18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS IN-SITU SYNTHESIS OF SPINEL WHISKERS REINFORCED ALUMINUM COMPOSITE Zhenyang Yu, Naiqin Zhao*, Enzuo Liu, Chunsheng Shi, Xiwen Du, and Jian Wang School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, 300072, P.R.China * Corresponding author ( nqzhao@tju.edu.cn ) Keywords : in-situ, spinel whisker, ball-milling pretreatment, aluminum matrix composite composites were less reported. In this work, the Abstract single crystal spinel (MgAl 2 O 4 ) whiskers were synthesized by in-situ method in aluminum matrix, MgAl 2 O 4 whisker reinforced Al matrix composite and the composite was prepared by powder was prepared by in-situ synthesis method after high- metallurgy method. The microstructure and the energy ball-milling pretreatment. The distribution mechanical properties have been conducted, and the and combination of the whiskers were characterized, hardness and compressive strength of the composite and the microstructure and mechanical properties of are investigated. The SEM images show that the the whisker reinforced Al matrix composite were MgAl 2 O 4 whiskers generate inside the matrix during systematically studied. The results indicate that a the calcination process. The whiskers have a well large number of MgAl 2 O 4 whiskers were in-situ combination with the Al matrix, and form a net generated in the Al matrix. The MgAl 2 O 4 whiskers structure, resulting in the effective enhancement in break through the Al matrix and exhibit excellent properties of the composite. distribution and interfacial bonding with the matrix, so the hardness and the compressive strength of the 2 Preparations composite were improved effectively. Aluminum powders, magnesium powder and boric 1 General Introduction acid were used as raw materials, mixed with a mass Ceramic one-dimensional material with high elastic ratio of 9:2:2. The mixture was then homogenized in modulus, tensile strength and low thermal expansion a stainless steel grinding chamber and milled in a properties could be used as reinforcements in planetary ball mill. composites to increase the wear resistance and The Mg content in the mixed powders is about strength of metal matrix [1-4]. In general, the metal 15.38% after the milling pretreatment, and the matrix composites (MMCs) [5-7] reinforced with additional pure Al powder was added to control the whiskers is synthesized by either powder metallurgy amount of whiskers formed during sintering. With or casting, but it is difficult to obtain well wettability the change of Al powder mass in the total mixture, between the ceramic reinforcement and the metal the MgAl 2 O 4 whiskers generated in the matrix is matrix by externally adding and mechanically about 1.5~15%. mixing, because some inevitable problems arise, Subsequently, the green samples were obtained by such as the agglomeration, poor interfacial reaction pressing the composite powders in a steel mold at and dispersion. Therefore, exploring a new room temperature, and sintered in argon atmosphere technology that can make the whiskers at 800 °C for 1 h. After the growth process of the reinforcement uniformly dispersed and strongly MgAl 2 O 4 whiskers, the samples were hot-extruded bonded with the matrix is needed for improving the to obtain the rod samples with 5 mm diameters. As a performance and realization of industrialization comparison, the pure Al and Mg-Al alloy samples application of the composites. were prepared under the same process. Recently, the in-situ generated reinforcement inside The phase identification was conducted using a the matrix has been applied in some MMCs [8-10], Rigaku D/max 2500V/pc automated X-ray such as Ti and Mg matrix composites, and has diffractometer in Cu K α radiation. The morphology shown good results for improving the wettability of the composites was characterized by a JEOL with the matrixes. However, the research of in-situ S4800 scanning electron microscope (SEM). growth of whisker reinforced aluminum matrix
