硅酸盐学报成绩优异
小学语文三年级上册・ 34 ・2013年
原位反应法制备Cr2AlC-Fe复合材料
陈新华，翟洪祥，王文娟，黄振莺
(北京交通大学，北京 100044)
摘要：采用原位反应法制备了Cr2AlC-Fe系复合材料，并采用热分析、X射线衍射、扫描电子显微镜和三点弯曲实验，研究了原位反应的烧结工艺对产物和显微结构的影响，以及对原料中Cr2AlC的含量对复合材料性能的影响。结果表明：通过高温原位反应，原料中Cr2AlC发生了分解，形成了网络状陶瓷增强结构，所制备的复合材料具有较好的强度和韧性，且随着Cr2AlC含量的增加，复合材料的强度也在增加，但断裂韧性逐渐下降。当Cr2AlC的体积分数达到30%时，复合材料的抗弯强度达1417MPa。
关键词：复合材料；原位反应；烧结；弯曲行为
中图分类号：TB333 文献标志码：A 文章编号：0454–5648(2013)01–
网络出版时间：网络出版地址：
Fabrication of Cr2AlC Fe-bad Composites by in-situ Reaction Method
CHEN Xinhua，ZHAI Hongxiang，WANG Wenjuan，HUANG Zhenying
(Center of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering,
Beijing Jiaotong University, Beijing 100044)
Abstract: A Cr2AlC-Fe composite, which could have potential applications in nuclear energy industry as engineering materials, was synthesized by an in-situ reaction method. The in-situ reactions between Cr2AlC and Fe at different temperatures and ratios were ana-lyzed by thermogravimetric analysis-differential thermal analysis, X-ray diffraction and scanning electron microscopy, respectively. The effect of s Cr2AlC content on the bending behaviors was investigated. The results show that Cr2AlC can in-situ react with Fe, and decompo to form chromium carbide. The synthesized composite exhibits a higher flexural strength and a greater fracture toughness at room temperature.
Key words: bad composites; in situ reaction; sintering; bending behaviors
1 Intr o duction
Combine with the unique desirable properties of a sin-gle metal or ceramic material and offtting each other's deficiencies, composites can be ud as the engineering materials for extreme environment of nuclear energy in-dustry, mining industry, chemical and metallurgical in-dustry and so on.[1–3] The reinforcing ceramic particulates include Al2O3, TiC, Si3N4, WC, etc..[4–8] Although tho traditional ceramic reinforcements own high strength, they have low fracture toughness, poor wetting, inconsis-tency of linear expansion coefficient with iron, and are hard to be machined by using normal tools.
Recently, Cr2AlC and related composites have at-tracted increasing attention,[9–13] which belong to a new type of ternary structures advance ceramics so called MAX pha. Compared with traditional ceramic rein-forcing agents, the kinds of ceramics posss unique crystal laminated structure as graphite. In the Cr2AlC crystal structure, the Cr atoms and C atoms form two common edges Cr6C tetrahedron with strong ionic bond are parated by Al atomic planes,[14] and the link be-tween Al atomic planes and Cr6C tetrahedron are weak Cr–Al metallic bond.[15–16] This structure lead the Cr2AlC pha combinational properties of both metals and ce-ramics, such as low density, high modulus, easy ma-chinability, good electrical and thermal conductivity, ex-cellent thermal shock and
high-temperature oxidation resistance, but also can react with Fe by the technique of in situ reaction method,[17–19] which is a process where
收稿日期：2012–04–23。修订日期：2012–09–25。基金项目：国家“863”项目(2006AA03Z527)。
第一作者：陈新华(1983—)，男，博士研究生。Received date:2012–04–23. Revid date: 2012–09–25. First author: CHEN Xinhua (1983–), male, Ph.D., Candidate. E-mail: 07116311@
第41卷第1期2013年1月
硅酸盐学报
JOURNAL OF THE CHINESE CERAMIC SOCIETY
Vol. 41，No. 1
January，2013
陈新华 等：原位反应法制备Cr 2AlC-Fe 复合材料
・ 35 ・
第41卷第1期
reinforcements are synthesized in metallic matrix by chemical reactions, and ensured the introduction of chemical bonding force in the interface of the reinforce-ments and the metal matrix, and the high performance bonding will make the external applied stress transferred from the matrix to the reinforcements. Zhang et al .[20] nod out that the reaction between Cu and Ti 3AlC 2 form TiC x and Cu (Al) above 950 ℃. Our previous work [21–24] shows that the incorporation of Ti 3SiC 2 or Ti 3AlC 2 with Cu increa the strength and modulus as well as wear resistance without the loss of conductivity and meet the requirements of applications in electrical sliding contacts in high-speed railway. In fact, similar reactions also exist between the Cr 2AlC with Fe matrix.
