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Materials Letters 61 (20

In-situ production of Fe–TiC composite

Wang Jing ⁎, Wang Yisan

School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, China

Received 21 November 2006; accepted 5 February 2007Available online 14 February 2007

Abstract

A TiC/Fe composite was produced by a novel process which combines in situ with powder metallurgy techniques. The microstructure of theFe–TiC composite was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD); with the help of differential thermal analysis(DTA), the reaction path of the Fe–Ti–C system was discussed. The results show that the production of an iron matrix composite reinforced byTiC particulates using the novel process is feasible. TiC particles exhibit homogeneous distribution in the α-Fe matrix. The reaction path is asfollows: first, allotropic change Feα→Feγ at 765.6 °C; second, formation of the compound Fe2Ti at 1078.4 °C because of the eutectic reactionbetween Ti and Fe; third, reaction between carbon and melted Fe2Ti causing formation of TiC at 1138.2 °C; finally, Fe3C formation due to theeutectic reaction between remanent C and Fe at 1146.4 °C.© 2007 Elsevier B.V. All rights reserved.

Keywords: Powder metallurgy; In-situ generation; Fe–TiC composite; Microstructure

1. Introduction

Composite materials with steel matrix and ceramic particlereinforcements provide scope for producing relatively inexpen-sive wear-resistant materials. Most of the work on iron-basedcomposites has involved TiC reinforcement, which is intro-duced in the iron matrix through a powder metallurgy (P/M)route [1]. The typical advantages of the route are raw materialsavings and low energy costs. In addition, P/M technique allowsa higher content of alloying elements and the addition of theceramic particles [2]. However, the materials produced via thistechnique generally suffer from the problem of contaminatedmatrix-reinforcement interfaces. From this view point, thetechniques involving the in-situ generation of the reinforcingphase have emerged as a preferred synthesis route for thesematerials. In-situ techniques involve a chemical reaction re-sulting in the formation of a very fine and thermodynamicallystable ceramic phase within a metal matrix. As a result, thereinforcement surfaces are likely to be free from gas absorption,oxidation or other detrimental surface reaction contamination,and the interface between the matrix and the reinforcement bond

⁎ Corresponding author. Tel./fax: +86 28 85402901.E-mail addresses: wjcd1994@163.com (Wang Jing),

wangyisan2002@163.com (Wang Yisan).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.02.011

therefore tends to be stronger [3]. Some of these technologiesinclude exothermic dispersion (XD), liquid–solid or liquid–liquid reactions, and self-propagation high-temperature synthe-sis (SHS). Among the several techniques available to synthesizemetal-matrix composites, SHS and casting technologies arewidely used to produce Fe–TiC composites. However, man-ufacturing of Fe/TiC composites using SHS will encounter largedifficulties handling the intrinsic porosity during the reaction[4]. Whereas Fe/TiC composites produced by casting will con-front two difficulties: on one hand, the distribution of TiC inthe Fe matrix is likely to be uneven because of the densitydifference between Fe and TiC; on the other hand, the volumefraction of TiC is limited because of reduction of the steel liquidfluidity at high TiC levels.

In the present work, we highlight a novel process in whichtraditional powder metallurgy plus in-situ techniques were usedto produce TiC/Fe matrix composite. This article gives par-ticular attention to the reaction path and the microstructure ofthe final product.

2. Experimental procedure

The starting powders were Ti, Fe and carbon black. Thepowders were mixed in a composition of 28 wt.% Ti, 7.2 wt.%C and 64.8 wt.% Fe. Powders mixing was undertaken by

Fig. 2. X-ray diffraction patterns of Fe–TiC composite sintered at 1420 °C (Cu-Kα radiation).

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planetary ball (QM-1SP, China) milling at 180 rpm for 24 h. Thejars for powders mixing were made of stainless steel. The ballsfor milling, which were made of stainless steel, were weighed,to achieve a ball to powder ratio of 6:1. After mixing, thepowder was characterised by X-ray diffraction (XRD) usingCu-Kα radiation. Samples of 16 mm diameter×10 mm heightwere manufactured by uniaxial die pressing at 350 MPa.Sintering was performed in the 1380–1440 °C range in a vac-uum furnace for 1 h followed by furnace cooling. The sintereddensity was determined by Archimedes' principle according toASTM C373-72.

