A c t a M a t e r i a l i a 5 9 ( 2 0 1 1 ) , p p . 3 9 2 7 -3 9 3 6 DO I : h t t p : / / d x . d o i . o r g / 1 0 . 1 0 1 6 / j . a c t am a t . 2 0 1 1 . 0 3 . 0 1 7 © 2 0 1 1 A c t a M a t e r i a l i a. E l s e v i e r L t d . I n s t i t u t i o n a l R e p o s i t o r y S t a b i l i t y o f n a n o s c a l e s e c o n d a r y p h a s e s i n a n o x i d e d i s p e r s i o n s t r e n g t h e n e d F e 1 2C r a l l o y V . d e C a s t r o a , ⇑ , E .A . M a r q u i s a , 1 , S . L o z a n o -P e r e z a , R . P a r e j a b , M . L . J e n k i n s a a D e p a r tm e n t o f M a t e r i a l s , U n i v e r s i t A b s t r a c t : T r a n sm i s s i o n e l e c t r o n m i c r o s c o p y a n d a t omp r o b e t om o g r a p h y w e r e u s e d t o c h a r a c t e r i z e o n a n e a ra t om i c s c a l e t h e m i c r o s t r u c t u r e a n d o x i d e a n d c a r b i d e p h a s e s t h a t f o rm d u r i n g t h e rm o m e c h a n i c a l t r e a tm e n t s o f a m o d e l o x i d e d i s p e r s i o n s t r e n g t h e n e d F e1 2 w t .% C r0 . 4 w t .% Y 2 O 3 a l l o y . I t w a s f o u n d t h a t s om e o f t h e Yr i c h n a n o p a r t i c l e s r e t a i n e d t h e i r i n i t i a l c r y s t a l l o g r a p h i c s t r u c t u r e b u t d e v e l o p e d a C re n r i c h e d s h e l l , w h i l e o t h e r s e v o l v e d i n t o t e r n a r y o x i d e p h a s e s d u r i n g t h e i n i t i a l p r o c e s s i n g . T h e Y a n d C rr i c h o x i d e p h a s e s f o rm e d r em a i n e d s t a b l e a f t e r a n n e a l i n g a t 1 0 2 3 K f o r 9 6 h . H ow e v e r , t h e n um b e r o f C rr i c h c a r b i d e s a p p e a r e d t o i n c r e a s e , i n d u c i n g C r d e p l e t i o n i n t h e m a t r i x .K e y w o r d s : OD S f e r r i t i c s t e e l ; N a n o p a r t i c l e s ; T r a n sm i s s i o n e l e c t r o n m i c r o s c o p y ; A t omp r o b e t om o g r a p h y . I n t r o d u c t i o nT h e s t r u c t u r a l c om p o n e n t s o f f u t u r e f u s i o n r e a c t o r s w i l l b e s u b j e c t e d t o h i g h h e a t l o a d s a n d n e u t r o n fl u x e s , im p o si n g d em a n d i n g r e q u i r em e n t s o n m a t e r i a l s u s e d i n s u c h a p p l i c a t i o n s , i n c l u d i n g l ow a c t i v a t i o n , a d e q u a t e s t r e n g t h a n d t o u g h n e s s , a n d h i g h sw e l l i n g a n d c r e e p r e s i s t a n c e [ 1 ] . Am o n g t h e m o s t p r om i s i n g c a nd i d a t e m a t e r i a l s a r e r e d u c e d a c t i v a t i o n f e r r i t i c /m a r t e n s i t i c (RA FM ) s t e e l s [ 2 ] . R e d u c e d a c t i v a t i o n i n s t e e l s i s a c h i e v e d b y r e p l a c i n g s l ow -d e c a y i n g e l em e n t s s u c h a s N b , M o a n d N i w i t h o t h e r s t h a t e x h i b i t a h i g h e r d e c a y r a t e o f i n d u c e d r a d i o a c t i v i t y , s u c h a s W a n d T a [ 3 ]. H i g h -t em p e r a t u r e t e n s i l e s t r e n g t h a n d c r e e p r e s i s t a n c e i n RA FM s t e e l s c a n b...