Hence, this paper intends to report on processing and property of Fe-bad Metal Matrix Composites (MMCs) using the Cr 2AlC, which is a type member of the MAX pha ceramics, as precursor in the raw material. By us-ing this in-situ reaction method, the Cr 2AlC ceramic par-ticles might decompo to fine chromium carbide rein-forcing agents. Different from the traditional direct syn-thesis methods, this method has veral advantages, such as the better link between the reinforcing agents with the matrix, and the easier distributed evenly of the reinforc-ing agents, and so on.
2 Experimental
Reduced iron powders (purity 99.5%, size < 74 μm, Beijing Chemical Reagent Company) and Cr 2AlC pow-ders (purity > 97%, average size 5.197 μm, the details can be found elwhere [10]) are mixed for 10 h in plastic cans to ensure homogeneous reactant mixtures. The Cr 2AlC particle powders size measurement was performed on lar diffraction particle size analyzer (Mastersizer 2000, Malvern, Britain). Then, the mixed powders were hot- presd (HP) at 1 300 ℃ under 30 MPa for 30 min in flowing argon gas. The flexural strength of the compos-ites was tested by three-point bending method by GB/T 6569–1986, and the fracture toughness was tested by single edge notched and three-point bending method (SENB). The final samples were cut into blocks with di-mensions of 3 mm × 4 mm × 36 mm for bending tests, of 4 mm × 6 mm × 36 mm for SENB tests, the both tests
were performed on a universal testing machine (ZWICK, Z005) at a loading speed of 0.5 mm/min. According to the standard force and displacement from the universal testing machine, the bending strength and fracture toughness of the compos-ites are calculated from following equations respectively:
tr 2
32FL
R bh =
(1) IC 2
32FL K bh =
(2)
where R tr and K IC are the flexural strength and fracture toughness of the composites respectively, F is the force required to the fracture, L is the distance between the fulcrums, b and h are the width and thickness of the sam-ples respectively, a is the crack length of the composites, here is 2.8 mm.
After the bending tests, the specimens were analyzed by a scanning electron microscopy (SEM)(JSM–6460).
3 Results and discussion星星点灯歌词
Figure 1 shows the TG–DTA curve of 30%Cr 2AlC/ 70%Fe system and the DTA curves of pure Fe. In the TG–DTA curve of 30%Cr 2AlC/70%Fe system, there are no obvious changes in the weight during the reaction between Cr 2AlC and Fe, which indicates that there is no obvious change in the weight during the reaction between Cr 2AlC and Fe. In the DTA curve 30%Cr 2AlC/ 70%Fe system, two endothermic peaks sat 763.5 ℃and 914.5 ℃can be obrved. For the endothermic peaks, compare to the DTA curves of pure Fe, one can notice that almost same endothermic peaks at 770.21and 915.24 ℃ are obrved in the DTA curves of Fe. Hence, it can be con-cluded that two endothermic peaks at 763.1 ℃and 913 ℃ are probably ascribed to the pha transition of Fe. Indeed, according to the Fe–Al pha diagrams,[25] it can be en the magnetic transition temperature (T c ) of Fe at 770 ℃and the transformation of α-Fe to γ-Fe at 912 ℃ are in good agreement with our results here. Above 914.5 , the ℃endothermic reaction is predominant and there was no new obvious peak. The different pha coposi-tions at different temperatures were made certain by the
analyzing of XRD.
Fig. 1 TG–DTA curves for 30% Cr 2AlC/70%Fe and DTA
curves for pure Fe powder in argon atmosphere
Figure 2 shows the XRD patterns of samples of 30%Cr 2AlC/70%Fe composites sintering at the tempera-tures from 1 000 ℃ to 1 400 under 30℃ MPa for 30 min. The results show that, when the sintering temperature is between 1 000 to 1 300 , the peaks from Cr ℃2AlC disap-
硅 酸 盐 学 报
・ 36 ・
2013年
Fig. 2 XRD patterns of Cr 2AlC powders and samples of
30%Cr 2AlC/70%Fe composites sintered at 1 000 to 1 400 ℃
peared and the main phas were stability of (Cr,Fe)7C 3 and Fe (Al). As the sintering temperature continued ri to 1 400 , the intensity of the diffraction peaks from Fe ℃(Al) was growing substantiall
y, and great contraction deformation has emerged in the samples after sintering at 1 400 ℃, this phenomenon indicating that the ceramic gregation had occurred at this high temperature. So the temperature from 1 000 to 1 300 ℃ is the appropriate sintering temperature range.
