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Some properties of a fibre-reinforced nickel-base alloy
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY A. W. H. MORRIS and A. BURWOOD-SMITH
Ministry o f Technology, National Gas Turbine Establishment, Pyestock, Farnborough, Hants (Great Britain) (Received: 13 April, 1970)
SUMMARY
Some improvement in tensile strength and creep-rupture strength has been demonstrated on reinforcing Nimocast 713C with 20 v/o of 0.050 in. diam. tungsten wire. A more significant enhancement in both strength and specific strength was achieved using tungsten-5% rhenium as the reinforcement. Theoretically, higher specific strength at elevated temperatures couM be obtained by reinforcement with molybdenum or niobium alloys provided that suitable barrier coatings can be developed to prevent interaction between matrix and reinforcement. The fatigue properties of Nimocast 713C in tension-tension loading are not measurably influenced by reinforcement with 0.050 (or 0"040) in. diam. tungsten wire, in contrast to previous reports on composites of a nickel-base alloy plus 0.010 in. diam. wires. This behaviour is interpreted in terms of the relative effectiveness of the fibre arrays of given volume fraction in arresting the propagating .fatigue cracks. A limited number of tests have shown that the composites are stable in thermal cycling conditions. Chemical interaction between matrix and reinforcement occurs to a degree which is considered acceptable onfibres of greater than 0.030 in. diam. The present data indicate that the choice of refractory metal wire diameter must be made on the basis of a compromise of fatigue strength, creep-rupture strength, and long term elevated temperature stability and the ability to retain alignment during infiltration. The feasibility of using extrusion as a fabrication technique has been demonstrated.
l.
INTRODUCTION
One aspect of the continuing development of air-breathing engines is the desire to operate at higher turbine entry temperatures. This requirement could be filled by 53
Fibre Scienceand Technology(3) (1970)--~ Elsevier Publishing Company Ltd, England--Printed in Great Britain
54
A.W.H.
MORRIS, A. BURWOOD-SMITH
blade materials of higher elevated temperature strength than that available in current cast and wrought nickel-base alloys. Such materials could be developed by the reinforcement of oxidation-resistant alloys by high strength whiskers or fibres. Several investigations have been reported for high temperature composite systems of both model and practical nature. The elevated temperature behaviour of tungsten-reinforced copper composites has been studied extensively by McDanels e t al. ~ in terms of metallurgical stability and analysis of stress-rupture and creep properties. The elevated temperature tensile properties of discontinuous tungstenwire-reinforced copper and copper-2 ~ chromium have also been investigated by Petrasek et al. a Both studies indicate the enhancement in elevated temperature strength which can be achieved by fibre reinforcement. However, there is a dearth of information concerning the properties of composite materials suitable for high temperature application in gas turbines where oxidation resistance is a paramount matrix property. Ellison and Harris 3 report that roll-bonded composites of Inconel 600 reinforced with tungsten wires exhibited improved short term tensile strength at elevated temperature and increased stress-rupture life. Appreciable increase in strength at elevated temperature is reported by Baskey 4 for composites of nickel alloy or titanium reinforced with tungsten and molybdenum wires. These composites were prepared by hot pressing powder plus aligned fibre compacts. At the National Gas Turbine Establishment, Dean 5 studied the reinforcement of nickel-base alloys with tungsten wire, fabricated by liquid metal infiltration, and showed substantial improvements in strength at temperatures greater than 900°C. However, impact properties were reduced at temperatures below 300°C due to the brittle reinforcement, while fatigue tests at temperatures up to 500°C showed that the reinforcement was beneficial. Refractory metal wire reinforced nickel-base alloy composites produced by slip casting and sintering have been examined by Petrasek et al.6 for use at 1093-1200°C. Significant improvements in stress-rupture strength were noted. Thus the potential of refractory metal wire reinforced nickel-base alloy composites has been demonstrated. However, these researches promote problems such as the interaction between matrix and reinforcement, noted particularly in the hot pressed or sintered composites, recrystallisation of the fibre and associated loss of strength, and control of filament distribution and orientation in a potential component. Because of the geometry and the need to completely encase the wires in matrix material, the latter problem is particularly relevant in a gas turbine blade application. The liquid metal infiltration technique was used in the present study since little or no interaction occurs between the nickel alloy matrix and tungsten wires. This paper describes an evaluation of the properties of cast composites of Nimocast 713C reinforced with either tungsten or tungsten-5~ rhenium wire of 0.040 or 0.050 in. diam. The results of trials to form nickel-base alloy composites by extrusion are also discussed.
SOME PROPERTIESOF A FIBRE-REINFORCEDNICKEL-BASEALLOY 2.
55
MATERIALSAND FABRICATIONTECHNIQUE
The choice of matrix material was governed largely by the requirements of high temperature strength and oxidation resistance, ductility and good castability. A high strength matrix is necessary since, although the matrix strength is low relative to the reinforcement, it must contribute significantly to the overall strength of a composite of low volume fraction reinforcement. The evaluation of low volume fraction fibre composites was necessary since turbine blade geometry and fibre diameter, and composite density limit the useful volume fraction to approximately 25 %. Nimocast 713C* was selected on the basis of continuity with previous work, 5 density and availability. Nimonic alloys 75t and 115:1: were chosen as additional matrices in the exploratory work on the extrusion of composites to facilitate an investigation of the effect of matrix strength on extrusion characteristics. In the anticipated application of high temperature composites to gas turbine blades, accurate alignment and spatial control of the reinforcement is essential to avoid exposure of the wire at the surface (rapid oxidation and loss in strength would ensue). A preliminary study showed that refractory metal wires of diameter less than 0-030 in. lacked sufficient rigidity to avoid distortion during casting. In addition it is contended that in the event of fibre-matrix interaction during service, the degradation of larger diameter fibres would be less detrimental to composite strength. Therefore a study was undertaken 7 to evaluate the relevant properties of candidate refractory metal wires for the reinforcement of nickel-base alloys, with particular reference to wires of 0.050 in. diam. Wire materials with potentially superior specific stress-rupture properties to tungsten were evaluated, because in the applications considered stress-rupture behaviour is of particular significance. The tensile strengths of these materials, determined at r o o m temperature and at 1100°C, are presented in Table 1. The superiority of tungsten-5 % rhenium alloy TABLE
1
TENSILE PROPERTIES OF SOME REFRACTORY METAL WIRES
Material
Tungsten Tungsten Tungsten-5 ~ rhenium Molybdenum TZM Molybdenum TZC Niobium SU16 Niobium SU31
Wire diameter (in.)
0"05 0.04 0.05 0.05 0.05 0.035 0'04
Ultimate tensile strength (MN/m z) Room temperature
1100°C
1655 1724 1696 1606 1544 896 1048
772 758 1076 600 896 ---
Ni C Cr Co Al Ti Mo W Nb Fe B Zr * Comp. Bal. 0.12 12.5 - - 6-1 0.8 4.2 - - 2-2 2.5 (max) 0.012 0.10 t Bal. 0.15 20.0 0.5 $ Bal. 0.15 15.0 15.0 5.0 4.0 3.5
56
A.W.H.
MORR1S~ A. BURWOOD-SMITH
at both temperatures is noted. The stress-rupture data obtained indicated that tungsten-5% rhenium, molybdenum TZM and niobium SU16 possessed sufficiently attractive properties to merit incorporation in nickel-base alloy matrices (Fig. 1). However, a serious interaction problem was encountered on incorporating the niobium and molybdenum wire in nickel-base alloys by casting. Thus the programme of work was conducted using wires of tungsten and tungsten-5 j%il rhenium only.
800
700
A
~
• ,', • 0 ¥ X
TUNGSTEN TUNGSTEN-5% RHENIUM MOLYBDENUM TZC MOLYBDENUM TZM NIOBIUM SU 31 NIOBIUM SU 16
600
5OO STRESS 4.0C MN/rn2 300
= ~ m
20C
10(;
I
I
lO
~00
I IO00
DURATION H R
Fig. 1.
Stress-rupture properties of wires tested in vacuum at 1100°C.
The composites evaluated were fabricated by liquid metal infiltration in vacuo since controlled casting minimised the degree of fibre-matrix interaction. Further investment casting is a standard method for production of gas turbine blades and the feasibility of producing reinforced blades by this technique has been demonstrated.8 The casting was performed in a Balzers high vacuum induction melting furnace, using heated shell moulds. The reinforcing wires were positioned by inserting their ends into the desired spatial array of holes in steel end plates (Fig. 2). These end plates were cemented into the fired shell mould, accurate alignment being facilitated by guides in the shell mould. A series of trials was performed to optimise casting conditions. A minimum metal casting temperature of 1500°C was found necessary to eliminate incomplete
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
57
penetration and void formation. In the optimised procedure the shell mould was heated to 800°C in vacuum in the casting chamber. In addition to casting, extrusion of blade shapes is an accepted method of producing uncooled and cooled turbine blades and is therefore of interest as a method of producing reinforced blades economically. Part of the work described in this paper was a feasibility study to establish extrusion process parameters.
-
-
SHELL MOULD
WIRES FURNACE
Fig. 2.
Diagram of shell mould assembly for test-piece preparation.
Billets 2 in. diam. and 9 in. long containing up to 35 v/o tungsten wire were prepared by the vacuum casting method. A flat-faced die and a 120 ° included angle die were used with nickel and mild steel clad billets. E-glass was used as a lubricant. The billets were extruded for N G T E by Henry Wiggin & Co. Ltd, and by the National Physical Laboratory.
3.
RESULTS AND DISCUSSION
3.1. Short-time tensile The tensile properties of tungsten-reinforced Nimocast 713C were determined using a Mayes Universal tensile testing machine. A loading rate of 1.6 M N m - 2/sec was applied to specimens of 1 in. gauge length (Fig. 3a). In selecting the spatial
58
A . W . H . MORRIS, A. BURWOOD-SMITH
distribution of the fibres the gas turbine application was considered. From considerations of composite density and the number of 0-050 in. diam. wires which can be inserted into a typical turbine blade per unit cross-section without exposure at the surface, a volume fraction of fibres of 20 vol % (v/o) was selected for evaluation. Thus test pieces were prepared with five 0.050 in. (or in some cases 0.040 in.)
O. 125" R
] ' Fig. 3(a).
-,--- I" - ~
Tensile and creep test-piece geometry.
diam. wires in the array shown in Fig. 3(a), such that the minimum distance between adjacent wires and between any wire and the surface was 0.025 in. The results of the evaluation are given in Table 2 and Fig. 4 in terms of ultimate tensile strength as a function of temperature. The variation of 0.2 % offset yield with temperature is shown in Fig. 5.
O' 2 5" P A R LL
O.BS"R, F-
3 '13 Fig. 3(b).
~Fatigue test-piece geometry.
