Synthesis and Characterization of AlMgB14 Hot Pressed Under Different Environments

AlMgB14 hot pressed under different environments from the elemental powders was investigated. The microstructure of the products was analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometer (XRD). The density, hardness and fracture toughness of AlMgB14 were also examined. The results show that single phase of MgAl2O4 is formed in hot pressed Al-Mg-B system in air. The addition of carbon powder cannot reduce the formation of MgAl2O4. Additional Al (10 wt.%) and Mg (10 wt.%) can effectively compensate for their loss in the formation of MgAl2O4 and enhance the production of AlMgB B14 under agron atmosphere. The AlMgB14 with 10 vol.% spinel MgAl2O4 owns a hardness of 28.42 GPa and a fracture toughness of 2.92 MPa·m . 1/2

The bulk and thin film of AlMgB 14 can be synthesized by different methods.Mechanical alloying and hot pressing were mostly used to prepare Al-Mg-B polycrystalline material [1,9].Kevorkijan and Okada adopted high temperature and normal pressure synthesis (HTS) to synthesize AlMgB 14 [10][11].In Roberts and Liu's study, electric field assisted sintering, named PECS and FAPAS, have been used to synthesize AlMgB 14 [12,13].In Sun's study, a two-step method was used, in which the Mg 6 AlB 14 disk was firstly prepared from Mg, Al and B powders at 850 °C under the protection of argon, and then AlMgB 14 disk was formed by the decomposition of Mg 6 AlB 14 sintered in vacuum [14].For preparing AlMgB 14 thin film, Tian used hot pressed Al 0.95 Si 0.05 MgB 14 as the target to obtain AlMgB 14 film by means of high-energy pulsed laser deposition [15].Wu prepared nanocomposite Al-Mg-B thin films on Si(100) substrates with a three target magnetron sputtering system [16].Yan used multiple unbalanced planar magnetrons equipped with two boron and one AlMg targets to obtain AlMgB film by sputter deposition [17].
Accompanying the formation of AlMgB 14 , MgAl 2 O 4 is inevitably produced due to the oxygen from raw materials and environments.There are few reports on the effect of sintering atmosphere on the synthesis of AlMgB 14 .In this paper, we started to prepare AlMgB 14 bulk materials from pure Al, Mg and B powders by hot pressing.Particularly, various content of hot pressed samples can be obtained via adjusting different atmosphere and conditions.carbon powder is attempted to adopt for the purpose of reduction of MgAl 2 O, we also consider to investigate the influence of addition of carbon powder to the element powders on the synthesis of AlMgB 14 .In contrast to the stoichiometric ratio of AlMgB 14 , extra amount of Al and Mg elemental powders were added to compensate their consumption in the reaction of MgAl 2 O 4 .It is, therefore, of great value to study the effects of the sintering atmosphere and conditions on the microstructure and properties of the hot pressed samples.

Experimental procedure
The raw materials are listed in Table I.The initial powders (the stoichiometric ratio of Al:Mg:B = 1:1:14) were subjected to high-energy milling in a planetary mill (QM-QX).The vials and the balls are made of cemented carbide; a ratio of ball to powder is controlled as 15.The elemental powders were introduced into the vials and sealed with argon in a glove box.The planetary ball mill was operated in dry condition at a rotation speed of 250 rpm with a milling time of 2,5,10 and 20 h.Different compositions of raw materials and sintering atmosphere for preparation of AlMgB 14 are listed in Table II and ball milled for 20 h.All those samples were then hot pressing under flowing Ar gas for 1 h at 1500 °C with unidirectional pressure of 30 MPa, and a dense compact was to be produced with the diameter of 30 mm and the thickness of 5 mm.The microstructure of the samples was examined by scanning electron microscope (Nova NanoSEM 430) with energy dispersive X-ray spectroscopy (EDX).The phases composition of hot pressed samples were also identified by an X-ray diffractometer (BRUKER D8 ADVANCE, Cu k radiation) with the scanning step size of 0.02 degrees from 20 degrees to 70 degrees.The density of the samples was calculated by the Archimedes method.The Vickers hardness (Hv) was determined on a microhardness tester (HX-1000TM/LCD) with the load of 10 kg and the dwelling time of 15 s, indentations were made in 10 random locations on each sample.The fracture toughness was measured and calculated by Shetty formula [18] as follow:

Tab. I
Where, H V is vikers-hardness, P is indentation load, l is the length of crack.

