Question

(a) Can oxygen be considered an alloying element for titanium? Explain.

(b) What is the effect of high oxygen content on the mechanical properties of titanium alloys?

(c) Unlike steel martensite, the formation of titanium martensite does not impart high hardness to the alloy. Explain why.

(d) What principal microstructure changes occur when titanium martensite is tempered at 500ºC?

Answer 1

a) Small amounts of oxygen can be considered as an economical alloying element that increases the strength of titanium. By controlling its content in the metal, it is possible to reach the optimal ratio of plasticity and strength characteristics of the material. In this case, it is very important to ensure uniform distribution and desirable form of oxygen in the metal. This is achieved by applying appropriate technologies for melting and heat treatment. For this purpose, the chamber electroslag remelting process can be used. In addition to refining in a controlled atmosphere, this process offers the possibility of additional alloying during remelting. The alloying step can be done from both the gas phase and by adding different alloying elements in the solid state.

b) The mechanical properties of titanium, such as tensile and fatigue strengths, can be enhanced with oxygen additions, but care must be taken not to compromise toughness and ductility. Hydrogen, at concentrations below the commercially accepted limit, can also be detrimental to some of the mechanical properties of titanium, particularly under multiaxial stress and at both high and low strain rates. Hydrogen decreases ductility and creep resistance without, however, lowering the tensile strength. Although some of these effects result from the formation of a hydride phase, transmission electron microscope evidence indicates that residual interstitial hydrogen lowers the energy for dislocation generation and propagation. These hydrogen effects are more pronounced in oxygen alloyed titanium than in high purity titanium.

c) Maraging steel contains an extremely low amount of carbon (0.03% maximum) and a large amount of nickel (17–19%) together with lesser amounts of cobalt (8–12%), molybdenum (3–5%), titanium (0.2–1.8%) and aluminium (0.1–0.15%). Maraging steel is essentially free of carbon, which distinguishes it from other types of steel. The carbon content is kept very low to avoid the formation of titanium carbide (TiC) precipitates, which severely reduce the impact strength, ductility and toughness when present in high concentration.