Effect of Heat Treatment on the Microstructure and Mechanical Properties of Selective Laser-Melted Ti64 and Ti-5Al-5Mo-5V-1Cr-1Fe

Additive manufacturing (AM) has shown the ability in processing titanium alloys. However, due to the unique thermal history in AM, the microstructure of AM-fabricated parts is metastable and non-equilibrium. This work was aiming to tailor the microstructure and to improve the mechanical properties of α+β Ti-6Al-4V alloy and metastable β Ti-5Al-5Mo-5V-1Cr-1Fe alloys by manipulating the post-process heat treatment. The results showed that Ti-6Al-4V exhibited a metastable α’ martensite microstructure in the as-fabricated condition, while a metastable β structure was formed in as-printed Ti-5Al-5Mo-5V-1Cr-1Fe. After post-process heat treatment, both lamellar and bimodal microstructures were obtained in Ti64 and Ti-5Al-5Mo-5V-1Cr-1Fe alloys. Especially, the Ti-6Al-4V alloy subjected to 950 °C annealing showed the lamellar structure with the highest fracture toughness of 90.8 ± 2.1 MPa.m1/2. The one cyclically heat-treated has excellent combined strength, ductility and fracture toughness attributed to the bimodal structure. In addition, similar observations of lamellar and bimodal microstructure appeared in the post-process heat-treated Ti-5Al-5Mo-5V-1Cr-1Fe alloy. This study demonstrated that heat treatment is an effective way to tackle the non-equilibrium microstructure and improve the mechanical properties of selective laser melting (SLM)-fabricated titanium alloys.

Characterization of recovery onset by subgrain and grain boundary migration in experimentally deformed polycrystalline olivine

To apprehend plate tectonics and the dynamics of the lithosphere–asthenosphere boundary, composed principally of olivine, we need to understand the mechanisms that control plastic deformation of olivine in the relevant temperature domain. After more than 50 years of laboratory studies and investigations on natural rocks, the interplay of several key parameters (e.g. temperature, pressure, vacancy concentration, dislocation densities, grain size, strain rate) controlling polycrystalline olivine plasticity remains difficult to assess. Here, we study four olivine polycrystals, which have been deformed in axial compression under a confining pressure of 300 MPa, at 1273 or 1473 K. Despite significant differences in mechanical properties (stress–strain curves), previous characterization by scanning (SEM) and transmission electron microscopy (TEM) did not reveal significant differences in dislocation microstructures which could explain these contrasted behaviours. We have undertaken automatic crystallographic orientation mapping (ACOM) analyses in TEM to increase the spatial resolution of characterization compared to previously obtained electron backscatter diffraction maps to further decipher the microstructures at nanoscale. With this novel technique applied to olivine, a noticeable difference in the onset of microstructural recovery has been identified between specimens deformed at 1273 and 1473 K. The microstructures of the olivine polycrystals deformed at 1473 K exhibit numerous curved grain and subgrain boundaries, advocating for recovery by boundary migration. In contrast, the microstructures of the olivine polycrystals deformed at 1273 K have significantly fewer subgrain boundaries and show more straight boundaries (i.e. closer to an equilibrium microstructure) than in the specimen deformed at 1473 K. Characterization by ACOM-TEM has permitted the identification of the onset of recovery, which is led by boundary migration even for very low macroscopic finite strains.

Acoustic and Microstructural Properties of Partially Molten Samples in the Ice–Ammonia System

We measured the ultrasonic properties and microstructure of two-phase binary mixtures of the ice–ammonia partial melt system, which was selected based on its importance for numerous planetary bodies. The equilibrium microstructure of ice–ammonia melt was examined using a light microscope within a cold room. The measured median dihedral angle between the solid and melt at 256 K is approximately 63°, with a broad distribution of observed angles between 10° and 130°. P-wave velocities in the partially molten samples were measured as a function of temperature (177 < T(K) < 268) and composition (1–6.4 wt % NH3). Vp decreases approximately linearly with increasing temperature and melt fraction. We compare the results of this study to those of other potential binary systems by normalizing the datasets using a vertical lever (TL–TE) and articulating the potential effects on the mechanical behavior and transport capabilities of partially molten ice in icy satellites.

Laser Surface Processing for Tailoring of Properties by Optimization of Microstructure

Laser surface processing involves modification of surface microstructure and/or composition of the near surface region of a component using a high power laser beam. The advantages of laser surface processing over conventional equilibrium surface processing include rapid processing rate, retention of non-equilibrium microstructure, alloying in liquid state and development of processed zone with superior properties as compared to the same developed by equilibrium processing route. Microstructure plays an important role to control the final properties of the tailored component. In the present contribution, with a brief introduction to laser, and its application, the microstructures developed under optimum conditions by different laser surface processing will be discussed with the corresponding improvement in properties. Finally, a brief review of the future scope of research in laser surface processing will be presented.

Methods for Defining the Concentration of Nanostructured Powders in Protective Gas and its Effect on the Microstructure of Deposit Metal

The effect on the deposited metal structure of nanostructured modifiers introduced into the weldpool has been studied. Methods have been developed for determining the concentration of nanostructured powders of tungsten, molybdenum and Al2O3 in protective gas and for defining their optimal concentration. The influence of nanopowders on the structure of deposited metal was examined in consumable electrode arc welding employing the austenitic steel (chemical composition: C – 0,12%, Cr – 18%, Ni – 10%,Ti – 1%) as deposit and 1.2-mm welding wire manufactured from the austenitic steel (chemical composition: C – 0,12%, Cr – 18%, Ni – 9%,Ti – 1%). Addition of nanostructured powders of tungsten, molybdenum and Al2O3 to the weldpool has shown positive effect on the structure of metal in arc welding. It is shown that introducing the powders decreases dendrite size and leads to the formation of a more equilibrium microstructure of the weld.