Figure 3 shows the XRD patterns of samples of Cr 2AlC-Fe mixtures with different Cr 2AlC contents after sintered at 1 300 ℃ under 30 MPa for 30 min. When the Cr 2AlC contents in the mixtures below 30%, no new phas can be detected besides Fe (Al) and (Cr, Fe)7 C 3. As the Cr 2AlC contents increasing to 50%, the diffraction peaks from Cr 7C 3 became higher. The main reason of the reactions can be contributed to the in-situ reaction be-tween Cr 2AlC and Fe. This in-situ reaction can be de-scribed as this: At high sintering temperature, Al atoms can strip from Cr 2AlC due to the weak Cr–Al metallic bond in Cr 2AlC, and dissolve in the metal matrix, form-ing Cr 7C 3 grains and Fe (Al) solid solution. At the same time, Fe will through into the ceramics particles by the Al vacancies. When the Cr 2AlC contents increasing to 50%, the liquid Fe are insufficient and results in some Cr 7C 3 pha remaining. The reaction can be described as fol-lowing:
负债经营Cr 2AlC + Fe →Cr 7C 3 + Fe(Al) (1)
Cr 7C 3 +
Fe →(Cr, Fe)7C 3 (2)
Figure 4 shows the microstructure of samples of 30%
Fig. 3 XRD patterns of samples of Cr 2AlC-Fe mixtures with
different Cr 2AlC contents after sintered at 1 300 ℃ for 30 min
Fig. 4 Microstructure of samples of 30%Cr 2AlC/70%Fe
sintered at 1 300 ℃ under 30 MPa for 30 min
Cr 2AlC/70%Fe sintered at 1 300 ℃ under 30 MPa for 30
min. Choo a notable ceramics area in a bulk 30%Cr 2·
幸福婚姻AlC/70%Fe composites’ polished surface, and its SEM
陈新华等：原位反应法制备Cr2AlC-Fe复合材料・ 37 ・第41卷第1期
image shows as Fig. 4(a). In the composites, the ceramic particulates are uniformly distributed in the
matrix, and the ceramic particulates with shapes of needle, spindle, and flake. Figure 4(b) is a reprentative SEM image of the bulk 30%Cr2AlC/70%Fe system composites’ polished surface which was etched in nital. This typical micro-graph exhibits that the composites with a compact texture, and the ceramic particulates are uniformly distributed in the Fe matrix with the average thickness size of 1.3μm, form a hard continuous skeleton. This structure can be contribute to the microstructure genetic effects from the superimpod shape of the platelet microcrystalline Cr2AlC in the of Cr2AlC polycrystalline grains.
The sample of 30%Cr2AlC/70%Fe sintered at 1300℃for 30min in argon atmosphere, and its flat well-polished surface analyzed by EDS are shown in Fig. 5. It clearly shows that the Cr2AlC particles almost turned into Cr7C3. We can also obrve that Fe had diffud into the Cr2AlC
particles, Al, Cr and C can be detected in the Fe matrix. Fig. 5 EDX of samples of 30%Cr2AlC/70%Fe sintered at
1300℃ under 30MPa for 30min
打喷嚏漏尿Figure 6 shows the relationship between the flexural strength, fracture toughness and the volume content of Cr2AlC for Cr2AlC-Fe composites sintered at 1300for
℃
30min. The flexural strength of the sample was incread greatly, but the fracture toughness was decread while raising Cr2AlC content. When the Cr2AlC content was 50% in the starting materials, the flexural strengths of 50%Cr2AlC/50%Fe can reach 1417.05MPa, however, its fracture toughness drop to 18MPa·m1/2. A most remark-able feature is the strengths of Cr2AlC-Fe composites not only much stronger than the strengths of pure Fe bulk, but
Fig. 6 Relationship between the flexural strength, fracture toughness and the volume content of Cr2AlC for
Cr2AlC- Fe composites sintered at 1300℃ for 30
min
also much stronger than the strengths of Cr2AlC bulk (which flexural strengths is about 378MPa). This phe-nomenon can be contributed to the hard continuous skeleton microstructure and strong interface bonding between the reinforce particulates and the metal matrix in the composites.
Figure 7 shows the typical bending site specimen photograph of 30%Cr2AlC/70%Fe sample sintered at 1300℃ for 30min after the bending tests. A lot of slip bands are formed in the matrix at about 45°with respect to draw axis. This result indicates that the interfacial bond between the ceramic reinforcements and metal matrix are strong. After in situ reaction, the chemical bond is formed in the interface between the chromium carbide particu-lates and matrix. During deformation, the ceramic parti-cles not only can resist the passing dislocations, but also can induce drag force to the grain boundaries of the matrix. As a result, the external applied stress will be transferred from the matrix to chromium carbide particulates, and un-doubtedly, the load bearing capacity of the composites with
Fig. 7 Typical bending site specimen photograph of 30%Cr2·AlC/70%Fe sample sintered at 1300℃ for 30min after
bending tests
硅酸盐学报
・ 38 ・2013年
more ceramic reinforcement will be greater during the period of elastic deformation.
4 Conclusions
The chromium carbide reinforces Fe (Al) composites can be prepared by in-situ reaction method usi
ng Cr2AlC powders as precursor. The in-situ reaction between Cr2AlC and Fe completed above 1000℃, and the products main phas keep stable in the 1000–1300℃ sintering tem-perature range. As the Cr2AlC content increasing, the flexural strength of the sample is incread greatly, but the toughness is decread.
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