Microstructures were examined by a JSM-5900LV scanningelectron microscopy (SEM). Phase identification was carriedout on a Philips X-ray diffractometer with Cu-Kα radiationoperated at 30 kV and 40 mA. Differential thermal analysis(DTA) of the specimen was performed using NETZSCH STA449C equipment. The sample was heated at 10 °C/min up to amaximum temperature of 1300 °C and cooled at the same rate toroom temperature. The entire test process was conducted inhigh-purity argon gas using a flow rate of 150 ml/min.

3. Results and discussion

3.1. Densification

The sintering behavior of the Fe–TiC composite is shown in Fig. 1. Itcan be seen that densification increases as the sintering temperatureincreases, reaching amaximum for the specimen sintered at 1420 °C,withdensification decreasing beyond this temperature. The reasons might beas follows: with the increase of sintering temperature, the wettabilitybetween liquid phase and TiC reinforcement is improved, whichintensifies dissolution–precipitation of TiC particles and increases theamount of liquid phase. As a result, densification increases. However,excess sintering temperature causes severe volatilization of metal(because of vacuum sintering), which results in the formation of voidsin the sintered samples, thereby decreasing densification.

3.2. Phase identification and XRD evaluation of Fe–TiC composite

The diffraction pattern for the Fe–TiC composite sintered at 1420 °C isshown in Fig. 2. The bcc-Fe binder was unchanged with respect to the

Fig. 1. Effect of sintering temperature on the apparent density of Fe–TiC composite.

original powder, while the formation of TiC solid solution and Fe3C isseen in the composite. From the composition of the initial powdermixture,it can be calculated that atomic ratio of C and Ti is 1. 03:1, that is, excesscarbon atoms are available, which accounts for the formation of Fe3C.

3.3. Microstructure of Fe–TiC composite

Fig. 3 shows the scanning electron micrograph of Fe–TiCcomposite. The gray areas are TiC particles and the lighter region isα-Fe matrix, while the dark areas are voids. It can be seen that TiCparticles are uniformly dispersed in the matrix. In addition, it can beobserved that some coarse TiC particles have formed which is attrib-uted to an Ostwald ripening process. Smaller particles dissolve due totheir higher dissolution potential (increased chemical potential), whilecoarser ones grow by material re-precipitation, thereby reducing theinterface area of the system.

3.4. Reaction path

The position of the DTA curve in relation to the abscissae (X-axis),indicates that endothermic effects dominate the sintering process over themeasured temperature range. The underlying phenomenon is reactionbetween the components of the system leading to the formation of theintermetallic compound TiFe [5]. In addition to the intermetal-lic compound, liquid phases appear. Depending on the temperature,

Fig. 3. Microstructure of Fe–TiC composite sintered at 1420 °C.

Fig. 4. Differential thermal analysis thermogram for the Fe–Ti–C system.

4395Wang Jing, Wang Yisan / Materials Letters 61 (2007) 4393–4395

the liquid phases have various chemical compositions and may bedescribed as products of the eutectic reactions: (a) between the solidsolution of iron in β-titanium and the intermetallic compound TiFe (at1085 °C) [5] and (b) between γ-iron and carbon (at 1147 °C) [6].

On the DTA graph in Fig. 4, three endothermic peaks and oneexothermic peak were observed. The first endothermic peak with amaximum at 765.6 °C arises from allotropic change Feα→Feγ. Theresult is in line with that reported in literature [7]. The decrease from912 °C (value for pure iron) is mainly caused by carbon diffusion in theiron. The second and third endothermic peaks (maxima at 1078.4 °C,1146.4 °C, respectively) are thought to arise from eutectic reactionsmentioned earlier. The exothermic peak with a maximum at 1138.2 °Cis attributed to exothermic reaction between melted Fe2Ti and carbon.

The DTA experiment did not yield the distinguishable thermaleffects that should occur from the allotropic change Tiα→Tiβ at

882 °C. This is in agreement with the lower latent heat of the allotropictransformation compared to that of the melting and reaction process inthe system.

4. Conclusions

Using a novel process which combines in-situ reaction withpowder metallurgy techniques, an iron base composite,reinforced by TiC particles was produced. The TiC particlesgenerated in situ are uniformly dispersed in the matrix. Thereaction path is as follows: first, allotropic change Feα→Feγat 765.6 °C; second, formation of the compound Fe2Ti at1078.4 °C because of the eutectic reaction between Ti and Fe;third, reaction between carbon and melted Fe2Ti causing theformation of TiC at 1138.2 °C; finally, formation of Fe3C due tothe eutectic reaction between remanent C and Fe at 1146.4 °C.

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