To evaluate the internal-notch sensitivity of the composite system in fatigue, a series of specimens was tested in which the centre wire of the array contained a circumferential notch at its mid-span. A notch profile of radius 0.0624 in. was used, the effective cross-sectional area of the wire being reduced by approximately 75 %. Prerequisite tensile data are shown in Table 2. A slightly lower ultimate tensile strength is noted in comparison to Nimocast 713C + 20 v/o unnotched tungsten wire (896 MN/m 2 cf 945 MN/m2).
59
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE A L L O Y
The present d a t a indicate t h a t reinforcement with the large d i a m e t e r tungsten wires is ineffective in increasing the ultimate tensile strength o f the nickel alloy, but does enhance the 0.2 % offset yield strength. A t r o o m t e m p e r a t u r e the c o m p o site strength is 95.3 % o f the theoretical rule-of-mixtures value, assuming the wire strength to be 1458 M N / m 2 as d e t e r m i n e d for wires leached f r o m a composite. TABLE 2 TENSILE BEHAVIOUR OF REINFORCED NIMOCAST 7 1 3 C
Material 713c
713c - 20 v/o w
713c ÷ 13 v/o w 713c + 20 v/o w Centre wire notched
0"2 % off- Ultimate Temperature tensile set yieM (MN/m2) strength (°C) (MN/mZ)
Elongation (%)
21 21 21 21 21 21 300 300 500 500 700 700 1000 1000 21 21 21
--726 ---732 724 780 763 788 788 318 303 ----
857 896 827 820 960 931 813 849 885 825 960 870 461 450 896 931 902
-8.6 7-0 2.5 -1-2 1.8 I-5 3.0 1-65 1.56 > 14.7 > 13.4 5 5 2
21
>741
896
3
UTS Reduction calculated in area (%) from rule of mixtures (MN/m z) 4.2 5.2 >3.1 -1.0 1.0 1.0 2.0 1.0 2-8 1.0 0.8 9.6 11.0 -1-2 1.0 1-0
----992 992 993 993 951 951 957 957 552 552 952 952 ---
M a c r o s c o p i c a n d microscopic e x a m i n a t i o n o f the fracture surface o f c o m p o s i t e s tested at r o o m t e m p e r a t u r e a n d 300°C shows some ductility in the tungsten wire. N o fibre pull out was observed b u t necking in the fibre h a d destroyed an otherwise g o o d b o n d in the i m m e d i a t e vicinity o f the fracture. A t 1000°C excessive o x i d a t i o n o f the tungsten prevented identification o f the fibre fracture mode. The reinforcem e n t o f N i m o c a s t 713C with tungsten wires reduces the ductility o f the alloy quite considerably, as represented by per cent elongation a n d per cent r e d u c t i o n in a r e a (Table 2). This e m b r i t t l e m e n t is tentatively explained below. The lack o f r e i n f o r c e m e n t o f N i m o c a s t 713C with tungsten wire at elevated t e m p e r a t u r e can be interpreted in terms o f l o a d transfer efficiency between m a t r i x a n d reinforcement (i.e. strength o f bond), a n d in the resistance to crack p r o p a g a t i o n . I f a fibre b r e a k s d u r i n g loading, the c o m p o s i t e m u s t continue to carry the a p p l i e d stress a n d this ability will d e p e n d on the l o a d transfer efficiency between m a t r i x and reinforcement. I f this factor is low the a p p l i e d l o a d m u s t be b o r n e b y the und a m a g e d fibres a n d the matrix, the b r o k e n fibre c o n t r i b u t i n g nothing in a region
60
A.W.H.
MORRIS, A. BURWOOD-SMITH
adjacent to the fracture. Further fibre fracture could act as a nucleant for matrix cracking and such cracking will diminish the load transfer capability and load bearing capacity of the matrix. It is postulated that the five-wire array of 0-050 in. diam. (or 0.040 in.) tungsten wires will not arrest matrix crack propagation (cf an array of finer wires of equivalent volume fraction). Thus it is conceivable that 1000
900
O
C~
© ~E 8O0 z ~E
LU CE
Too
© NIMOCAST 713 C + 20v/, TUNGSTEN ® NIMOCAST 713C PRESENT STUDY X NIMOCAST 713£ INCO DATA
600
500
0
I 200
I 400
I I 600 800 TEMPERATURE °C
I 1000
I 1200
Fig. 4. Variation of ultimate tensile strength of matrix and composites with temperature. subsequent to either matrix cracking or initial fibre fracture, failure of the composite will be in a brittle and catastrophic manner. Any weaknesses in the components of the composite, particularly the wire, will result in rapid premature failure of the specimen and could explain the negligible fibre reinforcement at elevated temperature. It is proposed that the 0 . 2 ~ yield stress would be insensitive to the abovementioned weaknesses.
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY 90(3
61
)
2
800
°
\
7O(: to z t~ to
td
;
60C
t~
O NIMOCAST 713C ,20Vl, TUNGSTEN
o
(~ NIMOCAST 713C PRESENT STUDY X NIMOCAST 713C INCO DATA 2 DENOTES TWO POINTS
i
o
t,~0C
30(:
I
I
I
I
I
I
200
400
600
800
1000
1700
TEMPERATURE °C
Fig. 5.
Variation of 0"2 % offset yield as a function of temperature.
3.2. Stability The metallurgical and thermo-mechanical stability of a composite is of prime importance in the high temperature application of such materials. Although tungsten and nickel-base alloys react together, it is possible to eliminate any interaction during liquid metal fabrication of composites by careful control of the casting parameters. Figure 6(a) shows a typical area in cast tungsten-Nimocast 713C. Similar results were obtained with tungsten-5% rhenium reinforcement. Further, electron-microprobe analysis confirmed the lack of interaction between tungsten and Nimocast 713C. This behaviour contrasts markedly with TZM molybdenum in nickel alloy matrices (Fig. 6(b)). Such extensive reaction was also
62
A. W. H. MORRIS, A. BURWOOD-SMITH
Fig. 6(a).
Fig. 6(b).
Tungsten wire in Nimocast 713C. As-cast condition. ( x 1000)
Molybdenum TZM wire in Nimocast 258. As-cast condition. ( x 70)
63
SOME PROPERTIES OF A FIBRE-REINFORCED N I C K E L - B A S E A L L O Y
found in using niobium wire and in neither case could it be avoided. In addition the molybdenum was observed to recrystallise, thereby reducing its strength considerably. Petrasek e t al. 6 have also reported such behaviour in pressed and sintered nickel alloy powder (Ni, W, Cr) plus molybdenum wire compacts. There is therefore a need for protective barriers if molybdenum (or niobium) is to be used as a reinforcement in superalloys. Some exploratory work has been undertaken at NGTE (Dean 9) to investigate potential barriers for molybdenum and niobium, namely tungsten metal and niobium oxide respectively. Such coatings did prevent interaction on fabrication of the composite but breakdown occurred after prolonged exposure at elevated temperature. The lack of interaction during composite fabrication does not indicate a chemically stable system and therefore the long term stability of the tungsten-nickel-base alloy composites was evaluated. Tungsten-reinforced Nimocast 713C specimens were heated at ll00°C in vacuo for periods of 97, 304, 617 and 990 hours. The temperature, which is higher than that experienced by typical turbine blades in service, was chosen to give an accelerated test and, in the event of minimal interaction, to remove the need for evaluation at lower temperatures. Specimens were examined metallographically and with the electron-probe micro-analyser. Figure 7 TABLE 3 ELECTRON-PROBE MICRO-ANALYSES FOR INTERACTION LAYER SHOWN IN FIG. 20
Percentage of element present Position Nickel A B C D
73'8 72.8 53-0 0'8
Tungsten
Chromium
->1"0 34'2 93.8
8'6 3"6 9'8 1'1
Aluminium 10"0 9.7 ---
shows the effect at the interracial zone in a Nimocast 713C + tungsten composite after various times at 1100°C and, in 7(c), the loci of the electron-probe analysis checks for nickel, tungsten, chromium and aluminium. The data shown in Table 3 suggest that nickel and chromium diffuse into the tungsten and that it is the diffusion of these two elements which controls the rate of growth of the interaction zone. The second apparent reaction zone is simply chromium-depleted matrix and has been tentatively identified as y' precipitate. These data can be fitted in to the standard diffusion equation x 2 = 4DT
+ C
where x is the depth of penetration (zone width), T is the time in seconds and D is the mass diffusion coefficient. The experimentally determined value of D = 1.66 x 10-13cm 2sec-1 compares with DNi in Ni-24.1 wt % W binary equal to 7.4 x 10- 12 cm 2 sec- 1.1o
64
A . W . H . MORRIS~ A. BURWOOD-SMITH
In addition to chemical stability, thermal stability is important, i.e. the effect of stresses generated during thermal cycling due to the mis-match in coefficients of expansion of the matrix and reinforcement (~-4.5 x 10-6/°C for tungsten" 17 x 10-6/°C for Nimocast 713C over the temperature range 20-1100°C). Fluidised bed thermal cycling units were used to study this effect. Tests were carried
(a)
97 HOURS t
lop
(b)
304 HOURS
(j)
gee HOURS
i
IMRE
IMIlUX
I*) Fig. 7.
Bff HOURS
The effect of exposure at 1100°C in vacuum of Nimocast 713C reinforced with tungsten, showing growth of interaction layer. ( × 1000)
SOME P R O P E R T I E S OF A F I B R E - R E I N F O R C E D N I C K E L - B A S E A L L O Y
65
out on cylindrical test pieces (0.5 in. diam. x 5 in. long) containing 20 v/o of 0.050 in. diam. wires and on simple aerofoils with the same volume fraction reinforcement. With a severe thermal cycle, i.e. T~ = 20°C, T z = 1050°C, there was considerable wire distortion in cylindrical test pieces after 200 cycles (5 rain immersion time). However, under conditions more representative of gas turbine blade operation, i.e. after 200 cycles with T 1 = 600°C, T 2 = 1050°C, and T 1 = 20°C, T 2 = 6 0 0 ° C , there was no measurable distortion of the specimen or the wires. Metallographic examination showed no degradation of the fibre-matrix bond or interface. Aerofoils subjected to 100 cycles of T 1 = 600°C, T 2 = 1050°C were unaffected except in one instance where matrix cracking at the leading edge gave rise to oxidation of a wire close to the surface. Oxide formation caused further expansion and propagation of the crack. This particular wire was less than the minimum recommended distance from the surface. 3.3. Impact properties Impact strength data for composites containing five 0.05 in. diam. wires, positioned as in the creep and tensile test pieces, were determined at temperatures up to 1000°C. Testing was carried out on unnotched test pieces (0-22 in. x 0.22 in. x 1.7 in.) in a Charpy-type machine. The results obtained were similar to previous data 5 obtained for composites containing higher volume fractions of smaller diameter tungsten wires. At room temperature the impact strength of the composite was lower than that of the unreinforced matrix due to the low ductility of the wires. The ductility of the wires increases with temperature and the composite was found to have a maximum impact strength at ~-500°C. The slight superiority in impact resistance of the composite relative to the matrix was maintained from 500°C up to 1000°C.
