The creep of alloy 617 at 700 °C: Material properties, measurement of strain and comparison between finite element analysis and digital image correlation Academic research paper on "Materials engineering"

Accepted Manuscript

The Creep of Alloy 617 at 700° C: Material Properties, Measurement of Strain and Comparison Between Finite Element Analysis and Digital Image Correlation

A. Narayanan, K. Dubey, C.M. Davies, J.P. Dear

PII: DOI:

Reference:

S0020-7683(17)30385-2 10.1016/j.ijsolstr.2017.08.021 SAS 9700

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International Journal of Solids and Structures

Received date: Revised date: Accepted date:

8 April 2017 27 July 2017 21 August 2017

Please cite this article as: A. Narayanan, K. Dubey, C.M. Davies, J.P. Dear, The Creep of Alloy 617 at 700° C: Material Properties, Measurement of Strain and Comparison Between Finite Element Analysis and Digital Image Correlation, International Journal of Solids and Structures (2017), doi: 10.1016/j.ijsolstr.2017.08.021

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Highlights

• Specimens of Alloy 617 have been tested at 700oC to evaluate creep prop-

The Creep of Alloy 617 at 700°C: Material Properties, Measurement of Strain and Comparison Between Finite Element Analysis and Digital Image Correlation

A. Narayanan^1-, K. Dubeya, C. M. Daviesa, J. P. Dear

aDepartment of Mechanical Engineering, Imperial College London, City and Guilds Building, South Kensington Campus, Exhibition Road, London, United Kingdom,

elect com

Abstract

Future generations of power plants, such as the Ultra-Super-Critical (USC) power plants, are being designed to be operated at more extreme pressures and temperatures in order to achieve higher efficiency. One candidate material for components is Inconel alloy grade 617, a nickel based superalloy, which is expected to possess better creep resistance in comparison to other types of alloys [1, 2]. At present there is little available data or information about the behaviour of this material at the temperature of interest (700oC) and hence there is a need to evaluate its properties under these conditions.

This paper details experimentation on Alloy 617 to evaluate its uniaxial behaviour under tension and creep at 700oC, using the results obtained to develop a creep damage model based on power law creep in conjunction with the Cocks-Ashby void growth approach [3] for creep in a multiaxial stress state. Finite Element (FE) simulations are compared to experimental results obtained byjDigital Image Correlation (DIC), which is used in order to validate the effectiveness of a power law creep damage model. Results made using a novel electrical strain sensor using ACPD principles supplement this work to draw parisons between the response of the sensor and the strain field experienced

Email addresses: a.narayanan@bristol.ac.uk (A. Narayanan), kanishkdubey@hotmail.com (K. Dubey), catrin.davies@imperial.ac.uk (C. M. Davies), j.dear@imperial.ac.uk (J. P. Dear)

1 Present Address: Department of Mechanical Engineering, University of Bristol, Queen's

Building, University Walk, Bristol, BS8 1TR

Preprint submitted to International Journal of Solids and Structures

August 22, 2017

cre< spe'

by the specimen.

Keywords: Creep, Alloy 617, Multiaxial stress state, Finite element studies

1. Introduction

Alloy 617 (also known as Inconel 617 [4]) is a nickel-chromium based sup alloy, of the composition of at least 44.5% nickel and 20% chromium (composition can be found in [4]). Superalloys are a class of material with superior

5 performance at elevated temperatures to conventional alloys, leading them to be popular for use in both aerospace and power generation applications, usually

Alloy 617 is known for good oxidation resistance and creep strength up to 1093°C, and its room temperature tensile properties have been observed to 10 improve significantly upon 1000 hours exposure to temperatures between 649° C and 870°C [5]. Most of the existing literature concerns its usage at temperatures well above those in plant condition and limited data exists on its properties. Recently, long term tests have been performed to determine creep properties of specimens extracted from thick-walled pipes [6], while work has shown that 15 modifying alloy 617 by adding more boron up to 60 ppm increases its creep rupture strength [7]. Nevertheless, there remains much more work to be done in order to obtain a detailed understanding of its behaviour.

In practice, components consist of networks of material joined by welds and which may have varying geometric features along the length. Consequently they 20 are more likely to experience a multiaxial stress state. Hence, data obtained from the uniaxial experiments is used in conjunction with typical power law ;reep models to model the deformation of a double-edged notch tension (DENT) cimen, where the notch simulates the presence of a stress raiser such as a defect or a weld. The suitability of the creep models for Alloy 617 is tested using two DENT specimens whose behaviour is experimentally analysed using Digital Image Correlation (DIC), a method that allows the strain field over the surface of a specimen to be determined. Research has been performed using

DIC to investigate the creep of a DENT specimen, showing its usefulness as a tool to measure creep strain [8] albeit at significantly lower temperatures than those considered here.

