Microstructure Of Ti6Al4V Research Articles

Modelling the micromorphology of heat treated Ti6Al4V forgings by means of spatial tessellations feasible for FEM analyses of microscale residual stresses

Due to finite thermal conductivity and the heterogeneous microstructure of Ti6Al4V, the temperature distribution within large components during thermal processing is highly heterogeneous on both, the macroscale and the microscale. To compute a spatial distribution of stresses at the microscale, a microdomain partition is prerequisite. By analysing representative micrographs, characteristic grain shapes are determined which serve as validation of numerically generated realistic microdomain partitions utilising the technique of spatial tessellations. By generalising the standard Voronoï tessellation, a more sophisticated tessellation, the Johnson–Mehl tessellation is introduced to capture these characteristics appropriately. The Johnson–Mehl cells grow isotropically around the kernels which result from an inhomogeneous Poisson point process, replicating the underlying phase evolution mechanism during thermal processing. In order to capture the anisotropy of the microstructure caused by preceding forging, a geometrical morphing is applied subsequently to the computation of the spatial tessellation. Comparison of the basic features of both, the experimentally derived micrographs and the numerically derived ones, reveals a good qualitative agreement.

Cavitation Erosion of Titanium Alloys.

Cavitation erosion was studied for various types of titanium alloys and for Ti-6 Al-4V with various heat treatments. The erosion resistance is the highest for β type alloy and decreases in order of α+β type alloy and pure titanium (α type). The resistance is macroscopically evaluated in terms of HV2/E (HV : Vickers hardness and E : Young's modulus), but the erosion proceeds depending on the microstructures. Heat treatments changes the microstructure of Ti-6 Al-4V drastically. Therefore, annealing at high temperature produces the large second phase and results in the accerelation of the erosion depth, in spite of low mass loss rate.

Effects of plasma immersion ion implantation of oxygen on mechanical properties and microstructure of Ti6Al4V

Plasma immersion ion implantation (PIII) is a surface treatment with increasing interest, as it offers the possibility of performing three-dimensional ion beam treatments, reducing the need for manipulation under vacuum to obtain a uniform treatment in geometrically complex parts. In this work the PIII process has been used to perform surface treatment on Ti6Al4V alloy. This Ti alloy is commonly used in aerospace and biomedical applications, due to its good combination of mechanical and chemical properties, such as strength to weight ratio, corrosion resistance or bioinertness. However, due to its poor tribological properties, the use of surface treatments to improve wear resistance or decrease friction coefficient is often recommended. PIII has been used to implant the surface of Ti6Al4V alloy with oxygen ions. The oxygen plasma was generated by electron cyclotron resonance microwave excitation, working at two different pressures. At the lower pressure plasma density was increased by means of an external ring magnet. High voltage pulses of −40 kV, at of 400 and 600 Hz pulse repetition rates, were applied. Elastic recoil detection (ERD) analysis showed retained doses in the range of 3×10 17 to 1×10 18 O atoms cm −2, with oxygen concentration values ca 65% in the near surface region. Surface mechanical properties such as hardness, wear and friction have been evaluated. Microindentation tests showed an increase of up to 100% in the surface hardness of the ion implanted samples compared to the non implanted material. Dry pin-on-disk tests with spherical ended UHMWPE pins showed a very significant increase in wear resistance in oxygen implanted Ti6Al4V samples. Scanning electron microscopy and optical profilometry showed an important roughening of the Ti alloy surface after PIII treatment under selected conditions.