Abstract:In this work the spark plasma sintering (SPS) technique has been explored as an alternative consolidation route for producing ultra-fine grained Fe-14Cr model alloys containing a dispersion of oxide nanoparti-cles. Elemental powders of Fe and Cr, and nanosized Y 2 O 3 powder have been mechanically alloyed in a planetary ball mill and rapidly sintered in a spark plasma furnace. Two alloys, with nominal compositions Fe-14%Cr and Fe-14% Cr-0.3%Y 2 O 3 (wt.%), have been fabricated and their microstructure and mechanical properties investigated. The results have been compared with those obtained for other powder metal-lurgy processed alloys of the same composition but consolidated by hot isostatic pressing. The SPS tech-nique under the present conditions has produced Fe-14Cr materials that apparently exhibit different microstructures yielding inferior mechanical properties than the counterpart material consolidated by hot isostatic pressing. Although the presence of a dispersion of Y-rich particles is evident, the oxide dis-persion strengthened (ODS) Fe-14Cr alloy consolidated by SPS exhibits poor tensile properties. The extensive decoration of the powder particle surfaces with Cr-rich precipitates and the residual porosity appear to be responsible for the impaired properties of this ODS alloy consolidated by SPS.

The electrical properties, optical-absorption characteristics, and positron-annihilation lifetimes have been determined for nominally pure ZnO single crystals that were thermochemically reduced in Zn vapor in the temperature range between 1100and 1500 K. Electrical-conductivity and Hall-effect measurements indicate that donors are produced as a result of the thermochemical reduction process. Additionally, optical measurements show that the reduction results in an increase in the optical absorption near the two fundamental absorption edges. Positron-annihilation studies reveal that a well-defined positron state having a lifetime of 169+2 ps exists in the reduced crystals, in contrast to the lifetime of 180+3 ps characteristic of colorless, high-resistivity as-grown crystals. The lifetime of 169 ps is attributed to positron annihilation in the bulk material. It is concluded that defects produced by thermochemical reduction of ZnO are not elcient positron traps -indicating that the defects either exist as interstitials or that they are positively charged.

W and W alloys are currently considered promising candidates for plasma facing components in future fusion reactors but most of the information on their mechanical properties at elevated temperature was obtained in the 1960s and 1970s. In this investigation, the strength and toughness of novel Y 2 O 3 doped W Ti alloys manufactured by powder metallurgy were measured from 25 °C up to 1000 °C in lab oratory air and the corresponding deformation and failure micromechanisms were ascertained from anal yses of the fracture surfaces. Although the materials were fairly brittle at ambient temperature, the strength and toughness increased with temperature and Ti content up to 600 °C. Beyond this tempera ture, oxidation impaired the mechanical properties but the presence of Y 2 O 3 enhanced the strength and toughness retention up to 800 °C.

The mechanical behavior of pure W, W-0.5 wt pct Y 2 O 3 , and W-4 wt pct Ti manufactured by powder metallurgy was studied from ambient temperature to 1000°C. Y 2 O 3 nanoparticles were added to inhibit grain growth and improve oxidation resistance, while Ti is a sintering activator. Strength and toughness were measured from the three-point bend tests in smooth and notched prismatic bars. The dominant deformation and failure micromechanisms were assessed as a function of temperature for each material through the analysis of the fracture surfaces and transverse cross sections in the scanning electron microscope. In particular, the role played by microstructure, dominant failure mechanisms, and oxidation rates in mechanical behavior were elucidated for each material in the entire temperature range. It was found that Y 2 O 3 improved the high-temperature properties by inhibiting oxidation. The Ti acted as a sintering activator, and the W-Ti alloy was fully dense and presented smaller grain size. Both factors enhanced the mechanical properties at ambient and intermediate temperatures, but the ductile-to-brittle transition temperature (DBTT) of this material was higher, leading to brittle behavior up to very high temperature.