ODS alloys meeting high temperature demands

ODS alloys meeting high temperature demands The worldwide search by materials scientists for high temperature materials capable of enhancing efficiency and reducing environment pollution, has led to the development of oxide dispersion strengthened alloys. K.H. Matucha and M.R~hle of Metallgesellschaft, Frankfurt, Germany, review the developments in this area.

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ntil the mid-1980s the development of oxide dispersion strengthened (ODS) materials was being advanced soley by private companies in the USA and the UK. Since then in Japan, as well as in Germany and Austria, research and development (R&D) programmes for these high temperature materials have started to receive government support. There are two groups of ODS alloys, namely iron based and nickel based alloys. Table 1 shows the chemical composition of some iron based ODS alloys. They contain chromium, aluminium, titanium a n d / o r molybdenum and additions of 0.25-0.75% yttrium oxide. Ferritic ODS alloys are basically heat resistant heating element alloys that have been additionally dispersion strengthened by oxides. Table 2 shows the composition of some nickel-based ODS s u p e r alloys. Obviously these are compositionally more complex than the iron based alloys. Excluding MA 754, MA 758 and PM 1000, the nickel based ODS alloys are gamma prime strengthened, the gamma prime content amounting to in excess of 50 vol%. The underlying principle is that the well known

Alloy

Cr

AI

Ti

Mo

M A 956

20

4.5

0, 5

-

0.5

MA 957

14

-

1.0

0.3

0.25

PM 2000

20

5.5

0.5

--

0.5

DT 2 9 0 6

13

--

2.5

1.5

0.75

DT 2203Y0.5

13

-

2.2

1.5

0,5

24 MPR November 1993

Y=Oa

gamma prime hardening is superimposed by the dispersion strengthening. In Ni-Cr alloys the chromium acts as a solid solution strengthener. This effect decreases drastically with increasing temperature. At temperatures above 900°C an essential strengthening contribution is achieved by the gamma prime phase. This can be cut only by dislocation pairs and therefore has a pronounced strengthening effect. At very high t e m p e r a t u r e s the g a m m a prime p h a s e is dissolved and therefore becomes ineffective. The idea of oxide dispersion strengthening is based on the introduction of very finely dispersed thermally stable oxides into the metallic matrix. These survive even at very high temperatures and impart a high creep resistance to these materials at temperatures almost up to the melting point. To avoid this creep resistance becoming impaired by grain b o u n d a r y sliding or by the formation of creep pores, the alloys need a coarse grained structure. The essential requirement for obtaining high creep resistance, however, is a high grain elongation grain aspect ratio (GAR). The effect of the alloying elements can be described as follows. Those elements dissolved in the matrix enhance strength by solid solution strengthening. These are chromium, molybdenum, t u n g s t e n and tantalum. This contribution is comparatively low at high temperatures. The more important effect of chromium is the formation of protective oxide scales. Aluminium and titanium form the strengthening gamma prime phase. This ordered phase gets stabilized by molybdenum, tungsten and tantalum. Thus the solvus temperature of the g a m m a prime p h a s e gets raised, preserving the strengthening effect up to higher temperatures. Yttrium oxide has to be present as a very fine dispersion with particle spacings below 100 nm to act as a strengthening factor. Yttrium oxide was selected because thermodynamically it is a very stable compound. At medium temperatures aluminium acts as a gamma prime former, before taking the role of a protective scale former at temperatures over 1000°C. The course grained structure with high GAR values in high gamma prime containing alloys has to be achieved by zone annealing. Figure 1 shows these elongated

