Evaluation of near-threshold fatigue crack propagation in Ti-6Al-4V Alloy with harmonic structure created by Mechanical Milling and Spark Plasma Sintering

Abstract

ABSTRACT. Titanium alloy (Ti-6Al-4V) having a bimodal "harmonic structure", which consists of coarsegrained structure surrounded by a network structure of fine grains, was fabricated by mechanical milling (MM) and spark plasma sintering (SPS) to achieve high strength and good plasticity. The aim of this study is to investigate the near-threshold fatigue crack propagation in Ti-6Al-4V alloy with harmonic structure. Ti-6Al-4V alloy powders were mechanically milled in a planetary ball mill to create fine grains at powder's surface and the MM-processed powders were consolidated by SPS. K-decreasing fatigue crack propagation tests were conducted using the DC(T) specimen (ASTM standard) with harmonic structure under the stress ratios, R, from 0.1 to 0.8 in ambient laboratory atmosphere. After testing, fracture surfaces were observed using scanning electron microscope (SEM), and crack profiles were analyzed using electron backscatter diffraction (EBSD) to discuss the mechanism of fatigue crack propagation. Threshold stress intensity range, Kth, of the material with harmonic structure decreased with stress ratio, R, whereas the effective stress intensity range, Keff, showed constant value for R lower than 0.5. This result indicates that the influence of the stress ratio, R, on Kth of Ti6Al-4V with harmonic structure can be concluded to be that on crack closure. Compared to the compact prepared from as-received powders with coarse acicular microstructure, K th value of the material with harmonic structure was low. This was because the closure stress intensity, K cl , in the material with harmonic structure was lower than that of the coarse-grained material due to the existence of fine grains. In addition, the effects of the grain size on the fatigue crack propagation behaviors of Ti-6Al-4V alloy were investigated for the bulk homogeneous material. The effects of the stress ratio and the grain size on the fatigue crack propagation of the material with harmonic structure were quantified.

Evaluation of near-threshold fatigue crack propagation in harmonic-structured CP titanium with a bimodal grain size distribution

“…The homogeneous fine-grained structure resulting from severe plastic deformation typically decreases the ductility of a metal [11,[14][15][16][17][18], including that of CP titanium [11,14,15], via the early induction of plastic instability. Previous investigations have demonstrated that a new microstructural design can improve both the strength and ductility (or toughness) of some metallic materials [8,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. As an example, Sergueeva et al [8] reported that both the strength and elongation of CP titanium were improved by high-pressure torsion followed by brief low temperature annealing.…”

“…As an example, Sergueeva et al [8] reported that both the strength and elongation of CP titanium were improved by high-pressure torsion followed by brief low temperature annealing. Our own research group [24][25][26][27][28][29][30][31] has developed a harmonic structure concept for the fabrication of titanium-based materials with high strength and ductility, based on powder metallurgy. This process generates a coarse grained structure surrounded by a network of fine grains.…”

Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure

“…Fatigue tests were interrupted after a certain number of cycles, and acetyl cellulose films were placed on the specimen surface using the replica method to examine fatigue crack initiation and propagation. Once the crack length was measured using optical microscopy, the stress intensity range, K, was calculated [27,28,32]; the aspect ratio, c/a, for small cracks was estimated as follows:…”

“…K-decreasing tests were also conducted to approach the fatigue threshold. The present study employed disk-shaped compact (DC(T)) specimens (2 mm thick) [28][29][30] in accordance with the ASTM standard. The tests were conducted in the ambient laboratory atmosphere under a stress ratio R of 0.1.…”

“…Our research group has designed a harmonic structure using powder metallurgy to sinter mechanically milled stainless steel powders [18][19][20][21][22][23], which improve both their strength and ductility by suppressing necking during tensile deformation [20]. In particular, we have focused on the fatigue properties under fourpoint bending [24][25][26][27] and near-threshold fatigue propagation of long cracks [28][29][30] in titanium-based materials with a bimodal harmonic structure. To achieve sufficient performance with the newly developed harmonic structured stainless steels, their fatigue properties and fatigue crack propagation behavior need to be examined.…”