Synchrotron X-ray diffraction investigation of the effect of cryogenic treatment on the microstructure of Ti-6Al-4V

Ti-6Al-4V was (intercritically) annealed at various temperatures in the range 1000-1250 K with intervals of 50 K, followed by cooling to room temperature at an average rate of approx. 1 K.s(-1). The heat treatment procedure was intended to systematically vary the microstructure and alter the thermal stability of the beta phase through the partitioning of the alloying elements between a and beta phases. The annealing treatment was followed by cryogenic treatment, CT, which consisted of immersion of the samples in boiling nitrogen for durations ranging from 5 min to 24 h, followed by re-heating in air. The heat-treated material was characterized ex-situ applying light optical microscopy (LOM), synchrotron X-ray diffraction (S-XRD), and hardness Vickers indentation. A set of samples not subjected to cryogenic treatment was taken as reference. LOM revealed that the material's microstructure after heat treatment consisted of a fraction of primary a grains and regions of lamellar alpha/beta structure. S-XRD showed that the fraction of retained beta was largest, approx. 7%, for the material treated at the highest applied annealing temperature, i.e. 1250 K, and decreased to 2% with a reduction of the annealing temperature. Hardness values varied in the range 300-330 HV and did not show a measurable effect of the annealing temperature. The applied techniques did not reveal any measurable effect of cryogenic treatment, neither on the microstructure, nor on the hardness. Additionally, CT had no measurable effect neither on the lattice parameters of the phases, nor on the density of crystallographic defects in the material. These observations are inconsistent with literature data, which report various effects of CT on the microstructure and on the mechanical properties of Ti-6Al-4V, and are attributed to a pronounced stability of the retained beta phase against its conversion into alpha' martensite during CT in the present heat treatment conditions.