Microstructure characterization of reinforcements in in-situ synthesized composites of Al-Zr-O system

ZHAO Yu-tao(ÕÔÓñÌÎ), DAI Qi-xun(´÷ÆðÑ«), CHENG Xiao-nong(³ÌÏþÅ©),LIN Dong-yang(ÁÖ¶«Ñó), CAI Lan(²Ì À¼)

(School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China)

Abstract: A novel in-situ reaction system Al-Zr-O was developed. In-situ Al₃Zr and Al₂O₃ particulates reinforced aluminum matrix composites were fabricated by the direct melt reaction technique in the Al-Zr-O system. Microstructures of the composites and crystal morphology of in-situ formed Al₃Zr and Al₂O₃ particulates were analyzed by scanning electron microscope(SEM) and transmission electron microscope(TEM). Results indicate that in-situ formed Al₃Zr and Al₂O₃ particles are finer and well distributed in aluminum matrix. Al₃Zr particulates with a tetragonal structure are mainly in the shape of polyhedron. A few of them are in the form of rectangle. The length/width ratio of the rectangular Al₃Zr is less than 2.0 and the maximum size is 2¦Ìm. In addition, a certain number of Al₂O₃ submicro particles with a hexagonal structure are also generated in this system. Furthermore, it is found that Al₃Zr crystal grows by the mechanism of twinning. The twin plane is (11[DD(-*2]-[DD)]4[DD(-*2]-[DD)]). The twinning direction is [22[TX-]1].The tensile tests show that the composites synthesized in the Al-Zr-O system exhibits high strength and ductility. There are a lot of ripples with fine particles on the fracture. The principal strengthening mechanisms for (Al₃Zr+Al₂O₃)_p/Al composites may include Orowan strengthening, grain-refining strengthening, solid-solution strengthening and dislocation strengthening.

Key words: in-situ synthesis; composites; Al-Zr-O; reinforcements CLC number: TB331; TG146.2

Document code: A

1 INTRODUCTION

The attractive physical and mechanical properties of aluminum matrix composites (AMCs), such as high specific modulus, strength, and thermal stability have been documented extensively^[1-4]. In recent years, much attention has been paid to the development of an effective fabrication process for aluminum matrix composite. In the in-situ fabrication process, the spontaneous reaction between the reactants is utilized to synthesize reinforcements in the aluminum matrix. Thus it is expected that the in situ formed composites may reveal not only excellent dispersion of fine reinforcing particles, but also high strength and thermodynamic stability. Some of in-situ techniques include XD^TM, DIMOX^TM, PRIMEX^TM, VLS, LSM, and SHS^{[5, 6]}. Especially, the direct melt reaction technique (DMR) is of simplicity, low cost and possibility of near net-shape forming and considered to be one of the most promising in-situ synthesis techniques of commercial production^[7]. Up to now, however, in-situ reaction systems are mainly concentrated on Al-Ti-X, for example, Al-Ti-O, Al-Ti-B and Al-Ti-C. In-situ formed reinforcements are only focused on a few particles such as Al₃Ti, Al₂O₃, TiB₂, TiC^[8].

The present work developed a novel in-situ reactive system Al-Zr-O, in which novel (Al₃Zr+Al₂O₃)_p/Al composites were fabricated by the direct melt reaction between zirconium oxychloride with molten aluminum. The dispersion behavior of particles formed by the in situ reaction process, the crystal morphology and growth mechanism of in situ formation of reinforcements in aluminum matrix were investigated, and the mechanical properties and strengthening mechanism of in-situ composites were discussed.

2 EXPERIMENTAL

The raw materials were pure industrial aluminum ingot (99.85%) and zirconium oxychloride (ZrOCl₂¡¤8H₂O) powder (99.92%). Zirconium oxychloride was pre-heated at 250¡æ for 3h to dehydrate the bounded water in it. At the same time, 3kg aluminum ingot was melted in RL100-60H graphite crucible in an electric furnace, and held at 800¡æ. Then 380g dehydrated ZrOCl₂ powder was added to the molten aluminum and pushed down into the melt. Subsequently liquid aluminum reacted with ZrOCl₂ to produce Al₃Zr and Al₂O₃. Finally, the composite melt was cast into permanent mould at 720¡æ and the (Al₃Zr+Al₂O₃)_p/Al composites were fabricated.

The as-cast composite ingots were sectioned into samples for SEM, XRD and TEM. The thin slices (d10mm¡Á0.5mm) for transmission electron microscope (TEM) were mechanically ground on 1000 grit silicon carbide paper, polished to approximately 60¦Ìm in thickness, and subsequently thinned using argon-ion beam at 5kV, 4mA at angles 30¡ã and 10¡ã . The foils prepared were carefully examined using JEM-2000EX transmission electron microscope equipped with a double-tilt holder and operated at 120kV.