Cr

AI

Co

Ti

Mo

W

Ta

Y2Oa

MA 6000

15

4.5

--

2.5

2.0

4.0

2.0

1.1

MA 760

20

6,0

--

--

2.0

3.5

--

0.9

MA 754

20

0.3

--

--

0.5

--

--

0.6

MA 758

30

0.3

--

--

0.5

--

--

0.6

PM 3000

20

6,0

--

--

2.0

3.5

--

0.9-1.1

PM 3000 S

17

6,0

--

--

2.0

3,5

2.0

0,9-1.1

PM

20

0.3

--

--

0.5

--

--

0.6

TMO2

5.9

4.2

9.7

0.8

-

12.4

4.7

1.1

Alloy 98

6.8

5.2

5.1

0.9

-

8.6

5.7

1.1

1000

grains, a d j u s t e d p a r a l l e l to t h e e x t r u s i o n d i r e c t i o n in PM 3030. The scale m a k e s clear t h a t t h e grain length is in the range of centimetres. The g a m m a p r i m e p h a s e a n d t h e finely d i s p e r s e d o x i d e s can be m a d e visible by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (Figure 2). F r o m t h i s it is p o s s i b l e to recognize small b l a c k p a r t i c l e s -- t h e oxides, a n d coarser, light grey polygons -- t h e g a m m a p r i m e phase, t h e l a t t e r being p r e s e n t in large a m o u n t s . The list of n i c k e l - b a s e d ODS alloys p r e s e n t e d in Table 2 is n o t c o m p l e t e -f u r t h e r alloys are c u r r e n t l y u n d e r development. It s h o u l d be p o i n t e d o u t t h a t t h e s a m e list e l a b o r a t e d in t h e m i d - e i g h t i e s w o u l d have only shown alloy MA 6000. D e v e l o p m e n t since t h e n h a s b e e n t a r g e t e d p r i m a r i l y at t h e i m p r o v e m e n t of t h e h o t gas corrosion r e s i s t a n c e for a p p l i c a t i o n s in gas turbines. F o r this r e a s o n the a l u m i n i u m c o n t e n t h a s been enhanced. Materials will only gain a c c e p t a n c e if t h e r e is a c o n v e n i e n t p r o d u c t i o n r o u t e a n d as t h e d i s p e r s o i d s in ODS alloys only act to s t r e n g t h e n if t h e m a i n s p a c i n g is well below 100 nm, t h e y can n o t be p r o d u c e d via t h e melting route. Therefore t h e s e alloys are p r o d u c e d using p o w d e r m e t a l l u r g y (PM). The p r o c e s s s t a r t s w i t h different p o w d e r s being weighed a n d m i x e d together. Elem e n t a l p o w d e r s can be u s e d as well as m a s t e r alloys. The first decisive s t e p is m e c h a n i c a l alloying (MA), d u r i n g which t h e h e t e r o g e n o u s p o w d e r m i x t u r e gets t r a n s formed into a p o w d e r in which e a c h p a r t i c l e has t h e s a m e c h e m i c a l c o m p o s i t i o n . Quality c o n t r o l of t h e MA p r o c e s s is i m p o r t a n t . E a c h p a r t i c l e m u s t be entirely m e c h a n i c a l l y alloyed, w h i c h can be j u d g e d using o p t i c a l microscopy. In spite of t h e small n u m b e r of p a r t i c l e s w h i c h can be

FIGURE 7: Coarse grained structure after recrystallization by zone annealing.

FIGURE 2: TEMmicrograph (dark field) of the austenitic ODS alloy PM 3030.

MPR November 1993 25

:@r

=rystalllzation 1350°C as a =ction of nperature and poreeof rmatlon ry different 'uctures velop Degree of deformation

FIGURE 3: Deformation chart for PM 2000 (D. Sporer 1993).

FIGURE 4: Extremely different recrystallization structure in PM 3030 heat treatment: zone annealing almost to the end of the rod and isothermal recrystallization at 1220°C.

centre end l of coil /

26 MPR November

tested it has turned out t h a t this is a convenient procedure for quality control, although for development work it is not sufficient. Only recently have tests been developed which allow a numerical description of milling quality. More convenient results can be achieved by X ray diffraction, using the fact t h a t with increasing milling time the lattice constant of iron based alloys also increases. After a certain period of time the lattice constant stops increasing indicating t h a t the powder is fully alloyed. The measuring of the Curie t e m p e r a t u r e can also be applied as a test method. This is based on the fact t h a t in the progress of the mechanical alloying the local composition changes according to the Curie temperature. Other measuring methods include DTA and DSC. MA is achieved by high intensity milling, and different equipment can be applied for example: • agitating a t t r i t o r s - - batch size up to 20 kg; • gravity mills -- batch size of more than 1 tonne possible; and • centrifugal mills -- batch size of 50 kg common, some 100 kg possible. Centrifugal mills are especially convenient as they allow short milling times and a rigid