This body of research has three complementary aims. The first is to measur and calculate mechanical properties of Alloy 617 at 700oC experimentally, with particular focus on uniaxial tensile and creep properties. This data is then used to evaluate creep constants for simulating the behaviour of the material. The importance of this is to establish an understanding of how it behaves at 700oC.

The second aim is to determine whether creep strain accumulated by specimens of Alloy 617 at this temperature can be accurately measured in-situ using a novel alternating current potential drop (ACPD) strain sensor [9, 10, 11]. This is essential as it enables the condition of the component material to be evaluated easily.

Finally, the third aim is to ascertain whether power law models for creep deformation provide an accurate prediction of the behaviour of Alloy 617, particularly when subject to a multiaxial stress field. This is critical to ensure that the typical models used within the field are appropriate.

2. Mechanics of Creep Deformation

Creep is generally modelled through various uses of power-law models to represent the relationship between different quantities. A representation of the entire creep curve of a material can be made using equation 1, combining the effects of all three stages of creep into one expression

ea = Aaana (1)

where ea is the average creep strain rate, Aa is the average creep coefficient nd na is the average creep stress index.

Similarly the secondary or minimum creep strain rate (es) can be represented using a power law expression of the form of equation 2,

£s = Asan"

where As and ns are the secondary creep coefficient and secondary creep 55 stress index respectively.

The various values of A and n are determined by regression fits to uniaxial creep data. In addition, if stress is plotted against time to rupture it also obey; a power law relationship given by equation 3

tf = Br ^ (3)

where Br and vr are constants to be found. This relationship can be used to predict the rupture behaviour of the material.

2.1. Modelling Creep Behaviour

nship can be use

he beha

The finite element (FE) model used to simulate the behaviour of Alloy 617 incorporates an elastic-plastic analysis of the material behaviour during loading (obtained from results presented within this paper), and simulates creep using 65 the average creep strain rate properties determined within this research (see table 5). The effect of creep damage is modelled using ductility exhaustion concepts as per equation 4

u = ^ (4)

where u is the damage, ec is the amount of creep strain predicted by the creep law used and f is the uniaxial creep failure strain measured from uniaxial creep tests. The damage parameter, w, is defined such that 0 < w < 1 and failure occurs when w approaches 1. The rate of damage accumulation, is related to equivalent creep strain rate by the relationship in equation 5

the equi

and the total damage at any instant is the integral of the damage rate in equation 5 thus given by

u = / udt (6)

75 In this body of work, the creep strain rate used is the average creep strain rate (ea), and so it is taken that ec = éa.

The damage accumulation in a multiaxial stress state depends on the growth and coalescence of voids at grain boundaries. In the vicinity of the crack tip the local (multiaxial) creep ductility, f, may be obtained for the material under . study ta. the Cod. and Ashby[3| model. The Cock^Ashby m°del d«„bes the evolution of creep damage due to the growth of voids at grain boundaries and is a typical model used for a variety of alloys[12, 13]. Due to the limited amount of research on alloy 617 at 700oC, the Cocks-Ashby model has been chosen due to its general wide use in the field. 85 The model describes the ratio of the multiaxial to uniaxial failure strain as a function of the triaxiality, h, as detailed in equation 7

f = smh[ § (n-M)]

f sinh[2( n-M )h] 1 J

where n is whichever stress index has been used in the calculation of steady state creep (either na or ns). The triaxiality describes the stress state present within the material as the ratio of the mean stress to the von Mises equivalent so stress (i.e. h = om/ovm).

When a portion of material becomes fully damaged (w = 0.999, to avoid numerical problems) as described equation 4, it no longer has the ability to take the load. Thus it is assumed that the stress components in all directions will be zero in the elements concerned, which is simulated in a similar manner to 95 [14, 15, 16, 17] whereby the material properties of the damaged element (i.e. its stiffness) are reduced to a near-zero value.

A one-eighth model was constructed, as the presence of planes of symmetry in all three orthogonal directions meant that symmetry conditions could be applied to faces in each direction. Hence, it was possible to minimise the ioo model size and therefore the number of elements within. A mesh refinement study showed that an element size of 62.5 ^.m at the notch root gave results of comparable accuracy and much shorter solving time to a more refined mesh.

