Martensite in titanium alloy

A product of titanium alloy formed by martensite transformation. The structures of, near-and + type titanium alloys after air cooling or rapid cooling from the phase region are martensite or mainly Martensite. The metastable titanium alloy can also produce martensite after deformation.

There are four types of martensite in titanium alloys: {334} single crystal hexagon martensite with habitus surface of {334} and no twin structure;{344} hexagonal Martensite of twin structure with habitus surface of {344}.Rhombic martensite derived from hexagonal structure; Face-centered cubic or face-centered square martensite.

(1) There are basically no twins in the hexagonal Martensite tablet of type {334} single crystal, and the phase between the matrix roughly follows the Bergus direction relation, that is, (0001) α ‖{110}β; <1120>α‖<111>β. The specific habitus surface on which the transition occurs is limited to those variables close to 90° with {110}, and the specific habitus surface on {110} changes to the base surface, i.e. the habitus surface is about 90° with the base surface. For example, the habitus surface of many low alloy titanium alloys martensite is approximately 87° from the base surface. This type of martensite is the most common and can be considered as an -Ti supersaturated solid solution containing a small amount of -stable elements. (2) There are high density dislocation, stacking dislocation and twins (usually {1011} twins) in {344} hexagonal Martensite tablets. Martensite and approximately follow (0001)α‖{110}β;<1120>α‖<111>'s orientation. The habitus surface is approximately 90° from the base surface. For example, the habitus surface of {344} Martensite in ti-5Mn alloy is approximately 83° from the base surface. This type of martensite can be considered as -Ti supersaturated solid solution with a higher concentration of -stable elements.

(3) Rhombic Martensite has three crystal axes with unequal axial lengths but perpendicular to each other. There may be no twins or twins in martensite tablets, approximately following {001}‖{110}β<110>‖<111>β's orientation. It can be considered that rhombic martensite is derived from hexagonal structure. When there are more alloying elements in the phase, the atomic arrangement changes slightly in the process of quenching and cooling, which destroys the hexagonal symmetry of phase and leads to the low rhombic symmetry with c axis smaller than B axis. Rhombic Martensite is found in titanium alloys with high -stable elements (such as titanium-vanadium, titanium-tantalum, titanium-niobium, titanium-molybdenum, titanium-tungsten, titanium-rhenium and other alloy systems).

(4) Plane-centered cubic or plane-centered tetragonal Martensite is twin martensite, and there are one or two sets of {111} twins in martensite tablets. This type of martensite was observed in the thin films of Ti-20V, Ti-11.6Mo, Ti-7Cr, Ti-5Mn and other alloys.

Transformation mechanism Martensite transformation is a non diffusive shear transformation. According to the theory of martensitic transformation crystal, the titanium alloy from a body centered cubic beta phase into a six-party and centered cubic or rhombic, face-centered tetragonal martensite, in addition to the shape of a uniform shear to generate the necessary change, also need to have a complementary shear, on the premise of no longer cause change shape, the structure of the generated by the uniform shear distortion into martensite matrix.

The transformation of martensite in titanium alloy is carried out by nucleation and crystal nucleus growth, and the martensite nucleus may be activated by heat. From the perspective of industrial production, the critical cooling rate of martensite formation and the temperature M_s at the beginning of martensite formation are two important parameters, which are mainly related to the properties and contents of alloying elements.

The effect of alloying element types and contents on M_s points is largely consistent with their stabilization of the phase, and their positions on the periodic table reflect some regularity (see figure).Zirconium and titanium are homozygous and homozygous, adjacent to each other in the periodic table, forming homology and homology phase diagrams with titanium, which is weakly stable phase and therefore slightly reduces M_s point. Niobium, vanadium, molybdenum, chromium, manganese and iron are body centered cubic structures, which stabilize phase and reduce M_s. The farther these elements are from titanium in the periodic table, the greater the stabilization of beta phase and the greater the decrease of M_s. Starting from iron, cobalt, nickel, copper and silver all have face-centered cubic structure, and the stabilizing effect on phase is decreasing. Aluminum stabilizes the alpha phase and thus causes M_s to rise. From tin to lead and bismuth, the increase in atomic diameter makes it more advantageous for solid solution to phase in a less dense reactor than for solid solution to phase, thus stabilizing the phase and reducing M_s.

The influence of alloy element type and content on the critical cooling rate is not obvious in the periodic table, but it is certain that elements in the compound phase with titanium tend to increase the critical cooling rate.