Investigation on the Durability of Ti-6Al-4V Alloy Designed in a Harmonic Structure via Powder Metallurgy: Fatigue Behavior and Specimen Size Parameter Issue

Abstract

In the present work, the four-point bending loading fatigue properties of a heterogeneously distributed grain size microstructure consolidated from Ti-6Al-4V alloy powder are studied. The microstructure involved here, a so-called “harmonic structure”, possesses quasi-spherical large grain regions (“cores”) embedded in a continuous fine grain region (“shell”). Unlike the previous reports dealing with this issue, the effect of the specimen size on the fatigue characteristics is also probed, since two distinct specimen configurations are considered. Furthermore, the obtained experimental data are compared with the corresponding fatigue results derived from homogeneous coarse grain counterparts. Contrary to homogeneous structure material, discrepancies on both the fatigue strength and the fatigue crack initiation aspects are found for the harmonic structure material. Consequently, the present work aims to clarify the underlining phenomena involved in the specimen size effect detected for Ti-6Al-4V designed in the harmonic structure. A less active interface surface between the core and the shell combined with a wider critical volume in the large size specimen should be the main reasons of the fatigue strength discrepancy.

Mechanical Properties of Spark Plasma Sintering-Processed Pure Ti and Ti-6Al-4V Alloys: A Comparative Study between Harmonic and Non-Harmonic Microstructures

“…Ti and Ti-6Al-4V alloys were all designed by the SPS process from a powder metallurgy route as originally proposed and developed by Prof. K. Ameyama's group [16][17][18][19]31]. By coupling MM and SPS process, HS samples can be obtained, compared to only considering SPS, which leads to conventional homogeneous or bimodal microstructures.…”

“…The resulting MM powders are referred to as Ti-6Al-4V-MM-90 ks, Ti-6Al-4V-MM-180 ks, and Ti-MM-360 ks, respectively. The combination of MM and SPS processes leads to samples with an HS design composed of coarse-grained regions (core) and a rim of fine-grained regions (shell) as described elsewhere [16][17][18][19]22]. The sintering parameters were as follows for all samples: applied uniaxial pressure of 50 MPa for half an hour considering a dwell temperature of 800 • C. An SPS 510-ML by NJS was considered.…”

“…Subsequently, SPS sintering will consolidate all of the powder by creating an interconnected network of the different particles' shells. This mechanism leads to a so-called harmonic structure [16][17][18][19].…”

Sigma Phase Stabilization by Nb Doping in a New High-Entropy Alloy in the FeCrMnNiCu System: A Study of Phase Prediction and Nanomechanical Response

“…Since these solid solutions are responsible for macroscopic strength, ductility, corrosion resistance, creep resistance, cryogenic, and high-temperature properties, it is necessary to accurately control and predict their formation to achieve a functionally designed alloy. In conventional alloys [1][2][3][4][5][6][7][8], efficient design has been performed using energy minimization systems such as Thermo-Calc based on the CALPHAD method [9][10][11][12]. However, for designing functional alloys, the prediction of microstructures remains a challenge, particularly in high-entropy alloys and complex concentrated alloys [13][14][15][16].…”