3.4. Creep rupture The creep-rupture properties of Nimocast 713C reinforced with tungsten or tungsten-5 % rhenium wire were evaluated and compared with the data determined for vacuum-cast Nimocast 713C. The tests were performed in standard creep test machines at 1000 and 1100°C using specimens of the geometry shown in Fig. 3(a). The reinforced specimens contained five equi-spaced wires of either 0.050 in. or 0.040 in. diam., yielding 20 v/o and 13 v/o reinforcement respectively. The data are presented in Table 4. From the stress to rupture behaviour shown graphically in Fig. 8 it is apparent that a substantial increase in rupture life can be achieved by reinforcement. The Nimocast 713C plus 20% tungsten-5% rhenium composite system exhibits the highest degree of reinforcement. The attractiveness of these two composite systems is tempered somewhat by the high density of tungsten which is apparent when a comparison is made on a specific stress basis (Fig. 9).
66
A. W. H. MORRIS, A. BURWOOD-SMITH
TABLE 4 CREEP-RUPTURE PROPERTIES OF NIMOCAST 7 1 3 C REINFORCED WITH TUNGSTEN AND TUNGSTEN-5 ~oo RHENIUM WIRE
Material
Density Stress Specific (kg/m3 . . . . . . ~2~ stress Temperature Duration x 10-4) t11/11¥/m ,(m 2 s 2) (°C) (h)
Minimum Elongation creep rate k(~oo/h) (~o)
713C + 20 v/o W (5 x 0'050 in. diam. wires)
1"03
76.5 92.4 108.3 123.4 139.3
7,400 9,000 10,500 12,000 13,500
1100 1100 1100 I100 1100
248.4 73-0 30'0 9"2 2.0
0'054 0'103 0-150 ---
14 19 14 19 13
713C + 20 v/o W (5 x 0.050 in. diam. wires)
1.03
123.4 139.3 154.4 169.6
12,000 13,500 15,000 16,500
1000 1000 1000 1000
439.5 196.0 68.8 38.3
0.003 0-009 0.019 0.050
3 4 4 6
713C -k 20v/o W-Re (5 x 0"050 in. diam. wires)
1.03
185.5 216.5 231.7 246.8 262.7
18,000 21,000 22,500 24,000 25,500
1000 1000 1000 1000 1000
288'7 115.3 74.0 45.4 32.0
0.016 0.040 0'040 0"069 0'091
t0 7 8 8 10
713C + 13 v/o W (5 x 0.040 in. diam. wires)
0.95
69'6 76.5 92.4 108.3 123-4
7,300 8,000 9,700 11,400 13,000
1100 1100 1100 1100 I100
212.9 102.0 19.5 7.6 3.1
0"049 0'060 0-149 --
18 18 14 16 13
713C + 13 v/o W (5 × 0.040 in. diam. wires)
0-95
146 154 158 216 247
15,300 16,200 16,600 22,700 26,000
I000 1000 1000 1000 1000
106'1 89.5 30"8 10.9 4'0
0.016 0.020 0'056 0.182
6 6 7 6 8
713C
0'80
103 101 117 124 124 138 152
12,900 12,200 14,600 15,500 15,500 17,200 19,000
1000 1000 1000 1000 I000 1000 1000
218.1 255.0 105.0 79.0 86.1 35.4 23"6
0.011 -0.015 0.019 0-019 0.045 0"070
12 17 11 14 18 17
713c
0.80
24 38 52
3,000 4,700 6,500
1100 1100 1100
>1736.0 336.7 48.0
0.013 0.034
0.4 10 19
T h e s e d a t a a g r e e w i t h p r e v i o u s w o r k o n the c r e e p - r u p t u r e b e h a v i o u r o f c o m p o sites I'2 a n d in p a r t i c u l a r n i c k e l - b a s e s u p e r a l l o y m a t r i x c o m p o s i t e s 3 - 6 . T h e s e i n v e s t i g a t i o n s i n v o l v e d finer r e i n f o r c e m e n t s ( D - ~ 0 . 0 1 0 i n . ) w h i c h , a l t h o u g h slightly stronger, are m o r e difficult to i n c o r p o r a t e i n t o a m a t r i x in a c o n t r o l l e d d i s t r i b u t i o n t h a n fibres o f 0.040 o r 0.050 in. d i a m . T h e p r e s e n t i n v e s t i g a t i o n indicates t h a t at least 85 ~ o f the t h e o r e t i c a l 100 h o u r r u p t u r e s t r e n g t h c a n be a c h i e v e d in a c o m p o s i t e utilising the larger d i a m e t e r wires. T h e s e c a l c u l a t i o n s are b a s e d on
67
SOME P R O P E R T I E S OF A F I B R E - R E I N F O R C E D N I C K E L - B A S E A L L O Y
rule-of-mixtures predictions using 100 hour rupture strength data for the components. The applicability of such a rule-of-mixtures calculation to rupture strength has been established by McDanels e t al. 1 for tungsten-reinforced copper. A linear relationship was found experimentally which can be expressed as (~), = (af),A I + (a~),A~
where (O')t is the stress to cause rupture in time t and subscripts c, f a n d m refer to composite, fibre and matrix respectively. In the present calculation the wire data 300
713C1 +3VolW (I000cC)
713C+20VolW-R(Ie O00°C)
" ~ 2(X IE
100
7
~ 1 3
C
+
~ 1 3
10 RUPTURE
F i g . 8.
~
TIME(HOURS)
3
100
C
1000)° (0 i 00oC)
1000
Stress-rupture behaviour at 1000°Cand 1100°Cof Nimocast 713C reinforced with tungsten and tungsten-5 ~ rhenium wire.
is for stress-rupture tests in vacuum, any degradation of the fibre caused by matrixfibre interaction being neglected. Degradation of the fibre by matrix-fibre interaction or primary recrystallisation may explain the achievement of only 85 ~o of the theoretical strength level. In addition to enhanced stress-rupture strength, the incorporation of tungsten or tungsten-5 ~ rhenium wires reduces the minimum creep rate of Nimocast 713C significantly. A typical composite creep curve is shown in Fig. 10 together with a comparative curve for the unreinforced matrix. Both curves exhibit the three characteristic stages. The marked reduction in minimum creep rate (~) on reinforcement is shown in Fig. 11 as a function of applied stress. Macro-examination of the fracture surfaces indicates that the composites failed transverse to the applied stress with no pull-out of the reinforcing wire. No surface cracking was evident except occasionally in the region immediately adjacent to the fracture surface. Microscopic examination confirmed this observation. Marked evidence of creep damage in the matrix was not observed except in the region
68
A.W.H.
MORRIS, A. BURWOOD-SM1TH
immediately adjacent to the fibre-matrix interface near the specimen fracture. Figure 12 shows typical cracking in the matrix adjacent to the fibre and indicates propagation of the crack away from the interface. Cavitation and cracking occur in the highly stressed region of the matrix immediately adjacent to the fibre, initiated by a crack in the fibre. Evidence of matrix creep deformation was more pronounced 27.0
\ 2/-..0
o
21"0
i
t/3
OVIoW Re
% ~-o
~so
12"0
9-0
10
Fig. 9.
100 RUPTURE TIME IN HOURS
1000
Specific stress-rupture properties o f reinforced Nimocast 713C at 1000°C.
in the region adjacent to the fracture surface and in the grip section in the more highly stressed Nimocast 713C + 20 vol ~ W-Re composites. The observation of localised creep deformation in the matrix was reported by Baskey4 in work on the behaviour of sintered Hastelloy X reinforced with 37 vol ~ of 0-010 in. diam. tungsten wire reinforcement. There is little information in the literature concerning the analysis of the variation of stresses and strains in a composite system subjected to long term loading. The distribution of the stresses in the fibre and matrix changes with time and, as the
69
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
3.5
3"0
2.5
/
2"0
/
713C &O M N / m z
v +20 I, W
1"5
/E.,. : oo.s ' i . / H o u r
/
1"0
/
/
/
/
~
_ ~
713C+20Vl, W
~
,4o~Im'
0.5
10
Fig. 10.
20
30
TIMEz,0HOURS 50
60
?0
80
Comparison of typical creep bchaviour of Nimocast 713C with and without tungsten
reinforcement (at 1100°C).
690
345 713C + 20V/o W- Re :£
/ E ] ~
D " " D /
v ~ 713c.13 low
207
__o...__~...o~:~ . ~ ~~° '~' xc~ / °
Q:
69
I 10"3
Fig. 11,
10-z MINIMUM CREEP RATE, % /HOUR
w I
I
10-I
100
Comparison of minimum creep rate for unreinforced and reinforced Nimocast 713 C for various stresses at 1000°C,
70
A.w.H.
Fig. 12(a).
MORRIS, A. BURWOOD-SMITH
Matrix cracking in stress-rupture test-piece of Nimocast 713C plus 20 vol. o~ of tungsten-5?/o rhenium wires after 32.5 h at 38.1 ksi and 1000~C. ( × 500)
Fig. 12(b).
Matrix cracking initiated at a fibre crack in the same test-piece. ( x 500)
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
71
behaviour of the constituent elements is not constant, numerical procedures to predict the creep properties of a composite are complex. Ellison and Landes 11 have reported an analysis of creep strains for a composite system, using the independent properties of the two constituents. Since creep strain data for tungsten wire was unavailable to them, data for bulk tungsten was used. The calculations are laborious and necessarily require computer aids. The computed stress-creep strain curves are similar to those obtained in the present work. McDanels et al. have developed an analysis of creep behaviour based on experimentally observed linearity in the relationship of log stress vs. log minimum strain rate. However, in order to predict the composite creep properties it is necessary to know the creep behaviour of the individual components. Unfortunately it is technically difficult to measure the stress-strain behaviour of filamentary material at high temperatures. The precise mechanism controlling the creep-rupture of tungsten wire/Nimocast 713C composites is difficult to define. The reduction in minimum creep rates observed on reinforcing the Nimocast 713C suggests that the stronger, more creep-resistant component, i.e. the wire, controls the behaviour. This is substantiated by the influence of volume fraction reinforcement on minimum creep rates as predicted by Ellison and Landes. The lack of evidence of creep deformation in the matrix of the composite, except at the matrix-fibre interface adjacent to the fracture surface, also suggests that the behaviour is controlled by the reinforcement. However, the onset of tertiary stage creep may be influenced to a greater extent by the deformation of the matrix, i.e. cavitation and cracking in the matrix will decrease the efficiency of load transfer and create a plane of weakness in the composite thus causing a localised stress concentration and consequently deformation of the fibres, ultimately leading to fracture in that plane. 3.5. Fatigue The tension-tension fatigue behaviour of Nimocast 713C was investigated, using the fatigue test piece geometry shown in Fig. 3(b), as a function of the volume fraction of tungsten wire reinforcement. The specimens were all ground with the gauge length being finished polished circumferentially using 000 emery. The tests were conducted in an Amsler Vibrophore fatigue machine at a frequency of approximately 7000 c/rain. Unless otherwise stated, the tests were performed with a ratio of static to dynamic load (R) of 3/2, and the stresses reported are maximum stress per cycle. Figure 13 shows the tension-tension fatigue behaviour of Nimocast 713C and Nimocast 713C + 20 v/o W, and in Fig. 14 a similar S/N curve is shown for 713C + 13 v/o W. Both series of composite specimens contained an equi-spaced five-wire array with volume fractions accordingly. Unreinforced Nimocast 713C exhibited an endurance limit (10 s cycles) of 207 MN/m 2 which results in a ratio of endurance limit to ultimate tensile strength of 0.24 (EL/UTS). The composites both showed an endurance limit of 179 MN/m / and EL/UTS of 0.21
72
A.W.H.
