Mechanochemical synthesis and structure of Ti-Al-B-based alloys

Abstract

The paper examines how the structure and phase composition of Ti-Al-B alloys evolve at various consolidation stages in the composite powder mechanochemical synthesis and subsequent sintering under pressure. Two powder alloys with different boron content are studied. The amount of aluminum in both initial powder mixtures is the same and corresponds to TiAl. The content of boron is selected so as to form an aluminide matrix with 10 and 25 vol.% borides. It is established that phases form in the mechanochemical synthesis in the following sequence: Ti + Al → Ti (Al) → TiAl 3 and Ti 3 Al → TiAl. Titanium borides are formed simultaneously with TiO 2 and TiAl or after them, which confirms that these processes are interrelated. The mechanochemical synthesis for hours in a planetary-ball mill results in the formation of micron particles that have agglomerated or conglomerated (sometimes layered) composite structures. X-ray analysis is used to study the phase evolution of Ti-Al-and Ti-Al-B alloys. It is shown that the presence of boron in mechanical alloying reduces the degree of amorphization and promotes the formation of fine crystalline structure. In addition, the presence of boron prevents the formation of metal oxides and a number of intermetallides. It is established that the sintered dispersion-hardened Ti-Al-B material consists of an aluminide matrix with micron and submicron inclusions of borides. The increase in boron content leads to a higher amount of boride inclusions. As a result, the distance between them decreases and thus microhardness increases.

Density functional study of AlBn clusters for n=1–14

“…For the following sizes of n = 9-12 the planar configurations are energetically favored in the most stable isomers. Open cage (bowl-like) and quasi-planar forms are the most stable clusters for AlB 13 and AlB 14 .…”

“…Because of its high melting point and hardness it has been the subject of many studies [13][14][15][16][17][18][19][20]. Depending on the synthesis condition and the amount of boron, it has the ability to influence the structure and phase composition of materials [13,14]. General results for pure, small sized boron clusters show that the relatively more stable structures are, mainly, the planar and quasi-planar nuclear arrangements compared to any of the three-dimensional forms.…”

Mechanochemistry: opportunities for new and cleaner synthesis

“…[63] Elemental combination has also been used for boron-containing alloys in the Ni-Nb-B [64] and Ti-Al-B systems. [65] The risk of atmospheric oxidation of the metals means that these reactions are carried out under inert gas, typically argon. They generally require relatively long milling times (24 to 300hrs).…”

Mechanical alloying and spark plasma sintering of the intermetallic compound Ti50Al50

“…As a rule, several problems are involved in obtaining nanostructured material from ultrafine particles; on the one hand, in order to retain a small grain size it is necessary to minimize recrystallization processes that are initiated at high temperatures, while on the other hand the production of dense and strong material requires quite high sintering temperatures [11]. Even if the use of [12] provided a high heating rate and a relatively minor holding time, but unusually high load and a high rate of cooling is necessary [13]. Furthermore, they are too costly to be applied to commercial applications, especially to a civil application such as the automobile industry.…”