Effect of milling time on microstructure of the mixed raw powder
The microstructure of the milled powder mixture is shown in Fig. 1.With the increase of milling time, the size of particles becomes finer and more homogeneous.X-ray patterns of the ball milled samples is shown in Fig. 2, which indicates that mechanical alloying cannot be found in the ball milled elemental powders with the milling time of 20 h.

Microstructure of the hot pressed samples
Fig. 3 shows the XRD patterns of hot pressed samples.Single phase MgAl 2 O 4 is formed without the protection of argon (Fig. 3d).Mg and Al powders are oxidized completely under 600 °C, however, the formation of AlMgB 14 is more than 1300 °C, and thus MgAl 2 O 4 is formed before the reaction of AlMgB 14 .It can also be seen from Fig. 3b that B 4 C and Al 3 C(BC 2 ) can be identified besides of MgAl 2 O 4 .The carbon powder is added for the purpose of reducing the effect of oxygen from the raw powders.However, it is found that carbon reacts with boron instead of oxygen in the process of hot pressing.
Under the protection of argon, the main phases of the hot pressed samples are AlMgB 14 and MgAl 2 O 4 .In the condition of adding extra amount of Al and Mg elemental powders, more AlMgB 14 is formed, which can be verified by the difference of the intensity of XRD peaks for AlMgB 14 , which was shown in Fig. 3a and Fig. 3c.Extra amount of Al and Mg powders can ensure the sufficient Al and Mg metal reacted with the boron powder, which can also be found in the quantitative analysis of crystal phases as shown in Table III.The formation of MgAl 2 O 4 is not only related to the presence of MgO and Al 2 O 3 but also a result of the presence of B 2 O 3 as shown in the following equations (1-2) [3]: (2) SEM and EDX were used to examine the microstructure of hot pressed AlMgB 14 .The brighter region marked with letter A in Fig. 4a is consisted of Al, Mg and O (Fig. 4b and Table IV), which is thought to be the spinel phase MgAl 2 O 4 [19].The darker region marked with letter B in Fig. 4a is mainly consisted of Al, Mg, and B as also seen in Fig. 4c and Table IV, which is the single-phase AlMgB 14.

Properties of AlMgB 14
Table V shows the density, hardness and fracture toughness of hot pressed AlMgB 14 .The average density of the samples was 2.538 and 2.695 g•cm -3 , respectively.AlMgB 14 with purity of 90 vol.% owns a higher hardness of 28.42 GPa and a fracture toughness of 2.92 MPa•m 1/2 .In the absence of an intentionally added second phase, the hardness for single cystals of AlMgB 14 has been reported to be in the range 20.2-28.6 GPa [11].The value obtained in this work is consistent with the references, higher hardness can be obtained by the addition of a second phase to the boride [20].The role of unintentional impurity, MgAl 2 O 4 , may explain in part the differences between them.The fracture toughness is shown to be relatively low, but it is also in agreement with the references.Studies have been attempted to improve the fracture toughness by the addition of a binder alloy [5].

Morphology of fracture surface of AlMgB 14
It can be seen from Fig. 5 that the AlMgB 14 owns a compact microstructure with lower porosity.The fracture surface is composed of a large number of pits and small steps, which shows a transgranular fracture mode.

Conclusions
Four kinds of conditions were designed to fabricate AlMgB 14 hard material.The effect of the source and milling time on the raw materials was examined; the reactant chemicals are only with no indication of product formation during the milling.Besides MgAl 2 O 4 , B 4 C and Al 3 C(BC 2 ) are formed when adding carbon powder into Al-Mg-B system.The addition of extra amount of Al and Mg can effectively increase the purity of AlMgB 14 .The hot pressed AlMgB 14 with 10 vol.% of MgAl 2 O 4 had a density of 2.695 g•cm -3 , a hardness of 28.42 GPa and a fracture toughness of 2.92 MPa•m 1/2 .The fracture surface is a typical transgranular fracture which is composed of a large number of pits and small steps.
Chemicals used for processing. 14.