Titanium hexagonal crystal structure

Titanium has two allotropic modifications (1) alpha form and (2) beta modification. The alpha form has a close-packed hexagonal crystal structure density 4.54 g/cm3 at 20°C and stable up to 882°C. It converts very slowly to a body-centered cubic beta form at 882°C. The density of the beta form is 4.40 g/cm3 at 900°C (estimated). The other physical properties are as follows ... 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

In the patent by Hill, an aUoy of titanium containing 13 wt%vanadium, 11 wt% chromium and 3 wt% aluminum was developed as a hydrogen transport membrane material [12]. In this alloy, the crystal structure of titanium, which is normally hexagonal below 1153 K (880 °C), is stabilized in its high-temperature body centered cubic allotropic form. The body centered cubic crystal lattice is preferred for hydrogen transport. This titanium alloy was found to have hydrogen permeability superior to that of pure palladium in the range 300-450 °C (573-723 K)... 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

Figure 22. Crystal structure of Ti3SiC2 (space group P6 /mmc). The close-packed titanium layers containing the carbon atoms are shown on the left-hand side. The edge-sharing Ti C octahedra are emphasized. On the right-hand side a (110) cut through the hexagonal cell is shown. The titanium and silicon atoms form close-packed layers with the stacking sequence hhhc where the silicon atoms correspond to the second h.

The structure of the catalyst surface was deduced from the crystal structure of a-TiCls (Arlman, 1964). The a modification of TiCla consists of stacks of sheets each containing two chloride layers with one titanium layer in between. Thus along the principal axis of the crystal two chloride layers alternate with one titanium layer. In a chloride layer the ions are in hexagonal closepacking. The titanium ions are arranged in regular hexagons with an empty Ti site at the center. 

The titanium aluminide Ti2AlNb with an ordered orthorhombic crystal structure rather than the ordered hexagonal DOiq structure of Ti3Al was stronger and has higher fractvire toughness than... 

MsSis type silicides have several crystal structures. MsSis type binary titanium and zirconium silicides, SisTis and SisZrs, have the hexagonal stmcture with space group of P6s/mcm with the MnsSis-type structure (D8g structure, hP16 or 16H-type). These silicides exhibit high melting points and low densities (below 6.0Mg/m ). 

An intermetallic compound, SisTisZrs, was designed and produced to be used at ultra-high temperatures for structural applications [1]. A part of zirconium atoms in SisZrs were substituted with titanium atoms to form SisTi2Zrs with the hexagonal MnsSis-type crystal structure. 

Since the number of slip systems is not usually a function of temperature, the ductility of face-centered cubic metals is relatively insensitive to a decrease in temperature. Metals of other crystal lattice types tend to become brittle at low temperatures. Crystal structure and ductility are related because the face-centered cubic lattice has more slip systems than the other crystal structures. In addition, the slip planes of body-centered cubic and hexagonal close-packed crystals tend to change at low temperature, which is not the case for face-centered cubic metals. Therefore, copper, nickel, all of the copper-nickel alloys, aluminum and its alloys, and the austenitic stainless steels that contain more than approximately 7% nickel, all face-centered cubic, remain ductile down to the low temperatures, if they are ductile at room temperature. Iron, carbon and low-alloy steels, molybdenum, and niobium, all body-centered cubic, become brittle at low temperatures. The hexagonal close-packed metals occupy an intermediate place between fee and bcc behavior. Zinc undergoes a transition to brittle behavior in tension, zirconium and pure titanium remain ductile. 

Titanium, zirconium and hafnium in normal conditions crystallize in the hexagonal close-packed structure (a modification) with a c/a slightly smaller than the ideal one c/a = 1.587 (Ti), 1.593 (Zr) and 1.581 (Hf). At high temperature they have the bcc W-type structure ((3 modification). High-pressure transformations are known (Tables 5.21-5.23). 

HCP (hexagonal close packing) A type of crystal lattice structure found in zinc, titanium, and cobalt, for example. 

The packing efficiency of both hexagonal and cubic closest packing is 74%, and the coordination number of both is 12. There is no way to pack identical spheres more efficiently. Most metallic elements crystallize in either of these arrangements. Magnesium, titanium, and zinc are some elements that adopt the hexagonal structure nickel, copper, and lead adopt the cubic structure, as do many... 

Metallurgical classification. The crystallographic structure of titanium exhibits a phase transformation from a low-temperature close-packed hexagonal arrangement (i.e., a-hcp, alpha-titanium) to a high-temperature form body-centered cubic crystal lattice (i.e., 6-bcc, beta-titanium) at 882°C. This transformation can be considerably modified by the addition of alloying elements (Table 4.52) to produce at room temperature alloys that have all alpha, all beta, or alpha-beta structures. 

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