High Tensile Ductility and Strength in Bulk Nanostructured Nickel

Abstract

Bulk nanostructured (NS) materials have relatively high strength but disappointingly low tensile ductility (elongation to failure) at ambient temperatures. [1][2][3][4] The limited ductility of NS materials has emerged as a particularly challenging issue in the study and application of this novel class of materials. Recently, a variety of strategies aimed at improving the poor ductility of NS materials have been reported; the results reveal varying degrees of success. [5,6] Despite some encouraging reports, the improvements in ductility remain quite limited, usually below 15% for most of the strategies, except possibly for bi-modal Cu (with a ductility of 65%) and ultrafine grained (UFG) Fe-Cr-Ni-Mn steel (ca. 30%). [7,8] In this Communication, we use cryomilling and subsequently quasi-isostatic (QI) forging processes (formerly known as Ceracon forging), to prepare bulk dense multimodal and bimodal NS Ni with tensile ductility of 42% and 49%, and yield strengths of 457 and 312 MPa, respectively. This combination of strength and ductility is much superior to those of the NS Ni prepared by electro-deposition (ED), [9][10][11][12][13][14][15][16] cryorolling, [17] equal-channel angular pressing (ECAP) and high pressure torsion (HPT) methods, [18] and cold drawing. [19] Microstructural analyses suggest that significantly reduced extrinsic processing artifacts, the presence of equilibrium high-angle grain boundaries (including twin boundaries), and multi-/bimodal grain size distributions are responsible for the measured high ductility. The high strength is argued to originate from several sources, including a high density of dislocations, UFGs, and from solid solution strengthening. Compared with other synthesis methods, the synthesis methodology described in the present work has no scale or material limitations, and therefore has important implications in terms of its potential for the large-scale fabrication of bulk NS metals, alloys, and composites that can be used in applications requiring both high ductility and strength. Bulk NS materials are usually synthesized by either a two-step approach involving the synthesis and consolidation of nanoparticles (e.g., via inert-gas condensation) [2,3] or nanocrystalline powders (e.g., via ball milling or cryomilling), [20] or a one-step approach such as severe plastic deformation (SPD). [21] In the case of NS materials prepared by the two-step approach, powder handling can yield extrinsic processing artifacts (such as porosity, incomplete bonding, impurities, and others). It is now well-established that these artifacts, when present, will cause premature failure under tensile stresses, sometimes even before the onset of yielding. [3] In a recent study, the material was consolidated in situ via an approach involving cryomilling followed by room-temperature milling. [22] In this case, the NS Cu prepared by this novel method was reported to have a uniform tensile elongation (strain before necking) of 14% and a high yield strength of 790 MPa. Despite these encouraging results, ...

Microstructure and mechanical behavior of ultrafine-grained Ni processed by different powder metallurgy methods

“…For the as-consolidated samples, there is a peak at approximately 60 in the misorientation distribution, which corresponds to S3 coincident-site lattice twin boundaries. 18,19 After compression at 10% strain, this peak disappeared for both samples, indicating the reduction of densities of twin boundaries. This observation is in agreement with the results of x-ray line profile analysis.…”

Strong Strain Hardening in Nanocrystalline Nickel

“…For simplicity, we hereafter refer to NC metals as those with ''largely clean'' grain interior, which contains no dominating twins, second phase particles or other structures. Note that improved strain hardening has been reported recently in nanostructured metals [5][6][7][8]. However, defined as metals with structural features less than 100 nm, they are not nanocrystalline because their grain sizes are usually larger than 100 nm.…”

Nanostructured Ti Consolidated via Spark Plasma Sintering

“…A similar approach was previously used for cryomilled materials, including Al, [15] Ni, [39] and Ti [13] alloys. A bimodal or multimodal structure can also be formed via thermomechanical treatments and again provide a high ductility.…”

“…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.…”