MORRIS, A. BURWOOD-SMITH
and 0-20 for 20 v/o W and 13 v/o W respectively. Therefore at room temperature the addition of large diameter tungsten wires does not enhance the fatigue strength of Nimocast 713C but, if anything, reduces it slightly. In addition to the presence of surface crack initiation points, a composite contains several potential internal stress raisers or crack sources. Therefore a series of tests was performed using composites having the centre wire of the five-wire array waisted to 25 % of its original cross-section at its mid-point. This notch should 700 6OO "'500 z
0 713C • 20rio W O 713C,20V/oW CENTRE WIRE NOTCHED ~SIGNIFIES NO FAILURE
O ~ L , N , ~
,,\
w400 ~~'0~ x
O
2oo
nn
c'~
-
-
7~3c
-
(3-" 100
I
106
I
107
I
,,
I
10e
NUMBER OF CYCLES TO FAILURE
Fig. 13.
T e n s i o n - t e n s i o n fatigue behaviour of N i m o c a s t 713C and Nimocast 713C plus 20 vol,
% of tungsten wire. act as an internal stress raiser and as such influence the fatigue behaviour of the composite. The results of these tests are shown in Fig. 13. It can be seen that at low stresses the presence of the notch does not influence the fatigue behaviour within the scatter of the data (EL/UTS = 0.20). At the stress level of 621 M N / m 2 some reduction in fatigue life could be attributed to the presence of a notched wire. Metallographic examination of the fatigue fractures indicated smooth regions adjacent to the specimen surface and striations indicative of surface initiated matrix cracking. This was confirmed by examination of longitudinal sections as shown in Fig. 15. This also shows a typical crack path through matrix and reinforcement, i.e. the wires do not arrest the propagating crack. Wire fractures were predominantly brittle in nature with striations characteristic of a fatigue-initiated failure. Some of the wires failed in a ductile manner, presumably when fatigue cracking had become so extensive that the undamaged section was unable to withstand the applied load.
73
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
In the fatigue study of conventional 'homogeneous' materials, the influence of the variation of ratio static to dynamic load R on fatigue life is well established (fatigue life increasing with increasing R). A series of tests was performed on 713C + 13% tungsten wire composites to establish the fatigue life at a given 700
- ~ SIGNIFIES NO FAILURE
600
z i
\\
g
x<%.
i 10 6
10 7
NUMBEROFCYCLESTO FAILURE
Fig. 14.
i
10 8
T e n s i o n - t e n s i o n fatigue b e h a v i o u r o f Nirnocast 713C plus 13 vol. % o f tungsten wire.
stress level as a function of R. These data are presented in Table 5. The stress mode was varied from tension-compression to tension-tension with R = 1, 1.5 and 4. The data are presented in terms of maximum stress v e r s u s number of cycles to failure, the case of tension-compression cycles being represented by the total TABLE 5 THE EFFECT OF VARIATIONOF RATIO STATIC" DYNAMIC LOAD (R) ON FATIGUELIFE OF A NIMONIC 713C + 13 v/o W COMPOSITE
Mode Tension-tension
Tension-compression
* U n r e i n f o r c e d matrix.
R
Maximum stress (MN/m 2)
1:1 1:1 3:2 3:2 4:1 4:1 ---
309 309 309 309 309 309 154 154
No. o f cycles to failure 1'2 1"4 4'4 3'5 6"4 1.4 2'4 2"9
x x × x × × x x
106 106 106 106 107 108 106 106
74
A. W. H. MORRIS, A. BURWOOD-SMITH
stress range ( + 154, - 154, = 309 MN/m2). The most deleterious form of fatigue is the tension-tension R = 1, and the least R = 4. which is in agreement with the findings for homogeneous materials. The present lack of reinforcement in fatigue is in contrast to the behaviour reported by Baskey and by Dean. Baskey reports the behaviour of Hastelloy X reinforced with 0.010 in. diam. TZM-molybdenum or ttmgsten-I o/~,,thoria wires. An endurance limit (10 6 cycles) in axial tension-tension fatigue -- 515 MN/m 2 was
Fig. 15.
Crack initiation in the matrix at the surface of a fatigue test-piece. ( ~ 100)
established for Hastelloy X + 35 v/o tungsten-thoria compared to -~ 412 MN/m 2 for unreinforced Hastelloy X. At elevated temperature the advantage to be gained from reinforcement is more impressive, an endurance limit "-~ 584 MN/m 2 at 1650°F being reported for 30-55 v/o reinforcement, which is approximately 3 times the fatigue strength of Hastelloy X. Dean reported improvements in the push-pull fatigue strength of Nimocast 258 on reinforcement with 0.010 in. diam. tungsten wire (40 v/o). The discrepancy between these studies and the present data could be explained on the basis of resistance to fatigue crack propagation. The mode of failure proposed can be appreciated if the tensile behaviour of a composite is first considered.
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
75
On loading, a composite has three-stage behaviour, namely: (1) Both components deform elastically (2) The fibre is elastic and the matrix is plastic (3) Both components behave plastically. In fatigue normally 1 and 2 only are involved. The degree of plastic deformation in the matrix will depend on the applied stress, and the magnitude of the elastic modulus of fibre (Es) and the difference in elastic moduli (AE) of the components. If E s and/or AE are low then plastic strain in the matrix will occur at lower stresses than in a system of high E s. Morris and Steigerwald ~2 demonstrated this point in a study of the fatigue of W-Ag and Steel-Ag composites. Extensive fatigue cracking occurred in the matrix only in the steel-Ag system in which E s and AE were low. Cratchley and Baker 13 found a similar behaviour in silica-reinforced aluminium composites tested in reversed cantilever beam fatigue. It is proposed that the W-Nimocast 713C composites behave similarly, i.e. fatigue cracking in the matrix is the predominant damage mechanism. Ham and Place 14 have proposed a fatigue mechanism which involves successive cycles of fatigue crack propagation in the matrix, arrest at a fibre and then cracking in the fibre. Thus in the present system involving a small number of large diameter fibres it is suggested that the array does not act as an effective crack stopper in comparison to an equivalent volume fraction array of 0.010 in. diam. tungsten wire array, hence the lack of reinforcement in fatigue. 3.6. Extrusion trials The extrusion of blade shapes is an accepted method for producing uncooled and cooled turbine blades, and was therefore of interest in the preparation of composite blades. Reinforced billets were extruded to examine the feasibility of co-extruding matrix and reinforcement without break-up of the reinforcement. Matrices of Nimonic 75, 115 and Nimocast 713C were used with reinforcement levels of 10 and 35 v/o tungsten wire. Preliminary trials with a fiat-faced die resulted in either complete inability to extrude or severe break-up of the wire. A 120° included angle die enabled satisfactory extrusion of all composites except Nimocast 713C + 35 v/o W at 1150°C. The optimum extrusion temperature was -'~ 1100°C for Nimonic 115 and ___1200°C for Nimocast 713C. Mild steel canning resulted in 'welding' to the die walls and necessitated the transfer to nickel, which was subsequently used successfully. Examples of the extrusions are shown in Fig. 16. To determine the effect of extrusion on the integrity of the reinforcement, the matrix was leached away by boiling in aqua regia. Part of such an extrusion is shown in Fig. 17. This illustrates that it is possible to co-extrude composite material of this type with a 4/1 reduction without break-up or serious displacement of the wires.
76
Fig. 16.
A. W. H. MORRIS, A. BURWOOI)-SMITtt
Unsuccessful (3 5 vol. ~ of wire) and acceptable (10 vol. 9~o)extrusions of 713C reinforced with tungsten wire.
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
77
Assessment of the mechanical properties of extruded nickel-base alloy composite material was limited by the lack of satisfactory material in the volume fraction reinforcement range of 2 0 ~ 0 ~ . However, results for Nimonic 75 + 40 v/o tungsten wire extruded at 1100°C indicate a 100 h rupture stress of 108 MN/m 2. This compares favourably with the stress rupture strength of cast composites.
Fig. 17. Satisfactory extrusion with the matrix removed by etching to show wire distribution.
ACKNOWLEDGEMENT The authors wish to thank Mr J. E. Restall for the electron-probe micro-analysis. REFERENCES 1. D. L. MCDANELS, R. A. SIGNORELU and J. W. WEETON, NASA-TN D-4173, September 1967. 2. D. W. I~T~SEK, R. A. SIGNORELLI and J. W. WEETON, NASA-TN-D-3886, March 1967.
78 3. 4. 5. 6. 7. 8. 9. 10. I 1. 12. 13. 14.
A . W . H . MORRIS, A. BURWOOD-SMITH E. G. ELL/SON and B. HARRIS, Appl. Mater. Res., 5 (1966) 33. R. H. BASKEY, Clevite Corp. Technical Report AFML-TR-67-196, September 1967. A. V. DEAN, J. Inst. Metals, 95 (1967) 79. D. W. PETRASEK, R. A. SIGNORELLIand J. W. WEETON, N,4SA-TN-D-4787, A. BURWOOD-SMITH. In press. Wiggin Nickel Alloys, No. 91, July 1968, 22. A. V. DEAN, unpublished work, National Gas Turbine Establishment, 1966. C. J. SMITHELLS, Metals Reference Book, Vol. 1I, Butterworth, London, 3rd edn., 1962. E. G. ELLISON and J. D. LANDES, Metals Engineering Conf., ASME, Houston, Texas, 1967. A. W. H. MORRIS and E. A. STE1GERWALD, Trans. TMS-A1ME, 239 (1967) 730. D. CRATCHLEY and A. A. BAKER, Metallurgia, 69 (1964) 154. R. K. HAM and T. A. PLACE, J. Mech. Phys. Solids, 14 (1966) 271.