Figure 1: Cross-section of specimen geometry u; mesh on cross-section of 3D model

Elements were 8 node, linear the notch is shown in figur Results are expresse

del of DENT specimen, showing

its, and the final mesh used around

tities measured along the notch throat, schemat-

gure 1. d as quanti

ically shown in figure 2. The distance along the throat, r, is normalised with respect to the distance between the notch root and the longitudinal centreline

of the speci:

3rial Proper

3. Material I -"roperty Determination

tion of curved Alloy 617 piping, 350 mm in length was provided, and own in figure 3. After fabrication the pipe had been annealed at 902° C for minutes before being cooled by water. It was then subject to a temperature of 700°C for approximately 17000 hours. The chemical composition of this particular pipe was provided by the supplier, and is listed in table 1.

Figure 2: Co-ordinate scheme used in expressing results ov it of notched specimens

ure 3: Section of Alloy 617 piping used for experimentation (shown alongside A4 sheet of paper as size comparison)

Table 1: Chemical composition of Alloy 617 pipe material provided (by % wt)

N1 Cr Co Mo C Si Al Fe Ti

54.27 22.34 11.70 9.26 0.06 0.06 0.97 0.87 0.43

;nsion to

115 3.1. Tensile Properties

Three cylindrical bar specimens of 8 mm diameter were loaded i: failure in a tensile testing machine, one at room temperature and two at 700° C. Displacement was measured using an Instron 2620-601 clip-on extensometer in room temperature tests or a Linear Variable Differential Transducer (LVDT) at 120 high temperature, and used to calculate strain. In the tensile test performed at 700°C, the specimen temperature was monitored using thermocouples attached according to ASTM Standard E633 [18].

3.2. Creep Properties

In order to determine the creep properties of Alloy 617, a series of six uni-125 axial creep tests at 700°C were performed. Tests involved first exposing the specimens to a uniform temperature field at 700°C, which was measured using thermocouples attached on different parts of the specimen according to the aforementioned ASTM Standard E633[18]. This temperature field was held for 24 h to ensure that it remained constant before a constant load, applied using 130 dead weights, was applied to the specimen. Each specimen was tested at a different load and allowed to creep until failure so as to obtain the material's response over a range of stresses.

Stress is expressed as the net section stress according to equation 8

-=W (8)

where W is the applied load and A0 is the initial cross-sectional area of the gauge region of the specimen. Loads were varied to give an applied stress from 270 to 340 MPa and rupture times in excess of 100 hours. Strain calculated from the extension of the gauge region of the specimen measured using a Linear

Variable Differential Transducer (LVDT). LVDTs were calibrated after each test in order to obtain an accurate displacement-voltage characteristic.

140 4. Creep Monitoring Systems

4.1. Digital Image Correlation Digital Image Correlation (DIC) is an optical method of meas

over an area on the surface of a material. It uses a random p; specks painted on the surface of the test subject and a came: 145 ture images while it undergoes loading. By tracking and compari

images during deformation, strain can be mapped across the surface.

Using a furnace with a porthole, a Single Lens Reflex (SLR) camera was positioned to be able to take pictures of the specimen surface during creep. High temperature paint capable of withstanding over 1000oC was sprayed over 150 the specimen surface to apply the speckle pattern.

Facet sizes were between 70 x 70 pixels and 100 x 100 pixels, with facet overlap being between 20 and 40 pixels. Facet size and overlap were adjusted depending on the size of paint spots in the speckle pattern and in order to produce the smallest error. Errors in DIC measurement were determined by 155 measuring the maximum fluctuation in reading at constant temperature with no loading (i.e. where no strain increase should occur), and error bars have been applied to each set of results in this manner.

4.2. The ACPD Sensor

The ACPD sensor used involves a 2 x 2 square set of probes that measures the electrical resistance of the material. The principle of operation involves applying a low frequency current of 2 Hz into two probes and measuring the voltage across the other two, before doing the same in the orthogonal direction. As the component strains, the separation between the probes (and hence the resistance) increases in the direction of the load and decreases in the transverse 165 direction. Therefore the change in resistance can be used to calculate the strain,

meaning the sensor functions as a high temperature strain gauge, at least until local material effects such as crack formation become significant[11].

