Tensile deformation and fracture properties of a 14YWT nanostructured ferritic alloy

Abstract

A new larger heat of a 14YWT nanostructured ferritic alloy (NFA), FCRD NFA-1, was synthesized by ball milling FeO and argon atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt.%) powders, followed by hot extrusion, annealing and cross rolling to produce a 12.5mmthick plate. NFA-1 contains a bimodal size distribution of pancake-shaped, mostly very fine scale, grains. The as-processed plate also contains a large population of microcracks running parallel to its broad surfaces. The small grains and large concentration of Y-Ti-O nano-oxides (NOs) result in high strength up to 800°C. The uniform and total elongations range from ≈ 1 to 8%, and ≈ 10 to 24%, respectively. The strength decreases more rapidly above ≈ 400°C and transitions to largely viscoplastic creep by ≈ 600°C. While the local fracture mechanism is ductile-dimple microvoid nucleation, growth and coalescence, perhaps the most notable feature of tensile deformation behavior of NFA-1 is the occurrence of periodic delamination, manifested as fissures on the fracture surfaces.

Characterization of polyhedral nano-oxides and helium bubbles in an annealed nanostructured ferritic alloy

“…Nanostructured ferritic alloys (NFAs) are a revolutionary class of Fe-Cr-based oxide dispersion strengthened (ODS) steels with many outstanding mechanical, thermal, and chemical properties for nuclear applications [1][2][3][4][5][6][7][8][9][10] . NFAs have high temperature strength and stability up to 900 °C [11 , 12] and unmatched radiation resistance, especially with respect to managing helium generated by nuclear transmutation reactions [1 , 13-18] .…”

Complete Status Report Documenting Development of Friction Stir Welding for Joining Thin Wall Tubing of ODS Alloys

“…The texture induced from hot rolling the 14YWT plate by 95% RIT to 1 mm thickness could have also played a role in the horizontal crack propagation via a delamination mechanism. This type of fracture mechanism was recently published by Alam et al [6] who showed that rolling produced a {100}<110> texture in the FCRD-NFA-1 (a new, larger heat of 14YWT) plate, which favors brittle cleavage in BCC alloys. It is not known how the cracks were nucleated, but along with the potential tensile residual stresses in the stir zone and the brittle cleavage texture, the stresses that were released from clamping of the 1 mm thick 14YWT plate may have initiated cracks near submicroscopic defects produced by the spinning pin tool during FSW.…”

A comparative assessment of the fracture toughness behavior of ferritic-martensitic steels and nanostructured ferritic alloys

“…Both the FM steels and the NFAs retain nearly constant strength over the temperature range of room temperature to about 300 ºC. In the low temperature region (≦ 300 ºC) ultrahigh yield stresses (>1 GPa) were measured for the majority of NFAs [48]. In particular, two alloys (i.e., 9Cr NFA LAP3C and 14Cr NFA 14YWT-SM11) demonstrated extra high strengths above 2 GPa at room temperature.…”

“…It was shown that in NFAs dislocation glide was highly active in deformation, and dimple-forming ductile fracture was predominant at liquid nitrogen temperature, where the grain boundary pileup of dislocations has relatively smaller role in strain hardening and thus the stress concentration at grain boundaries can be maintained below a critical level for separation [34]. The latest hardening mechanism studies [48][49][50][51] have also confirmed that the dominance of matrix hardening or intrinsic lattice hardening at cryogenic temperatures helps avoid the grain boundary decohesion mechanism that usually is the main cause for the low toughness in the high temperature region. Fourth, the decrease of fracture toughness at elevated temperatures or DSA effect is not obvious.…”

“…A series of studies on high temperature deformation and fracture mechanisms using 14YWT NFA have shown that microcracks tend to initiate and propagate along grain boundaries or grain aggregate boundaries by a low energy boundary decohesion mechanism, resulting in little plastic deformation in grains and thus low fracture toughness [31,35]. It was also shown that the low toughness characteristics of the nanograin alloys, which originated from weaklybonded grain boundaries, was exacerbated at high temperatures as the grain boundary hardening became the major load-bearing component [48][49][50][51]. The stress concentration due to the dislocation pileups at grain boundaries, which is the main feature in grain boundary hardening mechanism, can force to open the weakly-bonded grain boundaries at high temperatures.…”