Ministry o f Technology, National Gas Turbine Establishment, Pyestock, Farnborough, Hants (Great Britain) (Received: 13 April, 1970)
SUMMARY
Some improvement in tensile strength and creep-rupture strength has been demonstrated on reinforcing Nimocast 713C with 20 v/o of 0.050 in. diam. tungsten wire. A more significant enhancement in both strength and specific strength was achieved using tungsten-5% rhenium as the reinforcement. Theoretically, higher specific strength at elevated temperatures couM be obtained by reinforcement with molybdenum or niobium alloys provided that suitable barrier coatings can be developed to prevent interaction between matrix and reinforcement. The fatigue properties of Nimocast 713C in tension-tension loading are not measurably influenced by reinforcement with 0.050 (or 0"040) in. diam. tungsten wire, in contrast to previous reports on composites of a nickel-base alloy plus 0.010 in. diam. wires. This behaviour is interpreted in terms of the relative effectiveness of the fibre arrays of given volume fraction in arresting the propagating .fatigue cracks. A limited number of tests have shown that the composites are stable in thermal cycling conditions. Chemical interaction between matrix and reinforcement occurs to a degree which is considered acceptable onfibres of greater than 0.030 in. diam. The present data indicate that the choice of refractory metal wire diameter must be made on the basis of a compromise of fatigue strength, creep-rupture strength, and long term elevated temperature stability and the ability to retain alignment during infiltration. The feasibility of using extrusion as a fabrication technique has been demonstrated.
l.
INTRODUCTION
One aspect of the continuing development of air-breathing engines is the desire to operate at higher turbine entry temperatures. This requirement could be filled by 53
Fibre Scienceand Technology(3) (1970)--~ Elsevier Publishing Company Ltd, England--Printed in Great Britain
54
A.W.H.
MORRIS, A. BURWOOD-SMITH
blade materials of higher elevated temperature strength than that available in current cast and wrought nickel-base alloys. Such materials could be developed by the reinforcement of oxidation-resistant alloys by high strength whiskers or fibres. Several investigations have been reported for high temperature composite systems of both model and practical nature. The elevated temperature behaviour of tungsten-reinforced copper composites has been studied extensively by McDanels e t al. ~ in terms of metallurgical stability and analysis of stress-rupture and creep properties. The elevated temperature tensile properties of discontinuous tungstenwire-reinforced copper and copper-2 ~ chromium have also been investigated by Petrasek et al. a Both studies indicate the enhancement in elevated temperature strength which can be achieved by fibre reinforcement. However, there is a dearth of information concerning the properties of composite materials suitable for high temperature application in gas turbines where oxidation resistance is a paramount matrix property. Ellison and Harris 3 report that roll-bonded composites of Inconel 600 reinforced with tungsten wires exhibited improved short term tensile strength at elevated temperature and increased stress-rupture life. Appreciable increase in strength at elevated temperature is reported by Baskey 4 for composites of nickel alloy or titanium reinforced with tungsten and molybdenum wires. These composites were prepared by hot pressing powder plus aligned fibre compacts. At the National Gas Turbine Establishment, Dean 5 studied the reinforcement of nickel-base alloys with tungsten wire, fabricated by liquid metal infiltration, and showed substantial improvements in strength at temperatures greater than 900°C. However, impact properties were reduced at temperatures below 300°C due to the brittle reinforcement, while fatigue tests at temperatures up to 500°C showed that the reinforcement was beneficial. Refractory metal wire reinforced nickel-base alloy composites produced by slip casting and sintering have been examined by Petrasek et al.6 for use at 1093-1200°C. Significant improvements in stress-rupture strength were noted. Thus the potential of refractory metal wire reinforced nickel-base alloy composites has been demonstrated. However, these researches promote problems such as the interaction between matrix and reinforcement, noted particularly in the hot pressed or sintered composites, recrystallisation of the fibre and associated loss of strength, and control of filament distribution and orientation in a potential component. Because of the geometry and the need to completely encase the wires in matrix material, the latter problem is particularly relevant in a gas turbine blade application. The liquid metal infiltration technique was used in the present study since little or no interaction occurs between the nickel alloy matrix and tungsten wires. This paper describes an evaluation of the properties of cast composites of Nimocast 713C reinforced with either tungsten or tungsten-5~ rhenium wire of 0.040 or 0.050 in. diam. The results of trials to form nickel-base alloy composites by extrusion are also discussed.
SOME PROPERTIESOF A FIBRE-REINFORCEDNICKEL-BASEALLOY 2.
55
MATERIALSAND FABRICATIONTECHNIQUE
The choice of matrix material was governed largely by the requirements of high temperature strength and oxidation resistance, ductility and good castability. A high strength matrix is necessary since, although the matrix strength is low relative to the reinforcement, it must contribute significantly to the overall strength of a composite of low volume fraction reinforcement. The evaluation of low volume fraction fibre composites was necessary since turbine blade geometry and fibre diameter, and composite density limit the useful volume fraction to approximately 25 %. Nimocast 713C* was selected on the basis of continuity with previous work, 5 density and availability. Nimonic alloys 75t and 115:1: were chosen as additional matrices in the exploratory work on the extrusion of composites to facilitate an investigation of the effect of matrix strength on extrusion characteristics. In the anticipated application of high temperature composites to gas turbine blades, accurate alignment and spatial control of the reinforcement is essential to avoid exposure of the wire at the surface (rapid oxidation and loss in strength would ensue). A preliminary study showed that refractory metal wires of diameter less than 0-030 in. lacked sufficient rigidity to avoid distortion during casting. In addition it is contended that in the event of fibre-matrix interaction during service, the degradation of larger diameter fibres would be less detrimental to composite strength. Therefore a study was undertaken 7 to evaluate the relevant properties of candidate refractory metal wires for the reinforcement of nickel-base alloys, with particular reference to wires of 0.050 in. diam. Wire materials with potentially superior specific stress-rupture properties to tungsten were evaluated, because in the applications considered stress-rupture behaviour is of particular significance. The tensile strengths of these materials, determined at r o o m temperature and at 1100°C, are presented in Table 1. The superiority of tungsten-5 % rhenium alloy TABLE
1
TENSILE PROPERTIES OF SOME REFRACTORY METAL WIRES
Material
Tungsten Tungsten Tungsten-5 ~ rhenium Molybdenum TZM Molybdenum TZC Niobium SU16 Niobium SU31
Wire diameter (in.)
0"05 0.04 0.05 0.05 0.05 0.035 0'04
Ultimate tensile strength (MN/m z) Room temperature
1100°C
1655 1724 1696 1606 1544 896 1048
772 758 1076 600 896 ---
Ni C Cr Co Al Ti Mo W Nb Fe B Zr * Comp. Bal. 0.12 12.5 - - 6-1 0.8 4.2 - - 2-2 2.5 (max) 0.012 0.10 t Bal. 0.15 20.0 0.5 $ Bal. 0.15 15.0 15.0 5.0 4.0 3.5
56
A.W.H.
MORR1S~ A. BURWOOD-SMITH
at both temperatures is noted. The stress-rupture data obtained indicated that tungsten-5% rhenium, molybdenum TZM and niobium SU16 possessed sufficiently attractive properties to merit incorporation in nickel-base alloy matrices (Fig. 1). However, a serious interaction problem was encountered on incorporating the niobium and molybdenum wire in nickel-base alloys by casting. Thus the programme of work was conducted using wires of tungsten and tungsten-5 j%il rhenium only.
800
700
A
~
• ,', • 0 ¥ X
TUNGSTEN TUNGSTEN-5% RHENIUM MOLYBDENUM TZC MOLYBDENUM TZM NIOBIUM SU 31 NIOBIUM SU 16
600
5OO STRESS 4.0C MN/rn2 300
= ~ m
20C
10(;
I
I
lO
~00
I IO00
DURATION H R
Fig. 1.
Stress-rupture properties of wires tested in vacuum at 1100°C.
The composites evaluated were fabricated by liquid metal infiltration in vacuo since controlled casting minimised the degree of fibre-matrix interaction. Further investment casting is a standard method for production of gas turbine blades and the feasibility of producing reinforced blades by this technique has been demonstrated.8 The casting was performed in a Balzers high vacuum induction melting furnace, using heated shell moulds. The reinforcing wires were positioned by inserting their ends into the desired spatial array of holes in steel end plates (Fig. 2). These end plates were cemented into the fired shell mould, accurate alignment being facilitated by guides in the shell mould. A series of trials was performed to optimise casting conditions. A minimum metal casting temperature of 1500°C was found necessary to eliminate incomplete
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
57
penetration and void formation. In the optimised procedure the shell mould was heated to 800°C in vacuum in the casting chamber. In addition to casting, extrusion of blade shapes is an accepted method of producing uncooled and cooled turbine blades and is therefore of interest as a method of producing reinforced blades economically. Part of the work described in this paper was a feasibility study to establish extrusion process parameters.
-
-
SHELL MOULD
WIRES FURNACE
Fig. 2.
Diagram of shell mould assembly for test-piece preparation.
Billets 2 in. diam. and 9 in. long containing up to 35 v/o tungsten wire were prepared by the vacuum casting method. A flat-faced die and a 120 ° included angle die were used with nickel and mild steel clad billets. E-glass was used as a lubricant. The billets were extruded for N G T E by Henry Wiggin & Co. Ltd, and by the National Physical Laboratory.
3.
RESULTS AND DISCUSSION
3.1. Short-time tensile The tensile properties of tungsten-reinforced Nimocast 713C were determined using a Mayes Universal tensile testing machine. A loading rate of 1.6 M N m - 2/sec was applied to specimens of 1 in. gauge length (Fig. 3a). In selecting the spatial
58
A . W . H . MORRIS, A. BURWOOD-SMITH
distribution of the fibres the gas turbine application was considered. From considerations of composite density and the number of 0-050 in. diam. wires which can be inserted into a typical turbine blade per unit cross-section without exposure at the surface, a volume fraction of fibres of 20 vol % (v/o) was selected for evaluation. Thus test pieces were prepared with five 0.050 in. (or in some cases 0.040 in.)
O. 125" R
] ' Fig. 3(a).
-,--- I" - ~
Tensile and creep test-piece geometry.
diam. wires in the array shown in Fig. 3(a), such that the minimum distance between adjacent wires and between any wire and the surface was 0.025 in. The results of the evaluation are given in Table 2 and Fig. 4 in terms of ultimate tensile strength as a function of temperature. The variation of 0.2 % offset yield with temperature is shown in Fig. 5.
O' 2 5" P A R LL
O.BS"R, F-
3 '13 Fig. 3(b).
~Fatigue test-piece geometry.
