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Post annealing treatment is generally needed for additively manufactured titanium alloy to decompose metastable phases, alleviate residual stress, and improve ductility. In this work, in-situ electrical resistivity and line profile analysis of X-ray diffraction were utilized for monitoring phase transformation behaviors and dislocation evolutions of a laser powder bed fusion-built Ti-6Al-4V alloy under post annealing treatment. Besides, hardness and tensile tests were adopted for revealing the effects of phase transformation and dislocation evolutions on the mechanical properties. The results indicated that post annealing treatment decomposed martensitic α′ into lamellar α + β and eliminated dislocations efficiently. The martensite decomposition rate increased with the annealing temperature. Annealing at 700 °C for 4 h eliminated 98% of the dislocations, and further annealing has only a limited influence on the dislocation density. Annealing at 700 °C for 16 h is beneficial for achieving a high ductility of 10.3% owing to the favorable equilibrium lamellar α + β microstructure. These findings provide valuable insights for optimizing post annealing treatment procedures to enhance the mechanical properties of additively manufactured Ti-6Al-4V alloys. [ABSTRACT FROM AUTHOR]

Dynamic transformation is shown to take place in steels and titanium alloys at temperatures above and below the beta transus, respectively. Here the hard phase transforms into a softer phase during deformation. This unusual metallurgical phenomenon has previously been explained in terms of transformation softening (Ghosh et al., 2016). In the present work, compression tests were performed on samples of Zircaloy-4 at temperatures below the beta transus. The microstructures and microhardness measurements indicate that alpha transforms displacively into beta during deformation at temperatures down to about 75 °C below the transus. [ABSTRACT FROM AUTHOR]

Isothermal holding experiments were performed on a Ti-6Al-4V alloy after compression in the two-phase region. The forward transformation took place during compression while the reverse transformation was diffusion controlled when the samples were held after unloading at 940 °C, 970 °C, and 1000 °C. Time-temperature-reverse transformation (TTRT) curves are derived, which indicate that the rate of reverse transformation is independent of temperature when the holding time is 18 seconds. By contrast, it depends on temperature when the holding time is increased to 180 and 1800 seconds. [ABSTRACT FROM AUTHOR]

The present study concerns the deformation and fracture behavior of two ferrite–martensite dual phase steels (FMDP660 and FMDP780) with different phase fractions subjected to different strain rate (0.001 s−1 to 1000 s−1) tensile testing. For both steels, the yield strength (YS) monotonically increased with strain rates, whereas the values of ultimate tensile strength (UTS), uniform elongation (UE) and post-uniform elongation (PUE) were maintained stable at the low strain rate range (0.001–0.1 s−1), followed by a significant increase with strain rate at high strain rate levels (0.1–1000 s−1). The FMDP780 steel with a higher fraction of martensite possessed a stronger strain rate sensitivity of tensile strength and elongation (UE and PUE) values at the high strain rate stage, compared with the FMDP660 sample. The change of UTS and UE with different strain rates and phase fractions was highly related to the strain hardening behavior, which was controlled by the dislocation multiplication in ferrite, as validated by transmission electron microscopy (TEM). The fracture surface of the two steels was characterized by dimpled-type fracture associated with microvoid formation at the ferrite–martensite interfaces, regardless of the strain rates. The change of the dimple size and PUE value of the two steels with strain rates was attributed to the effect of adiabatic heating during tensile testing. [ABSTRACT FROM AUTHOR]

The dynamic transformation of alpha to beta is shown to take place in titanium at temperatures below the beta transus. The driving force for this transformation is the net softening associated with the formation of the lower flow stress beta phase. The obstacle to the transformation consists of the free energy difference between the phases as well as the work of dilatation during the phase change. Here the critical condition for transformation is defined as the moment when the driving force becomes equal to the obstacle energy. This approach is supported by data obtained from compression tests. [ABSTRACT FROM AUTHOR]