30 mm 1993

(P

control of milling atmosphere. Milled powders get sealed into vacuum tight canning and are consolidated. This is done by hot isostatic pressing (HIP) or by extrusion. HIPed ingots are transformed by hot rolling. S u b s e q u e n t recrystallization yields a highly elongated grain structure, provided t h a t the degree and t e m p e r a t u r e of deformation have been chosen correctly. For iron based ODS alloys, rod and wire have been p r o d u c e d a p p l y i n g different degrees and t e m p e r a t u r e s of deformation and subsequently annealed at high temperatures. Optical microscopy reveals different structures (Figure 3): the desired e l o n g a t e d g r a i n s t r u c t u r e ( t y p e 1) is achieved if a certain degree of deformation is s u r p a s s e d a n d t h e t e m p e r a t u r e of deformation lies within a specified area. If 70

I

60

50

40 o

30

,20

10

0

800

FIGURE 5: Creep rupture strength Rm/woo of PM 2000 for different structures.

the t e m p e r a t u r e of deformation T1. krit is surpassed annealing yields a different coarse grained structure (type 2A). Very high degrees and t e m p e r a t u r e s of deformation yield structure 2B, while low degrees of deformation yield structure type 3. In the area designated with 'normal grain growth' no secondary recrystallization takes place. In nickel-based superalloys the elongated grain structure is achieved by zone annealing. The process of secondary recrystallization is controlled on an empirical basis but is still not fully understood. Figure 4 shows how extremely different structures can be achieved by secondary recrystallization. This structure is the result of an e x p e r i m e n t t h a t i n t e r r u p t e d the zone annealing procedure of a round rod. This resulted in the coarse structure shown on the left side. Subsequently this sample was annealed isothermally. This illustrates that,

PM

SPECIAL

2800°C (W only)

FEATURE

up to 2000°C

1.800 ~ ~ 1.600

Metals (Nb, Mo, W, Ta)

¢ 1.400 .2 0~ 1.200 0m Q. ~" 1.000 =

(S|2N4, SIC)

ODS-Alloys co:nventional highly heat resistant alloys, iron-, cobalt- and nickel based

800

600 (9 Q. E 400

Heat resistant Steels Titanium Alloys A!uminiu m Alloys

200 i

t

i

i

i

I

Entire field of High Temperature Technology []

[]

for universal use

II

Application restricted or under development

FIGURE 6: Areas and temperatures of application for different groups of high temperature materials,

It is i n t e r e s t i n g to note t h a t t h e ' l o s s ' in c r e e p strength is again associated with a g a i n in d u c t i lity. This is important for applications where, for s t r e s s relaxation, a certain amount o f d u c t i l i t y is required so t h a t low cycle f a t i g u e is i m proved. The initial d e v e l o p m e n t of ODS alloys was targeted primarily at high temperature materials for blades and bur-

d e p e n d i n g on t h e p r e c e d i n g a n n e a l i n g t e m p e r a t u r e , very different s t r u c t u r e s can be formed. By controlling s t r u c t u r e it is p o s s i b l e to influence m e c h a n i c a l p r o p e r t i e s . The o u t s t a n d i n g high t e m p e r a t u r e p r o p erties of t h e ODS-alloys have b e e n widely r e p o r t e d . Besides a high r e s i s t a n c e a g a i n s t o x i d a t i o n a n d h o t gas c o r r o s i o n t h e s e m a t e r i a l s e x h i b i t an e x t r a o r d i n a r y high creep s t r e n g t h at high t e m p e r a t u r e s . At t e m p e r a t u r e s above 900°C ODS s u p e r alloys a r e s u p e r i o r to c o n v e n t i o n a l N i - b a s e d s u p e r a l l o y s a n d this s u p e r i o r i t y i n c r e a s e s strongly with i n c r e a s i n g t e m p e r a t u r e . The s t r e n g t h values given refer to t h e s t r u c t u r e o p t i m a l in r e s p e c t t o c r e e p resistance, t h a t m e a n s a high GAR value. For many application these extreme properties are not r e q u i r e d a n d m a y n o t even be d e s i r e d b e c a u s e of t h e i r high anisotropy. A newly d e v e l o p e d d e f o r m a t i o n c h a r t offers t h e p o s s i b i l i t y to a d j u s t t h e m e c h a n i c a l p r o p e r t i e s of t h e m a t e r i a l s by a t a r g e t e d s t r u c t u r e d e v e l o p m e n t . This is e x p l a i n e d by F i g u r e 5, w h i c h s h o w s t h a t t h e c r e e p r e s i s t a n c e p r o p e r t i e s achieved by t a r g e t e d s t r u c t u r e d e v e l o p m e n t cover a wide range.

