titanium

Ductile fracture of Titanium alloy            Ti-6Al-4V and it’s microstructure

Titanium is a lustrous transition metal with a silver colour,low density(as half that of the steel), high strength with higher toughness and corrosion resistance to aqua medium. Although titanium is the ninth abundant metal on earth , it has major disadvantage in its processing and to get a pure metal. So we look into Titanium alloys.

Ti-6Al-4V :

It is an alpha-beta titanium alloy with the 4.5% of venedium and 6.75% of Aluminum(at max.) and with few percentages of Fe, O, C, N, H, Y. It has a wide range applications in racing and aerospace , marine ,chemical, and gas turbine industries.

Since it is the mixture of alpha and beta alloy of the same alloy, it attains the properties like higher strength,goid creep resistance and low ductility. The ratio of alpha and beta decides whether the material shows ductile fracture or brittle fracture. Since here we discuss about only ductile not the brittle , so the ratio alpha/beta will be greater than 1.

Ductile fracture of Ti-6Al-4V :

  Ductile fracture test is conducted on several different specimen types including plane stress(thin flat specimens and for thick flat specimens individually), axisymmetric(like spheres) and with combined loads. Theories attribute the failure to void nucleation , coalescence and eventually growth which then leads to complete failure. The failure point of interest for each of the specimen type is taken from the center most point because the void nucleation is assumed to begin here. The study includes the plastic strain localization since it is a dual phase alloy and we can observe shear band and vertical split band types of failures too.

                                                              microstructure of ductile failure in Ti-6Al-4V alloy

High cycle fatigue has been identified as one of the prime cause of turbine engine failure in military aircrafts from the initiation of cracks. The Titanic ship also broken into 2 due to the use of less alpha Ti-6Al-4V alloy than beta which induced brittle fracture.

Conclusion :

The fracture toughness of Titanium alloys is governed by the initiation and propagation, resistance characteristics of the micro crack generated from the precrack tip before maximum load, irrespective of the alloy type and testing temperature, and the failure will occur due to the coalescence of voids at the center of the specimen.