To evaluate the internal-notch sensitivity of the composite system in fatigue, a series of specimens was tested in which the centre wire of the array contained a circumferential notch at its mid-span. A notch profile of radius 0.0624 in. was used, the effective cross-sectional area of the wire being reduced by approximately 75 %. Prerequisite tensile data are shown in Table 2. A slightly lower ultimate tensile strength is noted in comparison to Nimocast 713C + 20 v/o unnotched tungsten wire (896 MN/m 2 cf 945 MN/m2).
59
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE A L L O Y
The present d a t a indicate t h a t reinforcement with the large d i a m e t e r tungsten wires is ineffective in increasing the ultimate tensile strength o f the nickel alloy, but does enhance the 0.2 % offset yield strength. A t r o o m t e m p e r a t u r e the c o m p o site strength is 95.3 % o f the theoretical rule-of-mixtures value, assuming the wire strength to be 1458 M N / m 2 as d e t e r m i n e d for wires leached f r o m a composite. TABLE 2 TENSILE BEHAVIOUR OF REINFORCED NIMOCAST 7 1 3 C
Material 713c
713c - 20 v/o w
713c ÷ 13 v/o w 713c + 20 v/o w Centre wire notched
0"2 % off- Ultimate Temperature tensile set yieM (MN/m2) strength (°C) (MN/mZ)
Elongation (%)
21 21 21 21 21 21 300 300 500 500 700 700 1000 1000 21 21 21
--726 ---732 724 780 763 788 788 318 303 ----
857 896 827 820 960 931 813 849 885 825 960 870 461 450 896 931 902
-8.6 7-0 2.5 -1-2 1.8 I-5 3.0 1-65 1.56 > 14.7 > 13.4 5 5 2
21
>741
896
3
UTS Reduction calculated in area (%) from rule of mixtures (MN/m z) 4.2 5.2 >3.1 -1.0 1.0 1.0 2.0 1.0 2-8 1.0 0.8 9.6 11.0 -1-2 1.0 1-0
----992 992 993 993 951 951 957 957 552 552 952 952 ---
M a c r o s c o p i c a n d microscopic e x a m i n a t i o n o f the fracture surface o f c o m p o s i t e s tested at r o o m t e m p e r a t u r e a n d 300°C shows some ductility in the tungsten wire. N o fibre pull out was observed b u t necking in the fibre h a d destroyed an otherwise g o o d b o n d in the i m m e d i a t e vicinity o f the fracture. A t 1000°C excessive o x i d a t i o n o f the tungsten prevented identification o f the fibre fracture mode. The reinforcem e n t o f N i m o c a s t 713C with tungsten wires reduces the ductility o f the alloy quite considerably, as represented by per cent elongation a n d per cent r e d u c t i o n in a r e a (Table 2). This e m b r i t t l e m e n t is tentatively explained below. The lack o f r e i n f o r c e m e n t o f N i m o c a s t 713C with tungsten wire at elevated t e m p e r a t u r e can be interpreted in terms o f l o a d transfer efficiency between m a t r i x a n d reinforcement (i.e. strength o f bond), a n d in the resistance to crack p r o p a g a t i o n . I f a fibre b r e a k s d u r i n g loading, the c o m p o s i t e m u s t continue to carry the a p p l i e d stress a n d this ability will d e p e n d on the l o a d transfer efficiency between m a t r i x and reinforcement. I f this factor is low the a p p l i e d l o a d m u s t be b o r n e b y the und a m a g e d fibres a n d the matrix, the b r o k e n fibre c o n t r i b u t i n g nothing in a region
60
A.W.H.
MORRIS, A. BURWOOD-SMITH
adjacent to the fracture. Further fibre fracture could act as a nucleant for matrix cracking and such cracking will diminish the load transfer capability and load bearing capacity of the matrix. It is postulated that the five-wire array of 0-050 in. diam. (or 0.040 in.) tungsten wires will not arrest matrix crack propagation (cf an array of finer wires of equivalent volume fraction). Thus it is conceivable that 1000
900
O
C~
© ~E 8O0 z ~E
LU CE
Too
© NIMOCAST 713 C + 20v/, TUNGSTEN ® NIMOCAST 713C PRESENT STUDY X NIMOCAST 713£ INCO DATA
600
500
0
I 200
I 400
I I 600 800 TEMPERATURE °C
I 1000
I 1200
Fig. 4. Variation of ultimate tensile strength of matrix and composites with temperature. subsequent to either matrix cracking or initial fibre fracture, failure of the composite will be in a brittle and catastrophic manner. Any weaknesses in the components of the composite, particularly the wire, will result in rapid premature failure of the specimen and could explain the negligible fibre reinforcement at elevated temperature. It is proposed that the 0 . 2 ~ yield stress would be insensitive to the abovementioned weaknesses.
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY 90(3
61
)
2
800
°
\
7O(: to z t~ to
td
;
60C
t~
O NIMOCAST 713C ,20Vl, TUNGSTEN
o
(~ NIMOCAST 713C PRESENT STUDY X NIMOCAST 713C INCO DATA 2 DENOTES TWO POINTS
i
o
t,~0C
30(:
I
I
I
I
I
I
200
400
600
800
1000
1700
TEMPERATURE °C
Fig. 5.
Variation of 0"2 % offset yield as a function of temperature.
3.2. Stability The metallurgical and thermo-mechanical stability of a composite is of prime importance in the high temperature application of such materials. Although tungsten and nickel-base alloys react together, it is possible to eliminate any interaction during liquid metal fabrication of composites by careful control of the casting parameters. Figure 6(a) shows a typical area in cast tungsten-Nimocast 713C. Similar results were obtained with tungsten-5% rhenium reinforcement. Further, electron-microprobe analysis confirmed the lack of interaction between tungsten and Nimocast 713C. This behaviour contrasts markedly with TZM molybdenum in nickel alloy matrices (Fig. 6(b)). Such extensive reaction was also
62
A. W. H. MORRIS, A. BURWOOD-SMITH
Fig. 6(a).
Fig. 6(b).
Tungsten wire in Nimocast 713C. As-cast condition. ( x 1000)
Molybdenum TZM wire in Nimocast 258. As-cast condition. ( x 70)
63
SOME PROPERTIES OF A FIBRE-REINFORCED N I C K E L - B A S E A L L O Y
found in using niobium wire and in neither case could it be avoided. In addition the molybdenum was observed to recrystallise, thereby reducing its strength considerably. Petrasek e t al. 6 have also reported such behaviour in pressed and sintered nickel alloy powder (Ni, W, Cr) plus molybdenum wire compacts. There is therefore a need for protective barriers if molybdenum (or niobium) is to be used as a reinforcement in superalloys. Some exploratory work has been undertaken at NGTE (Dean 9) to investigate potential barriers for molybdenum and niobium, namely tungsten metal and niobium oxide respectively. Such coatings did prevent interaction on fabrication of the composite but breakdown occurred after prolonged exposure at elevated temperature. The lack of interaction during composite fabrication does not indicate a chemically stable system and therefore the long term stability of the tungsten-nickel-base alloy composites was evaluated. Tungsten-reinforced Nimocast 713C specimens were heated at ll00°C in vacuo for periods of 97, 304, 617 and 990 hours. The temperature, which is higher than that experienced by typical turbine blades in service, was chosen to give an accelerated test and, in the event of minimal interaction, to remove the need for evaluation at lower temperatures. Specimens were examined metallographically and with the electron-probe micro-analyser. Figure 7 TABLE 3 ELECTRON-PROBE MICRO-ANALYSES FOR INTERACTION LAYER SHOWN IN FIG. 20
Percentage of element present Position Nickel A B C D
73'8 72.8 53-0 0'8
Tungsten
Chromium
->1"0 34'2 93.8
8'6 3"6 9'8 1'1
Aluminium 10"0 9.7 ---
shows the effect at the interracial zone in a Nimocast 713C + tungsten composite after various times at 1100°C and, in 7(c), the loci of the electron-probe analysis checks for nickel, tungsten, chromium and aluminium. The data shown in Table 3 suggest that nickel and chromium diffuse into the tungsten and that it is the diffusion of these two elements which controls the rate of growth of the interaction zone. The second apparent reaction zone is simply chromium-depleted matrix and has been tentatively identified as y' precipitate. These data can be fitted in to the standard diffusion equation x 2 = 4DT
+ C
where x is the depth of penetration (zone width), T is the time in seconds and D is the mass diffusion coefficient. The experimentally determined value of D = 1.66 x 10-13cm 2sec-1 compares with DNi in Ni-24.1 wt % W binary equal to 7.4 x 10- 12 cm 2 sec- 1.1o
64
A . W . H . MORRIS~ A. BURWOOD-SMITH
In addition to chemical stability, thermal stability is important, i.e. the effect of stresses generated during thermal cycling due to the mis-match in coefficients of expansion of the matrix and reinforcement (~-4.5 x 10-6/°C for tungsten" 17 x 10-6/°C for Nimocast 713C over the temperature range 20-1100°C). Fluidised bed thermal cycling units were used to study this effect. Tests were carried
(a)
97 HOURS t
lop
(b)
304 HOURS
(j)
gee HOURS
i
IMRE
IMIlUX
I*) Fig. 7.
Bff HOURS
The effect of exposure at 1100°C in vacuum of Nimocast 713C reinforced with tungsten, showing growth of interaction layer. ( × 1000)
SOME P R O P E R T I E S OF A F I B R E - R E I N F O R C E D N I C K E L - B A S E A L L O Y
65
out on cylindrical test pieces (0.5 in. diam. x 5 in. long) containing 20 v/o of 0.050 in. diam. wires and on simple aerofoils with the same volume fraction reinforcement. With a severe thermal cycle, i.e. T~ = 20°C, T z = 1050°C, there was considerable wire distortion in cylindrical test pieces after 200 cycles (5 rain immersion time). However, under conditions more representative of gas turbine blade operation, i.e. after 200 cycles with T 1 = 600°C, T 2 = 1050°C, and T 1 = 20°C, T 2 = 6 0 0 ° C , there was no measurable distortion of the specimen or the wires. Metallographic examination showed no degradation of the fibre-matrix bond or interface. Aerofoils subjected to 100 cycles of T 1 = 600°C, T 2 = 1050°C were unaffected except in one instance where matrix cracking at the leading edge gave rise to oxidation of a wire close to the surface. Oxide formation caused further expansion and propagation of the crack. This particular wire was less than the minimum recommended distance from the surface. 3.3. Impact properties Impact strength data for composites containing five 0.05 in. diam. wires, positioned as in the creep and tensile test pieces, were determined at temperatures up to 1000°C. Testing was carried out on unnotched test pieces (0-22 in. x 0.22 in. x 1.7 in.) in a Charpy-type machine. The results obtained were similar to previous data 5 obtained for composites containing higher volume fractions of smaller diameter tungsten wires. At room temperature the impact strength of the composite was lower than that of the unreinforced matrix due to the low ductility of the wires. The ductility of the wires increases with temperature and the composite was found to have a maximum impact strength at ~-500°C. The slight superiority in impact resistance of the composite relative to the matrix was maintained from 500°C up to 1000°C.