3 RESULTS AND DISCUSSION

3.1 X-ray diffraction and microstructure

Fig.1(a) shows the X-ray diffraction pattern of the composite synthesized by the direct melt reaction between zirconium oxychloride and molten aluminum. It is illustrated that the reinforced phases in the Al-Zr-O system are Al₃Zr and ¦Á-Al₂O₃. The metallurgical reactions in the molten aluminum are as follows:

2ZrOCl₂=ZrCl₄(g)+ZrO₂(s)(1)

3ZrCl₄(g)+4Al(l)=3[Zr]+4AlCl₃(g)(2)

3ZrO₂+4Al(l)=3[Zr]+2Al₂O₃(3)

[Zr]+3Al(l)=Al₃Zr(4)

Fig.1(b) shows the SEM microstructure of (Al₃Zr+Al₂O₃)_p/Al composites. It is indicated that the in-situ synthesized Al₃Zr and Al₂O₃ particles are well distributed in the aluminum matrix. The maximum size of those particles is 2¦Ìm. The morphology of them is mainly polyhedron.

3.2 Crystal morphology and growth

Fig.2 shows the crystal morphology, TEM diffraction pattern and growth model of Al₃Zr reinforcement. There are two shapes of Al₃Zr crystal. One is polyhedral (Fig.2(a)), the other is rectangular (Fig.2(b)). The length/width ratio of the rectangle is in the range of 1.5-2.0. There is a faceted growing tendency on the surface of the polyhedral and a twin growing on the surface of the rectangular. The interface between Al₃Zr particulates and Al matrix is smooth, clean and there is no reaction product. Moreover, the observations in many samples show that the dislocation density of aluminum matrix nearby the polyhedral is higher than that of the matrix nearby the rectangular. The TEM diffraction pattern of the twinning is shown in Fig.2(c). According to the diffraction pattern, it is determined that the twin plane is (114), the growth direction of the twin is [221]. The twinning growth model of Al₃Zr crystal is illustrated in Fig.2(d).

What is the relation between the twin and the growing morphology of Al₃Zr intermetallics? Based on experimental observation and crystal growth theory^[9], Al₃Zr intermetallic compound grows in the form of facet. The atomic arrangement on the interface of liquid/solid is smooth. Thus single molecule is difficult to accumulate up on the smooth surface of Al₃Zr crystal. However, the twin occurs because of the atomic dismatch. It results in a very pronounced reentrant edge or groove. The diffusing Al₃Zr molecules from the liquid melt are easy to attach to the groove. It may be concluded that the twin plane reentrant edge(TPRE)^[10] is important here for the growth kinetics of Al₃Zr crystal. Although only one twin can be observed in Al₃Zr crystal, it is fact that there are four closely packed planes in Al₃Zr crystal, such as (114), (114), (114) and (114), according to the analysis of Al₃Zr crystal stereogram. The twinning phenomenon may take place on one or several

Fig.1  XRD pattern(a) and SEM microstructure(b) of composite synthesized in Al-Zr-O system

Fig.2  Crystal morphologies and growth model of Al₃Zr

closely packed planes. So it can result in one twin or several twins. Under the observation of Al₃Zr morphology by SEM and TEM, Al₃Zr reinforcement grows in the shape of rectangle when only one twin is caused, whereas Al₃Zr reinforcement grows in the form of polyhedron when multiple twins are produced.

Fig.3 shows the crystal morphology and TEM diffraction pattern of Al₂O₃ particle. It shows that the crystal morphology of Al₂O₃ particulate is approximately equiaxial, and Al₂O₃ crystal is of hexagonal structure.

3.3 Mechanical properties of composite

The mechanical properties of the composite are shown in Fig.4(a). The result indicates that the tensile strength of (Al₃Zr+Al₂O₃)_p/Al composites is enhanced greatly with the increasing of volume fraction of particles. However, the elongation of the composite is decreased with the increasing of volume fraction of particles when the volume fraction of particles is larger than 4%. The fracture morphology is shown in Fig.4(b). Many particles adhere to the matrix. It is indicated that the interfacial bonding strength is high. The fracture of this composite is the mixture of brittleness and toughness. The principal strengthening mechanisms for (Al₃Zr+Al₂O₃)_p/Al composites may include Orowan strengthening, grain-refined streng-thening, solid-solution strengthening and dislocation strengthening. Linear summation of such terms may be used to predict yield strength and the results are

¦Ò_composite=¦¤¦Ò_Orowan+¦¤¦Ò_grain+ ¦¤¦Ò_solution+¦¤¦Ò_dislocation(5)

Fig.3  TEM micrographs of in-situ Al₂O₃ particulate

Fig.4  Tensile property and fracture morphology of (Al₃Zr+Al₂O₃)_p/Al composites at ambient temperature

3.3.1 Orowan strengthening

Orowan strengthening results from interaction between the dislocation and the dispersed particles. When the composite bears the load the plastic deformation of the material is caused. Al₃Zr and Al₂O₃ hard particles act as obstacles to hinder the motion of dislocations nearby particles in the matrix. The Orowan strengthening effect of particles on the matrix is enhanced gradually with the increasing of the particulate volume fraction. The following expression is used^[11]:

where G is the shear modulus of the matrix, b is the Burger¬ðs vector, ¦Í is the Poisson ratio, ¦Ë is the edge-to-edge particulate spacing, and D is the average particulate diameter. The particulate spacing, ¦Ë, can be expressed in terms of the volume fraction (¦Õ) of dispersed particles and the average particulate diameter by^[11]

which can be calculated with the microstructural parameters obtained from the TEM results. As concerned as for 12% (Al₃Zr+Al₂O₃)_p/Al composite, where G=26900MPa, b=2.8¡Á10^-10m, ¦Í=0.34 and D=0.5¦Ìm, the Orowan strengthening effect can be calculated by Eqns.(6) and (7) and the result is ¦¤¦Ò_Orowan=23.01MPa.