Zhenyang Yu, Naiqin Zhao, Enzuo Liu, Chunsheng Shi, Xiwen Du, and Jian Wang 3 Results and discussion Fig.4 plots the hardness of the composites After sintered at 800 °C, a large number of whiskers containing different amount of MgAl 2 O 4 whiskers. were generated in the Al matrix, and the Al The hardness of the composite increased linearly composite was obtained. The whiskers can be with the amount of the whisker rising until the observed in the fracture after the samples is broken. amount of the MgAl 2 O 4 whisker in the composite The typical morphologies of the whisker products achieves 10.5%. Because the MgAl 2 O 4 whiskers in- are shown in Fig.1. It can be seen that the straight situ generated in the Al matrix have a well whiskers with diameter of 0.2 to 2 µm dispersing in combination and distribution, the hard ceramic the aluminum matrix, and all the whiskers have a whiskers play an enhanced role in the soft matrix. flat top and six smooth sides. Every whisker has However, when the content of the whiskers is more uniform size from top to the end, and crystallizes than 10.5%, the whiskers tend to form the clusters, well. and the distribution of the reinforcement becomes To identify the phase of the whiskers, the composite uneven, so parts of the enhancement of the hard was grinded to collect the whisker. The XRD pattern whisker is ineffective. The curve of hardness of the of the products is shown in Fig.2. The diffraction composite tends to smooth, when the composite peaks in the pattern exhibit a good agreement with contains 15% MgAl 2 O 4 whiskers, the hardness is the standard spectra of the spinel (JCPDS 21-1152) nearly 130HV, which reached the highest value. and aluminum (JCPDS 04-0787) in both the Fig.5 shows compressive stress-strain curve of the reflection profile and intensity, and no peaks related composites. Compared with the pure Al and 5%Mg- to by-products was detected. Al alloy samples, the addition of whiskers enhanced During the sintering process, the MgAl 2 O 4 whisker mechanical properties of the composite significantly. generation process is probably as follows. With the increasing of the content of whisker, the H 3 BO 3 → B 2 O 3 + H 2 O (1) strengthen efficiency of the MgAl 2 O 4 whisker on the B B 2 O 3 + Al + MgO → MgAl 2 O 4 + B (2) Al matrix increased linearly due to in-situ B B 2 O 3 +Al + Mg → MgAl 2 O 4 + B (3) synthesized whiskers having perfect distribution in The molten B 2 O 3 involved in chemical reactions and the matrix. Though the rising of the content of the supplied a liquid-phase reaction environment for the whisker may lead the whisker to form the clusters, it nucleation and generation of the whiskers. The not affects the enhancement of mechanical MgAl 2 O 4 ceramic whiskers were generated in the properties, and the composite may showed the better semi-molten Al matrix, so it is easy to grow into the performances caused by the effects of the new soft matrix. The Fig.3 shows the whiskers in the Al framework formed by whiskers. The compressive matrix, which indicated a well combination and strength of the composite can achieve 400MPa, three wettability between the whiskers and the matrix. times than that of the pure Al, with the content of the Because the MgAl 2 O 4 whiskers were in-situ MgAl 2 O 4 whiskers increased to 15%. synthesized in the matrix, and a matching between 4 Conclusions whisker and Al matrix interface at atomic level could be achieved. So the MgAl 2 O 4 whiskers can be Homogeneously dispersed MgAl 2 O 4 whisker covered tightly by the parts of Al matrix, and it reinforcement was in-situ fabricated in Al matrix by shows a small contact angle at the interface between powder metallurgy using ball-milled composite the whiskers and the matrix in Fig.3. The well powders of metal and boric acid, which provides a wettability between the reinforcement and the matrix new way for in-situ generation of whiskers in the Al is important for the composite as the matrix need matrix. The MgAl 2 O 4 whiskers have excellent transfer the stress to the reinforcement through the distribution and wettability with the matrix, so the interface under load and the well combination hardness and mechanical properties of the promotes the transfer efficiency. The in-situ composites were improved effectively. The hardness generated MgAl 2 O 4 whiskers have excellent and compressive strength of the composite contained distribution and interfacial bonding with the matrix, 15% MgAl 2 O 4 whiskers can reach 130HV and so the hardness and the mechanical properties of the 400MPa, respectively. composite were improved effectively.
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