3.4. Creep rupture The creep-rupture properties of Nimocast 713C reinforced with tungsten or tungsten-5 % rhenium wire were evaluated and compared with the data determined for vacuum-cast Nimocast 713C. The tests were performed in standard creep test machines at 1000 and 1100°C using specimens of the geometry shown in Fig. 3(a). The reinforced specimens contained five equi-spaced wires of either 0.050 in. or 0.040 in. diam., yielding 20 v/o and 13 v/o reinforcement respectively. The data are presented in Table 4. From the stress to rupture behaviour shown graphically in Fig. 8 it is apparent that a substantial increase in rupture life can be achieved by reinforcement. The Nimocast 713C plus 20% tungsten-5% rhenium composite system exhibits the highest degree of reinforcement. The attractiveness of these two composite systems is tempered somewhat by the high density of tungsten which is apparent when a comparison is made on a specific stress basis (Fig. 9).
66
A. W. H. MORRIS, A. BURWOOD-SMITH
TABLE 4 CREEP-RUPTURE PROPERTIES OF NIMOCAST 7 1 3 C REINFORCED WITH TUNGSTEN AND TUNGSTEN-5 ~oo RHENIUM WIRE
Material
Density Stress Specific (kg/m3 . . . . . . ~2~ stress Temperature Duration x 10-4) t11/11¥/m ,(m 2 s 2) (°C) (h)
Minimum Elongation creep rate k(~oo/h) (~o)
713C + 20 v/o W (5 x 0'050 in. diam. wires)
1"03
76.5 92.4 108.3 123.4 139.3
7,400 9,000 10,500 12,000 13,500
1100 1100 1100 I100 1100
248.4 73-0 30'0 9"2 2.0
0'054 0'103 0-150 ---
14 19 14 19 13
713C + 20 v/o W (5 x 0.050 in. diam. wires)
1.03
123.4 139.3 154.4 169.6
12,000 13,500 15,000 16,500
1000 1000 1000 1000
439.5 196.0 68.8 38.3
0.003 0-009 0.019 0.050
3 4 4 6
713C -k 20v/o W-Re (5 x 0"050 in. diam. wires)
1.03
185.5 216.5 231.7 246.8 262.7
18,000 21,000 22,500 24,000 25,500
1000 1000 1000 1000 1000
288'7 115.3 74.0 45.4 32.0
0.016 0.040 0'040 0"069 0'091
t0 7 8 8 10
713C + 13 v/o W (5 x 0.040 in. diam. wires)
0.95
69'6 76.5 92.4 108.3 123-4
7,300 8,000 9,700 11,400 13,000
1100 1100 1100 1100 I100
212.9 102.0 19.5 7.6 3.1
0"049 0'060 0-149 --
18 18 14 16 13
713C + 13 v/o W (5 × 0.040 in. diam. wires)
0-95
146 154 158 216 247
15,300 16,200 16,600 22,700 26,000
I000 1000 1000 1000 1000
106'1 89.5 30"8 10.9 4'0
0.016 0.020 0'056 0.182
6 6 7 6 8
713C
0'80
103 101 117 124 124 138 152
12,900 12,200 14,600 15,500 15,500 17,200 19,000
1000 1000 1000 1000 I000 1000 1000
218.1 255.0 105.0 79.0 86.1 35.4 23"6
0.011 -0.015 0.019 0-019 0.045 0"070
12 17 11 14 18 17
713c
0.80
24 38 52
3,000 4,700 6,500
1100 1100 1100
>1736.0 336.7 48.0
0.013 0.034
0.4 10 19
T h e s e d a t a a g r e e w i t h p r e v i o u s w o r k o n the c r e e p - r u p t u r e b e h a v i o u r o f c o m p o sites I'2 a n d in p a r t i c u l a r n i c k e l - b a s e s u p e r a l l o y m a t r i x c o m p o s i t e s 3 - 6 . T h e s e i n v e s t i g a t i o n s i n v o l v e d finer r e i n f o r c e m e n t s ( D - ~ 0 . 0 1 0 i n . ) w h i c h , a l t h o u g h slightly stronger, are m o r e difficult to i n c o r p o r a t e i n t o a m a t r i x in a c o n t r o l l e d d i s t r i b u t i o n t h a n fibres o f 0.040 o r 0.050 in. d i a m . T h e p r e s e n t i n v e s t i g a t i o n indicates t h a t at least 85 ~ o f the t h e o r e t i c a l 100 h o u r r u p t u r e s t r e n g t h c a n be a c h i e v e d in a c o m p o s i t e utilising the larger d i a m e t e r wires. T h e s e c a l c u l a t i o n s are b a s e d on
67
SOME P R O P E R T I E S OF A F I B R E - R E I N F O R C E D N I C K E L - B A S E A L L O Y
rule-of-mixtures predictions using 100 hour rupture strength data for the components. The applicability of such a rule-of-mixtures calculation to rupture strength has been established by McDanels e t al. 1 for tungsten-reinforced copper. A linear relationship was found experimentally which can be expressed as (~), = (af),A I + (a~),A~
where (O')t is the stress to cause rupture in time t and subscripts c, f a n d m refer to composite, fibre and matrix respectively. In the present calculation the wire data 300
713C1 +3VolW (I000cC)
713C+20VolW-R(Ie O00°C)
" ~ 2(X IE
100
7
~ 1 3
C
+
~ 1 3
10 RUPTURE
F i g . 8.
~
TIME(HOURS)
3
100
C
1000)° (0 i 00oC)
1000
Stress-rupture behaviour at 1000°Cand 1100°Cof Nimocast 713C reinforced with tungsten and tungsten-5 ~ rhenium wire.
is for stress-rupture tests in vacuum, any degradation of the fibre caused by matrixfibre interaction being neglected. Degradation of the fibre by matrix-fibre interaction or primary recrystallisation may explain the achievement of only 85 ~o of the theoretical strength level. In addition to enhanced stress-rupture strength, the incorporation of tungsten or tungsten-5 ~ rhenium wires reduces the minimum creep rate of Nimocast 713C significantly. A typical composite creep curve is shown in Fig. 10 together with a comparative curve for the unreinforced matrix. Both curves exhibit the three characteristic stages. The marked reduction in minimum creep rate (~) on reinforcement is shown in Fig. 11 as a function of applied stress. Macro-examination of the fracture surfaces indicates that the composites failed transverse to the applied stress with no pull-out of the reinforcing wire. No surface cracking was evident except occasionally in the region immediately adjacent to the fracture surface. Microscopic examination confirmed this observation. Marked evidence of creep damage in the matrix was not observed except in the region
68
A.W.H.
MORRIS, A. BURWOOD-SM1TH
immediately adjacent to the fibre-matrix interface near the specimen fracture. Figure 12 shows typical cracking in the matrix adjacent to the fibre and indicates propagation of the crack away from the interface. Cavitation and cracking occur in the highly stressed region of the matrix immediately adjacent to the fibre, initiated by a crack in the fibre. Evidence of matrix creep deformation was more pronounced 27.0
\ 2/-..0
o
21"0
i
t/3
OVIoW Re
% ~-o
~so
12"0
9-0
10
Fig. 9.
100 RUPTURE TIME IN HOURS
1000
Specific stress-rupture properties o f reinforced Nimocast 713C at 1000°C.
in the region adjacent to the fracture surface and in the grip section in the more highly stressed Nimocast 713C + 20 vol ~ W-Re composites. The observation of localised creep deformation in the matrix was reported by Baskey4 in work on the behaviour of sintered Hastelloy X reinforced with 37 vol ~ of 0-010 in. diam. tungsten wire reinforcement. There is little information in the literature concerning the analysis of the variation of stresses and strains in a composite system subjected to long term loading. The distribution of the stresses in the fibre and matrix changes with time and, as the
69
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
3.5
3"0
2.5
/
2"0
/
713C &O M N / m z
v +20 I, W
1"5
/E.,. : oo.s ' i . / H o u r
/
1"0
/
/
/
/
~
_ ~
713C+20Vl, W
~
,4o~Im'
0.5
10
Fig. 10.
20
30
TIMEz,0HOURS 50
60
?0
80
Comparison of typical creep bchaviour of Nimocast 713C with and without tungsten
reinforcement (at 1100°C).
690
345 713C + 20V/o W- Re :£
/ E ] ~
D " " D /
v ~ 713c.13 low
207
__o...__~...o~:~ . ~ ~~° '~' xc~ / °
Q:
69
I 10"3
Fig. 11,
10-z MINIMUM CREEP RATE, % /HOUR
w I
I
10-I
100
Comparison of minimum creep rate for unreinforced and reinforced Nimocast 713 C for various stresses at 1000°C,
70
A.w.H.
Fig. 12(a).
MORRIS, A. BURWOOD-SMITH
Matrix cracking in stress-rupture test-piece of Nimocast 713C plus 20 vol. o~ of tungsten-5?/o rhenium wires after 32.5 h at 38.1 ksi and 1000~C. ( × 500)
Fig. 12(b).
Matrix cracking initiated at a fibre crack in the same test-piece. ( x 500)
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
71
behaviour of the constituent elements is not constant, numerical procedures to predict the creep properties of a composite are complex. Ellison and Landes 11 have reported an analysis of creep strains for a composite system, using the independent properties of the two constituents. Since creep strain data for tungsten wire was unavailable to them, data for bulk tungsten was used. The calculations are laborious and necessarily require computer aids. The computed stress-creep strain curves are similar to those obtained in the present work. McDanels et al. have developed an analysis of creep behaviour based on experimentally observed linearity in the relationship of log stress vs. log minimum strain rate. However, in order to predict the composite creep properties it is necessary to know the creep behaviour of the individual components. Unfortunately it is technically difficult to measure the stress-strain behaviour of filamentary material at high temperatures. The precise mechanism controlling the creep-rupture of tungsten wire/Nimocast 713C composites is difficult to define. The reduction in minimum creep rates observed on reinforcing the Nimocast 713C suggests that the stronger, more creep-resistant component, i.e. the wire, controls the behaviour. This is substantiated by the influence of volume fraction reinforcement on minimum creep rates as predicted by Ellison and Landes. The lack of evidence of creep deformation in the matrix of the composite, except at the matrix-fibre interface adjacent to the fracture surface, also suggests that the behaviour is controlled by the reinforcement. However, the onset of tertiary stage creep may be influenced to a greater extent by the deformation of the matrix, i.e. cavitation and cracking in the matrix will decrease the efficiency of load transfer and create a plane of weakness in the composite thus causing a localised stress concentration and consequently deformation of the fibres, ultimately leading to fracture in that plane. 3.5. Fatigue The tension-tension fatigue behaviour of Nimocast 713C was investigated, using the fatigue test piece geometry shown in Fig. 3(b), as a function of the volume fraction of tungsten wire reinforcement. The specimens were all ground with the gauge length being finished polished circumferentially using 000 emery. The tests were conducted in an Amsler Vibrophore fatigue machine at a frequency of approximately 7000 c/rain. Unless otherwise stated, the tests were performed with a ratio of static to dynamic load (R) of 3/2, and the stresses reported are maximum stress per cycle. Figure 13 shows the tension-tension fatigue behaviour of Nimocast 713C and Nimocast 713C + 20 v/o W, and in Fig. 14 a similar S/N curve is shown for 713C + 13 v/o W. Both series of composite specimens contained an equi-spaced five-wire array with volume fractions accordingly. Unreinforced Nimocast 713C exhibited an endurance limit (10 s cycles) of 207 MN/m 2 which results in a ratio of endurance limit to ultimate tensile strength of 0.24 (EL/UTS). The composites both showed an endurance limit of 179 MN/m / and EL/UTS of 0.21
72
A.W.H.