Results obtained are expressed as the ratio of the longitudinal resistance to the transverse resistance in order to negate effects of temperature. Thi 170 resistance ratio is then normalised with respect to the initial measurement producing a quantity called the normalised resistance ratio and given the symbol

The probes used in this sensor consist of stainless steel dowel pins we

to the surface of the specimen, with chromel wires attached to the pins to form 175 the connection to the sensing units. A multiplexer switches between inputs, so measurements across multiple sets of probes can be made. An array of probes (2 x 10 long) has been applied along the gauge region of one DENT specimen, with two 2x2 sets affixed to the other one.>The first configuration is designed to measure the variation in response progressively further away from the notch. 180 These probe configurations are shown schematically in figure 4.

5. Results

5.1. Measurement of Tensile Properties

The stress-strain curves for Alloy 617 at both room temperature and 700°C are shown in figure 5, with corresponding tensile properties given in table 2. 185 The elastic modulus at room temperature is greater than that quoted in [4] by 10%, while both the 0.2% proof stress (00.2) and ultimate tensile strength (UTS, outs) are significantly greater. At 700°C, the effect of temperature has been to reduce both the proof stress and modulus, with the modulus at 700° C being 76% the room temperature value and the 0.2% proof stress decreasing by 29.5%. Furthermore, the UTS at this temperature is just over half that at room temperature. When compared to the data sheet value[4], the former is 6% greater than the listed value of 166 GPa at 700°C, which is to be expected when one considers that the room temperature experimental value of 233 GPa is 8% greater than the corresponding data sheet value.

Probes at noti

Remote pr<

■ „■SJ . . .

Figure 4: Geometry of specimens tested with DIC and ACPD systems, showing schematics of a) specimen N-1 and b) specimen N-2

Table 2: Tensile properties of Alloy 617 at room temperature

Temperature Elastic Modulus, E (GPa) 0.2% Proof Stress (MPa) UTS (MPa)

✓ Room Temperature 233 580 1253

700oC 169 379 690

Table 3: Details of testing on completed uniaxial creep specimens; ¿s is secondary creep strain rate and ¿a is average creep strain rate

Stress (MPa) Time to Failure (h) Failure Strain (%) es (h-1) ea (h-1)

270 2535 5.46 1.31 x 10-5 2.12 x 10-5

279 1034 5.37 2.78 x 10-5 5.23 x 10-5

315 619 8.53 5.50 x 10-5 1.38 x 10-4

325 220 9.68 2.39 x 10-4 4.41 x 10-4

330 402 19.40 1.11 x 10-4 4.83 x 10-4

340 385 18.72 1.55 x 10-4 4.86 x 10-4

Rupture Stress Index, vr Br (MPaVrh)

10.20 1.52 x 102

Table 4: Rupture constant 617 at 700oC

195 5.2. Measurement of Creep Prop

Six uniaxial creep tests have been performed and are listed in table 3. Strain-time histories for all specimens are plotted in figure 6, with strain calculated from displacement measured using a Linear Variable Differential Transducer (LVDT). It should be noted that the general trend for three of the specimens 2oo (tested at 325, 330 and 340 MPa) show very similar trends with a high degree of overlap. Indeed, the latter two specimens display almost exactly the same curve. The specimen tested at 325 MPa has failed before the specimens tested at 330 MPa and 340 MPa and accumulated half as much creep strain.

Results suggest that in this material, the creep ductility cannot be assumed 05 to be constant over the stress range of the tests, which is borne out by figure 7a. he stress-rupture characteristic for the six completed tests is in figure 7b, with rupture properties displayed in table 4

The relationship between strain rate and applied stress determined from experiments is shown in figure 7c and figure 7d, and the creep constants derived

Table 5: Steady state creep constants using secondary and average creep strain rates for Alloy 617

Creep Coefficient, A (MPa1/np) Creep Stress Index, n

Secondary/minimum creep rate ¿s (h 1) 1.05 x 10 36 Average creep rate ëa (h-1) 1.72 x 10-38 13.64

Table 6: Details of tests performed on DENT samples; <Jnet is applied stres; 0.2% proof stress

To 2 is the

Specimen Temperature (°C) anet 0"net/o"o.2 Test Du

N-1 700 300 0.73

N-2 700 300 0.73

—577—

t Durati &

ration (h

210 are listed in table 5 using a regression fit to the data on a set of logarithmic axes. Results have been plotted alongside results from similar work performed recently[6].

