previousl y tested sam- ples containe d more excess Al2O3 content, and agglomeration ofAl2O3 becomes serious in TiAl/Al 2O3 composites, which resulted in a decrease of the mechanical properties.

The typical crack propagation patterns of the (Ti,Cr)Al/Al2O3

composite are given in Fig. 7. The intergranula r, transgranul ar

Fig. 3. SEM images of surface and fracture of (Ti,Cr)Al/Al2O3 composite with different contents of Al2O3 (a1 and a) 0 wt.%; (b1 and b) 1 wt.%; (c1 and c) 3 wt.%; (d1 and d)12 wt.%.

18 J. Zhu et al. / Journal of Alloys and Compounds 567 (2013) 15–20

and delaminating fractures together with crack-bridg ing, branch- ing and deflection are coexisted. The strengthening effects of these composites are credited to a combination of solid solution and incorporation of second phase of Al2O3. First, the in situ formed Al2O3 particles with high hardness results in a higher deformation resistance and residual stress tougheni ng. The tougheni ng of the composite is a result of diminishing the residual compressive stress intensity at the tip of the deflected crack. Secondly, the fineprecipitated Al2O3 particles hinder the growth of the TiAl matrix grains by locating at the TiAl grain interfaces (Fig. 3), the grain refinement not only enhances the strength, but also improves the toughness. Thirdly, the fine colony grain strengthening plays an

important part in strengthening the composites, which can beestimate d by Hall–Petch equation [21]. With the increasing ofAl2O3 content, both the matrix and reinforce ment size decrease obviously, and the volume fraction increase gradually , resulted inthe strength increasing.

In addition, the previous investigatio ns have revealed that solid solution of Cr in TiAl could modify its chemical composition and the valence electron concentr ation (VEC) by substituting Cr for Ti, because of the smaller size and one more valence electron ofCr. These enhanced the weak bond of metal and Al, and thus mod- ify the microstructure of the composites , which could result inhigher mechanical propertie s [18]. In this study, Cr was produced

Table 3Summary of Typical properties of (Ti,Cr)Al/Al2O3 composite(with 3 wt.% Al2O3),together with those from [20] for comparison.

Properties TiAl–Al2Ti4C2–Al2O3–TiC (Ti,Cr)Al/Al2O3

Source [20] This work Density (g cm�3) – 4.45 Hardness (KN mm�1/2) – 3.75 Flexural strength (MPa) 236 634.6 KIC (MPa m1/2) 5.9 9.79

Fig. 4. Vickers hardness and density of (Ti,Cr)Al/Al2O3 composites.

Fig. 5. Compression strength of (Ti,Cr)Al/Al2O3 composites with different Cr2O3

contents.

Fig. 7. Typical crack propagation patterns of the (Ti,Cr)Al/Al2O3 composite (3 wt.%Al2O3).

Fig. 6. Flexural strength and fracture toughness of (Ti,Cr)Al/Al2O3 composites.

J. Zhu et al. / Journal of Alloys and Compounds 567 (2013) 15–20 19

by the reaction between Al and Cr2O3 and dissolved in TiAl to form (Ti,Cr)Al solid solution, and as a result, its mechanical properties were reinforced.

4. Conclusion s

(Ti,Cr)Al/Al2O3 composites were fabricated from the powder mixtures of Ti, Al and Cr2O3 by reactive hot pressing method. The in situ formed Al2O3 in the TiAl composites leads to the obvious refinement of the matrix grains. The addition of Cr2O3 remarkabl yimproved the flexural strength, fracture toughness and compres- sion strength of the TiAl based composites due to the generation of Al2O3 particles and the solid solution of Cr in the TiAl matrix.When the addition of Cr2O3 is 4.47 wt.% (3 wt.% Al2O3), the compos- ite possesses the highest flexural strength, fracture toughness and compress ion strength of 634 MPa, 9.78 MPa m1/2 and 1032 MPa,which are improved by 78%, 80.5% and 18%, respectively .

Acknowled gements

This work was supported by the National Foundation of Natural Science, China (51272145, 5117109 6), the Natural Foundation ofShaanxi Province, China (2010JM6014), Innovation Fund of Xi’an Science and Technology Program (CXY1124(3)), China, and the Graduate Innovation Fund of Shaanxi University of Science and Technolo gy.

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