MORRIS, A. BURWOOD-SMITH
and 0-20 for 20 v/o W and 13 v/o W respectively. Therefore at room temperature the addition of large diameter tungsten wires does not enhance the fatigue strength of Nimocast 713C but, if anything, reduces it slightly. In addition to the presence of surface crack initiation points, a composite contains several potential internal stress raisers or crack sources. Therefore a series of tests was performed using composites having the centre wire of the five-wire array waisted to 25 % of its original cross-section at its mid-point. This notch should 700 6OO "'500 z
0 713C • 20rio W O 713C,20V/oW CENTRE WIRE NOTCHED ~SIGNIFIES NO FAILURE
O ~ L , N , ~
,,\
w400 ~~'0~ x
O
2oo
nn
c'~
-
-
7~3c
-
(3-" 100
I
106
I
107
I
,,
I
10e
NUMBER OF CYCLES TO FAILURE
Fig. 13.
T e n s i o n - t e n s i o n fatigue behaviour of N i m o c a s t 713C and Nimocast 713C plus 20 vol,
% of tungsten wire. act as an internal stress raiser and as such influence the fatigue behaviour of the composite. The results of these tests are shown in Fig. 13. It can be seen that at low stresses the presence of the notch does not influence the fatigue behaviour within the scatter of the data (EL/UTS = 0.20). At the stress level of 621 M N / m 2 some reduction in fatigue life could be attributed to the presence of a notched wire. Metallographic examination of the fatigue fractures indicated smooth regions adjacent to the specimen surface and striations indicative of surface initiated matrix cracking. This was confirmed by examination of longitudinal sections as shown in Fig. 15. This also shows a typical crack path through matrix and reinforcement, i.e. the wires do not arrest the propagating crack. Wire fractures were predominantly brittle in nature with striations characteristic of a fatigue-initiated failure. Some of the wires failed in a ductile manner, presumably when fatigue cracking had become so extensive that the undamaged section was unable to withstand the applied load.
73
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
In the fatigue study of conventional 'homogeneous' materials, the influence of the variation of ratio static to dynamic load R on fatigue life is well established (fatigue life increasing with increasing R). A series of tests was performed on 713C + 13% tungsten wire composites to establish the fatigue life at a given 700
- ~ SIGNIFIES NO FAILURE
600
z i
\\
g
x<%.
i 10 6
10 7
NUMBEROFCYCLESTO FAILURE
Fig. 14.
i
10 8
T e n s i o n - t e n s i o n fatigue b e h a v i o u r o f Nirnocast 713C plus 13 vol. % o f tungsten wire.
stress level as a function of R. These data are presented in Table 5. The stress mode was varied from tension-compression to tension-tension with R = 1, 1.5 and 4. The data are presented in terms of maximum stress v e r s u s number of cycles to failure, the case of tension-compression cycles being represented by the total TABLE 5 THE EFFECT OF VARIATIONOF RATIO STATIC" DYNAMIC LOAD (R) ON FATIGUELIFE OF A NIMONIC 713C + 13 v/o W COMPOSITE
Mode Tension-tension
Tension-compression
* U n r e i n f o r c e d matrix.
R
Maximum stress (MN/m 2)
1:1 1:1 3:2 3:2 4:1 4:1 ---
309 309 309 309 309 309 154 154
No. o f cycles to failure 1'2 1"4 4'4 3'5 6"4 1.4 2'4 2"9
x x × x × × x x
106 106 106 106 107 108 106 106
74
A. W. H. MORRIS, A. BURWOOD-SMITH
stress range ( + 154, - 154, = 309 MN/m2). The most deleterious form of fatigue is the tension-tension R = 1, and the least R = 4. which is in agreement with the findings for homogeneous materials. The present lack of reinforcement in fatigue is in contrast to the behaviour reported by Baskey and by Dean. Baskey reports the behaviour of Hastelloy X reinforced with 0.010 in. diam. TZM-molybdenum or ttmgsten-I o/~,,thoria wires. An endurance limit (10 6 cycles) in axial tension-tension fatigue -- 515 MN/m 2 was
Fig. 15.
Crack initiation in the matrix at the surface of a fatigue test-piece. ( ~ 100)
established for Hastelloy X + 35 v/o tungsten-thoria compared to -~ 412 MN/m 2 for unreinforced Hastelloy X. At elevated temperature the advantage to be gained from reinforcement is more impressive, an endurance limit "-~ 584 MN/m 2 at 1650°F being reported for 30-55 v/o reinforcement, which is approximately 3 times the fatigue strength of Hastelloy X. Dean reported improvements in the push-pull fatigue strength of Nimocast 258 on reinforcement with 0.010 in. diam. tungsten wire (40 v/o). The discrepancy between these studies and the present data could be explained on the basis of resistance to fatigue crack propagation. The mode of failure proposed can be appreciated if the tensile behaviour of a composite is first considered.
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
75
On loading, a composite has three-stage behaviour, namely: (1) Both components deform elastically (2) The fibre is elastic and the matrix is plastic (3) Both components behave plastically. In fatigue normally 1 and 2 only are involved. The degree of plastic deformation in the matrix will depend on the applied stress, and the magnitude of the elastic modulus of fibre (Es) and the difference in elastic moduli (AE) of the components. If E s and/or AE are low then plastic strain in the matrix will occur at lower stresses than in a system of high E s. Morris and Steigerwald ~2 demonstrated this point in a study of the fatigue of W-Ag and Steel-Ag composites. Extensive fatigue cracking occurred in the matrix only in the steel-Ag system in which E s and AE were low. Cratchley and Baker 13 found a similar behaviour in silica-reinforced aluminium composites tested in reversed cantilever beam fatigue. It is proposed that the W-Nimocast 713C composites behave similarly, i.e. fatigue cracking in the matrix is the predominant damage mechanism. Ham and Place 14 have proposed a fatigue mechanism which involves successive cycles of fatigue crack propagation in the matrix, arrest at a fibre and then cracking in the fibre. Thus in the present system involving a small number of large diameter fibres it is suggested that the array does not act as an effective crack stopper in comparison to an equivalent volume fraction array of 0.010 in. diam. tungsten wire array, hence the lack of reinforcement in fatigue. 3.6. Extrusion trials The extrusion of blade shapes is an accepted method for producing uncooled and cooled turbine blades, and was therefore of interest in the preparation of composite blades. Reinforced billets were extruded to examine the feasibility of co-extruding matrix and reinforcement without break-up of the reinforcement. Matrices of Nimonic 75, 115 and Nimocast 713C were used with reinforcement levels of 10 and 35 v/o tungsten wire. Preliminary trials with a fiat-faced die resulted in either complete inability to extrude or severe break-up of the wire. A 120° included angle die enabled satisfactory extrusion of all composites except Nimocast 713C + 35 v/o W at 1150°C. The optimum extrusion temperature was -'~ 1100°C for Nimonic 115 and ___1200°C for Nimocast 713C. Mild steel canning resulted in 'welding' to the die walls and necessitated the transfer to nickel, which was subsequently used successfully. Examples of the extrusions are shown in Fig. 16. To determine the effect of extrusion on the integrity of the reinforcement, the matrix was leached away by boiling in aqua regia. Part of such an extrusion is shown in Fig. 17. This illustrates that it is possible to co-extrude composite material of this type with a 4/1 reduction without break-up or serious displacement of the wires.
76
Fig. 16.
A. W. H. MORRIS, A. BURWOOI)-SMITtt
Unsuccessful (3 5 vol. ~ of wire) and acceptable (10 vol. 9~o)extrusions of 713C reinforced with tungsten wire.
SOME PROPERTIES OF A FIBRE-REINFORCED NICKEL-BASE ALLOY
77
Assessment of the mechanical properties of extruded nickel-base alloy composite material was limited by the lack of satisfactory material in the volume fraction reinforcement range of 2 0 ~ 0 ~ . However, results for Nimonic 75 + 40 v/o tungsten wire extruded at 1100°C indicate a 100 h rupture stress of 108 MN/m 2. This compares favourably with the stress rupture strength of cast composites.
Fig. 17. Satisfactory extrusion with the matrix removed by etching to show wire distribution.
ACKNOWLEDGEMENT The authors wish to thank Mr J. E. Restall for the electron-probe micro-analysis. REFERENCES 1. D. L. MCDANELS, R. A. SIGNORELU and J. W. WEETON, NASA-TN D-4173, September 1967. 2. D. W. I~T~SEK, R. A. SIGNORELLI and J. W. WEETON, NASA-TN-D-3886, March 1967.
78 3. 4. 5. 6. 7. 8. 9. 10. I 1. 12. 13. 14.
A . W . H . MORRIS, A. BURWOOD-SMITH E. G. ELL/SON and B. HARRIS, Appl. Mater. Res., 5 (1966) 33. R. H. BASKEY, Clevite Corp. Technical Report AFML-TR-67-196, September 1967. A. V. DEAN, J. Inst. Metals, 95 (1967) 79. D. W. PETRASEK, R. A. SIGNORELLIand J. W. WEETON, N,4SA-TN-D-4787, A. BURWOOD-SMITH. In press. Wiggin Nickel Alloys, No. 91, July 1968, 22. A. V. DEAN, unpublished work, National Gas Turbine Establishment, 1966. C. J. SMITHELLS, Metals Reference Book, Vol. 1I, Butterworth, London, 3rd edn., 1962. E. G. ELLISON and J. D. LANDES, Metals Engineering Conf., ASME, Houston, Texas, 1967. A. W. H. MORRIS and E. A. STE1GERWALD, Trans. TMS-A1ME, 239 (1967) 730. D. CRATCHLEY and A. A. BAKER, Metallurgia, 69 (1964) 154. R. K. HAM and T. A. PLACE, J. Mech. Phys. Solids, 14 (1966) 271.