FIGURE 7: Joining of this PM 2000 loading rack was done with riveting.

ner c h a m b e r s

in gas turbines, a n a r r o w s e c t o r of high t e m p e r a t u r e technology. New fields of a p p l i c a t i o n s have since e m e r g e d (Figure 6) as, like h e a t r e s i s t a n t steels a n d c o n v e n t i o n a l superalloys, t h e ODS alloys a r e n o t only able to cover t h e e n t i r e range of high temperature technology but also operate at a considerably higher temperat u r e level t h a n c o n v e n t i o n a l m a t e r i a l s . M a t e r i a l s c a p a b l e of h a n d l i n g still h i g h e r t e m p e r a t u r e s , like c e r a m i c s a n d refractory metals, only cover s e l e c t e d sectors of ODS

FIGURE 8: Pre-combustion chamber and insert pin for a Diesel engine (courtesy of Daimler-Benz AG).

a p p l i c a t i o n s ; w h i c h include: • m a t e r i a l s t e s t i n g -- e q u i p m e n t for high t e m p e r a t u r e tensile a n d creep testing; • apparatus construction --burner-nozzles, h e a t shield, h e a t e x c h a n g e r s ; • f u r n a c e s - c o n v e y o r rolls, c o n v e y o r belts, l o a d i n g racks; • glass i n d u s t r y -- m o u l d s , agitators;

MPR November 1993 27

r o t a t i o n in a high t e m p e r a t u r e furnace a r e also possible. One d r a w b a c k of ODS alloys is t h a t j o i n i n g t e c h n i q u e s involving t h e liquid s t a t e of t h e m a t r i x l e a d to coalescence of t h e dispersoids and thus make dispersion s t r e n g t h e n i n g ineffective. T h u s w e l d i n g m u s t be a v o i d e d ( F i g u r e 7) o r confined to lower s t r e s s e d a r e a s of p a r t s . F i g u r e 8 shows a p r e c o m b u s t i o n c h a m b e r a n d a f i n g e r for d i e s e l engines. T h e s e a r e in c o m m e r c i a l u s e now. T h e t i p o f t h e c h a m b e r was j o i n e d by friction welding. For materials application commercial a s p e c t s p l a y a m a j o r role. The p r o d u c t i o n costs for iron b a s e d ODS m a t e r i a l s m a d e in G e r m a n y a r e Figure 9. When p r o d u c t i o n was s t a r t e d in 1989 c o m p a r a t i v e l y s m a l l lots were p r o c e s s e d into s h e e t a n d rod. Main e x p e n s e s were t h o s e for m a t e r i a l , MA, consolidation and transformation. By switching to large lots p r o d u c t i o n costs can be c o n s i d e r a b l y reduced. This r e q u i r e s t h e e l a b o r a t i o n of t h e p r o d u c t i o n r o u t e for large lots a n d large d i m e n s i o n s . This h a s been d o n e successfully. []

100

80

60

40

20

0

small lots

large lots

small lots

large lots

FIGURE g: Production cost of semi-finished products of iron b a s e d ODS-alloys for • different lots s i z e s (small lots: 10 to 100 kg; large lots: 500 to 1000 kg). •

m o t o r vehicles -- diesel p r e c o m b u s t i o n chamber; a n d

gas t u r b i n e s -- nozzles, b u r n e r c h a m bers, blades. A p p l i c a t i o n s of m a t e r i a l s can involve machining, t r a n s f o r m i n g o r j o i n i n g technology. S i m p l e p a r t s can be p r o d u c e d by m a c h i n i n g w i t h high precision, while c o m p l e x p a r t s -like a universal j o i n t for t r a n s m i s s i o n of a

is available to you -

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ABRA FLUID AG has outstanding achivments in SINTER-HIP and CIP pressing, for reserach and production purposes. We offer tailor made equipment in a quality which sets the standard worldwide. For more information please call US DOW. SINTER HIP oress ]OPa - 200MPa 2000°C

ArSRA 28 MPR November 1993

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