Precipitation in MA/ODS Ferritic Alloy MA957

Abstract

Some mechanically-alloyed (MA), oxide dispersion strengthened (ODS) ferritic alloys have exhibited dramatically improved high temperature creep properties compared to other ferritic alloys. These alloys are fabricated by ball milling powders of the prealloyed metal with an oxide powder. Atom probe tomography (APT) has revealed that some of these MA/ODS alloys [e.g., 12YWT] contain a high number density of Ti-, Y-, O-enriched particles that are stable to at least 1300°C (~85% T m ) [1]. In this paper, a commercial MA/ODS ferritic alloy MA957 is characterized by APT to determine if these ultrafine particles are a common feature of these MA/ODS ferritic alloys.The commercial MA957 alloy used in this study had a nominal composition of Fe-14 wt % Cr, 0.9% Ti, 0.3% Mo and 0.25% Y 2 O 3 [Fe-14.8 at. % Cr, 0.17% Mo, 1.0% Ti, 0.13% Y and 0.19% O] and contained trace levels of Al, Mn, Si, B and C. Specimens were cut from an extruded tube. This alloy was characterized in the as-received state and after annealing for 1 h at 1300 °C. TEM revealed partial recovery of the dislocation structure but no recrystallization had occurred during the high temperature anneal.The solute distribution in this MA957 alloy is shown in the atom maps in Fig. 1. A high number density (~2 x 10 24 m -3 ) of ultrafine Ti-, Y-and O-enriched particles are evident in the as-received condition, Fig. 1a. The number density of the particles decreased by an order of magnitude to ~2 x 10 23 m -3 after the 1 h at 1300°C anneal, Fig. 1b. The average Guinier radius of the particles, r G , was determined with the use of the maximum separation method [2] to be 1.2 ± 0.4 and 1.7 ± 0.7 nm, respectively for the as-received and 1 h at 1300 °C conditions. These results indicate that some coarsening of the particles had occurred during the anneal. A representative atom map of a 2-nmthick slice through the center of one of the particles in the annealed material is shown in Fig. 2. The Y atoms were predominantly found in the central region and surrounded with a Ti-and O-enriched shell, as apparent by the TiO 2+ ions. This differing solute distribution is also evident in the atom maps shown in Fig. 1. In both conditions, r G (Y) was approximately 90% of the overall r G value. The compositions of the individual particles were determined by the envelope method [2] with a grid spacing of 0.1 nm. The average compositions and the solute partitioning factors are given in Table 1. The oxygen content in the matrix was estimated to be <0.14 and <0.19 at. % O, respectively for the as-received and annealed conditions. These estimates are upper bounds due to the possible presence of Mo 3+ and Ti 3+ ions superimposing with the TiO 2+ and O + ions, respectively. These results are similar to previous APT studies of a 3 wt% W, Mo-free MA/ODS 12YWT ferritic alloy [1]. [3] [1] D.

Radiation response of a 9 chromium oxide dispersion strengthened steel to heavy ion irradiation

“…2. These nanoclusters were similar to those previously observed by atom probe tomography in mechanically alloyed, oxygen dispersion strengthened (MA/ODS) 12YWT, 14YWT and MA957 alloys [3,8,[10][11][12][13][14][15][16]. The average Guinier diameter, 2r G , of these nanoclusters was estimated from the maximum separation method to be 2r G = 4.6 nm.…”

Stability of ferritic MA/ODS alloys at high temperatures

“…Some mechanically alloyed (MA), oxide dispersion strengthened (ODS) ferritic alloys fabricated from powders of a prealloyed mixture of iron, chromium, yttrium, titanium, and tungsten and yttria have exhibited dramatically improved high temperature creep properties at temperatures of up to 850 8C [1][2][3][4][5][6][7][8][9][10][11][12]. This temperature is significantly higher than the traditionally accepted operating range of ferritic alloys.…”

“…Atom probe tomography characterizations of a MA/ODS 12YWT alloy with a nominal composition of Fe-12.3 wt% Cr-3% W-0.39% Ti-0.25% Y 2 O 3 [Fe-13.3 at.% Cr, 0.92% W, 0.46% Ti, 0.13% Y and 0.19% O] revealed the presence of high number densities of 2-4 nm diameter titanium-, yttrium-and oxygen-enriched particles in both the asprocessed and crept conditions [7][8][9][10][11][12]. These ultrafine particles were found to be present in the ferrite after high temperature creep for up to 14,500 h at 800 8C (O50% T m ).…”

“…The aim of this study was to investigate the stability of these titanium-, yttrium-and oxygen-enriched particles during high temperature annealing. Previous studies indicated that tungsten had an important role in the improved mechanical properties [6][7][8][9][10][11][12]. The tungsten-free commercial MA957 alloy was included in this study to ascertain the importance of tungsten or whether other alloying elements such as molybdenum had similar beneficial effects.…”

Nanometer scale precipitation in ferritic MA/ODS alloy MA957

“…Some similar tungsten-containing mechanically-alloyed ODS ferritic alloys (designated 12YWT) have shown remarkable improvement in their creep properties at temperatures up to 850°C compared to the molybdenum-containing MA957 alloy [3][4][5][6][7]. The intragranular microstructure of the 12YWT alloy was found to contain a high number density of ultrafine Ti-, Y-and O-enriched particles that were resistant to coarsening during high temperature creep or isothermal annealing at up to 1300°C [8][9][10][11][12][13]. The purpose of this study was to investigate the nanometer scale microstructure in the MA957 with atom probe tomography to determine whether this molybdenum-containing alloy also exhibited similar ultrafine Ti-, Y-and O-enriched particles and to determine the stability of the nanometer size particles during high temperature annealing treatments close to the melting point of the alloy.…”