Mechanical Behavior of Cryomilled CP-Ti Consolidated via Quasi-Isostatic Forging

Abstract

Commercially pure (CP) Ti (Grade 2 with chemical composition 0.190 wt pct O, 0.0165 wt pct N, 0.0030 wt pct C, and 0.013 wt pct Fe) was cryomilled in liquid argon and liquid nitrogen for 8 hours. The influence of the milling environment on the chemistry, grain size, and grainboundary structure of CP-Ti was studied by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and chemical analysis. The results show that the final average grain size obtained after 8 hours of cryomilling was~20 nm, for both liquid nitrogen and liquid argon cryomilling environments. Grains were observed to be heavily deformed and they did not reveal well-defined boundaries between them. Liquid nitrogen and liquid argon cryomilling environments led to differences in the final powder chemistry. Cryomilling in liquid nitrogen resulted in Ti powders with~2 wt pct nitrogen, which caused embrittlement that in turn affected the mechanical behavior of the consolidated materials. Cryomilling in liquid argon resulted in powders with slightly higher oxygen levels than those from liquid nitrogen experiments; this was attributed to the use of stearic acid (CH 3 (CH 2 ) 16 COOH) as a process control agent (PCA). The cryomilled powders, in the form of various compositional blends from the argon and nitrogen milling experiments, were subsequently consolidated via quasi-isostatic (QI) forging, for mechanical behavior studies. The mechanical testing results showed that the QI-forged 85 pct as-received +15 pct liquid-argon-cryomilled powder blend exhibited~30 pct elongation to fracture, with a yield strength (YS) of 601 MPa and an ultimate tensile strength (UTS) of 711 MPa. In the case of 100 pct liquid-argon-cryomilled and QI-forged material, the YS, UTS, and elongation values were 947 and 995 MPa and 4.32 pct, respectively. The mechanical behavior was discussed in terms of the operative microstructure mechanisms. The enhanced ductility noted in the blended powders was discussed in terms of the presence of a bimodal microstructure.

Nanostructured Ti Consolidated via Spark Plasma Sintering

“…Powder contamination is an inherent characteristic of cryomilling, and it may arise either from the processing media or from the atmosphere. [14,16,33,41] Carbon black, used as PCA, represents the origin of increased C content, whereas the steel milling media is the source for Fe contamination. The increased amounts of N and O are attributable to atmospheric contamination during powder handling and transport prior to chemical analysis.…”

“…The microstructure and mechanical behavior of cryomilled CP-Ti was studied previously by Ertorer et al [13,14] In these studies, balanced combinations of strength and ductility were reported. Also in these studies, quasi-isostatic (QI) forging was used as a consolidation and secondary deformation method.…”

“…[7][8][9][10][11] This trend is illustrated by reports of CP-Ti with combinations of high tensile strength and good ductility (900-1500 MPa ultimate tensile strength [UTS] and 6-25 pct elongation to failure) as reported in different studies when processed via high-pressure torsion, [9] equal-channel angular pressing (ECAP), [7,12] and cryomilling. [13,14] For example, Semenova et al [7] reported up to 1150 MPa room temperature tensile yield strength (YS) and 1240 MPa UTS with a tensile elongation value of 11 pct, using ECAP followed by a heat treatment step.…”

“…In roomtemperature tensile tests, a YS of 840 MPa, a UTS of 902 MPa, and an elongation to failure value of 27.5 pct were measured for these samples. More detailed information related to cryomilled Ti powders and bulk materials is available in related publications by Sun et al [16,17] and Ertorer et al [14] In view of the aforementioned results, the objective of the present work is to investigate the consolidation behavior, microstructure evolution and mechanical behavior of cryomilled CP-Ti powders consolidated via the spark plasma sintering (SPS) technique. In the last-two decades, SPS has emerged as an interesting consolidation approach given its reported ability to densify metal and ceramic powders fully in a short time interval, thereby helping retain the starting microstructure.…”

“…The selection of a 70 pct cryomilled + 30 pct as-received mixture was motivated by numerous reports on improved ductility in bimodal and multimodal metals, [15,[34][35][36][37][38] as well as earlier experiments on QI-forged cryomilled Ni and Ti. [13,14,39,40] The blended powders were degassed in a closed stainless steel can under vacuum (~10 À3 Pa) at 623 K (350°C) to eliminate moisture and other volatile species using a Varian Turbo-V 70D (Varian Inc., Santa Clara, CA) turbo molecular pump equipped with a Terranova Model 934 vacuum gage controller (Duniway Inc., Mountain View, CA) and a tube furnace. The degassed can was opened and powders were stored in an argon glove box until SPS processing to minimize possible atmospheric contamination.…”