3.3.2 Grain-refined strengthening

The experimental observations indicate that Al₃Zr reinforcing phase can reduce significantly the grain size of aluminum matrix with the increasing of the particulate volume fraction as shown in Fig.5. According to the analysis of Al₃Zr crystal structure, polyhedral Al₃Zr particles act as the heterogeneous nucleation catalyst for aluminum. The grain-refined strengthening effect of Al₃Zr particulate is improved by the increasing of the volume fraction of polyhedral Al₃Zr particles via the Hall-Petch type of Eqn.(8)^[12]:

¦Ò_grain=¦Ò₀+kd^-1/2(8)

where ¦Ò_grain is the yield strength contribution from grains, ¦Ò₀ is the friction stress, d is the grain size and k is the material constant. Thus, the grain-reinforced strengthening effect of 12%(Al₃Zr+Al₂O₃)_p/Al composite can be obtained from Eqn.(9) and the value is ¦¤¦Ò_grain=22.13MPa.

¦¤¦Ò_grain=k(d^-1/2₁-d^-1/2₂)¡Ökd^-1/2₁(9)

where k=0.07MPa¡¤m^1/2 and d₁=10¦Ìm.

Fig.5  Morphologies of ¦Á(Al) phase in (Al₃Zr+Al₂O₃)_p/Al composite with different particulate volume fractions

3.3.3 Solid-solution strengthening

When a foreign zirconium (Zr) atom dissolves in the matrix aluminum (Al), it may act as an atomic-sized obstacle to the motion of dislocations. Because the volume of foreign Zr atom (0.023272nm³) is larger than that of Al atom (0.016603nm³), a misfit strain field will be produced around the Zr atom that may interact with the dislocation strain field. The analysis of the strain field-dislocation interaction is given by^[13]

where G is the elastic shear modulus, x_f is the mole fraction of the foreign atoms and ¦Å is the fractional difference in zirconium and aluminum atom diameters. So the solid-solution strengthening effect of 12% (Al₃Zr+Al₂O₃)_p/Al composite can be evaluated by Eqn.(10) and the result is ¦¤¦Ò_solution=35.79MPa. Where G=26900MPa, ¦Å=0.119, x_f=0.05%.

3.3.4 Dislocation strengthening

In many of metal matrix composites, dislocations are generated in the matrix upon cooling or quenching temperature from the processing or solution temperature, due to a mismatch of the coefficient of thermal expansion (CTE) between the matrix and reinforcements. The higher dislocation density increases the strength of the matrix. The amount of dislocation generation is affected by CTE, particle size, particle volume fraction, and matrix strength. The dislocation strengthening effect of 12% (Al₃Zr+Al₂O₃)_p/Al composite can be evaluated by^[14]

where A is the total surface areas of particles, G is the shear modulus of the matrix, b is the Burger¬ðs vector, and ¦¤¦Ñ is the increment of dislocation density in the matrix because of particles. Thus, the dislocation strengthening result of 12%(Al₃Zr+Al₂O₃)_p/Al composite is ¦¤¦Ò_dislocation=32.07MPa. Where A=0.83, G=26900MPa, b=2.8¡Á10^-10m, ¦¤¦Ñ=2.52¡Á10¹³m^-2.

According to Eqn.(5), the total yield strength of 12%(Al₃Zr+Al₂O₃)_p/Al composite is calculated by ¦Ò_composite=¦¤¦Ò_Orowan+¦¤¦Ò_grain+¦¤¦Ò_solution+¦¤_dislocation=113.0MPa, whereas the real yield strength of this composite is measured to be 112.4MPa by the tensile test. So the difference in yield strength of 12% (Al₃Zr+Al₂O₃)_p/Al composite between the elevated value and the tested value is very approximate.

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(Edited by YUAN Sai-qian)

Foundation item: Project(50471050) supported by the National Natural Science Foundation of China; Project(00170) supported by the Key Foundation of the Ministry of Education of China; Project(BE2002039) supported by Jiangsu Provincial Key Project of Science and Technology; Project(JH02-039) supported by Jiangsu Provincial Development Project of High and New Technology of China

Received date: 2004-08-10; Accepted date: 2004-10-28

Correspondence: ZHAO Yu-tao, Professor; Tel: +86--; E-mail: Zhaoyt@ujs.edu.cn