5.3. Measurement of Creep Strain

Two double-edged notch tension (DENT) specimens have been tested for the 215 purpose of evaluating the capabilities of the ACPD sensor at 700° C as well as observe the evolution of the strain field they experience, with conditions given in table 6. Experiments involved a similar setup to that specified in the previous section in terms of application of temperature and load. The sensors used are

described in previous sections. Each specimen had a speckle pattern applied ie surface facing the camera (through the porthole) and at least one set of four

the surfa< ACPD pr

probes attached to the reverse side. Before presenting the results for N-1 and N-2, it should be mentioned that although in table 6 the time to failure is specified as 194 h and 577 h respectively, a large crack formed at one of the notches at 154 h in the case of the former and 225 479 h in the case of the latter. These cracks grew visibly until failure occurred. It is also noted that one test lasts almost 400 h longer than the other, which

is an example of the kind of variability in material performance during creep exacerbated by the fact these were accelerated creep tests.

The strain fields measured using DIC for specimen N-1 have been shown for 230 t = 0 h, t = 97 h (t/tf = 0.5) and t = 135 h. These times have been represent the period before creep occurred, halfway through the test and a few hours before a crack became visible by the naked eye respectively. Similar strain fiel for specimen N-2, obtained using DIC, are displayed for t = 0 h, t = 18 and t = 356 h. The latter has accumulated more strain overall as it ci or a 235 longer period of time.

In this work, the limitations of the experimental apparatus meant that only one notch could be focused on, therefore the difference in strain fields between the two notches could not be quantified. However, as large-scale cracking initiated at one notch, it is possible that there was an asymmetric strain field

240 produced prior to this. This would support prior work by Gariboldi[8] on alu-

nitial str

cn eventually a—la.e m„K ,« °„e „oUi, «he „«he,.

To investigate the consistency in behaviour prior to large-scale cracking, figure 11 has plotted measurements of strain along the throat of the notch for 245 both notched specimens of Alloy 617 at 97 h and 135 h into each. At both time points there is good consistency between the two specimens of Alloy 617, although N-2 has more strain close to the notch root at t = 97 h. Subsequently the behaviour differs between the two as the crack appears in one notch of N-1 soon afterwards. This early cracking of one specimen compared to the other 250 could be an example of material variability in creep.

As previously detailed in figure 4, specimen N-1 had an array of ACPD probes welded over the entire length of its gauge region, while specimen N-2 d one set attached at the notch and one set located at a distance of 20 mm away to provide a remote field measurement. Results are shown for both spec-255 imens in figure 10, expressed using the normalised resistance ratio (£). There is clearly a diminishing resistance response as the distance from the notch increases. However, it can be seen that after 6 mm, there is no immediately visible

minium showing that although the initial strain field may have symmetry, strain

increase in the gradient of the creep curve at the end of the steady-state portion of the graph. This is to be expected, as the resistance change is geometry driven 260 until tertiary creep[19, 20] and the presence of an elastic stress field further away from the notch means that strain increases are negligible in comparison to the near-notch region. The set of probes located 6 mm away from the notch : enough to it to detect the strain localisation and crack formation occurri Finally, the resistance ratio is shown to accelerate well in advance 265 crack formation, supporting previous experiments conducted[11] that show the ability of the ACPD sensor to sense the onset of the accelerated portion of the material life in advance of conventional strain measurement tools.

6. Comparison with Finite Element Mod

Figure 12 compares experimental results for specimen N-2 to predicted re-270 sults from FE simulations at four different time points. It is seen that in the early stages there is a discrepancy between the FE and the experimental results. This is expected as the average creep strain rate method represents the entire creep curve using one constant strain rate. Therefore predictions using the model underestimate creep strain due to the initial high strain rate of pri-275 mary creep. As time progresses, the discrepancy reduces until final failure of the model at t = 236 h, where failure has been defined in this case as the point of time when the first element becomes fully damaged according to equation 4. This is an important observation as failure will occur within the tertiary creep regime and therefore if the model can accurately match with experimental data later points in the test it means that it remains suitable for use on this

7. Discussion

Creep constants for Alloy 617 have been calculated and presented using data from six tests to provide preliminary data for finite element modelling. In com-285 parison to recent work performed by Knezevic et al [6], it can be seen that the

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Strain

Figure 5: True stress-true strain curves for Alloy 617 at room temperature and 700oC

♦ a = 340 Mpa

o - 330 MPa

+ a = 325 MPa

a = 315 MPa

• a = 279 MPa

• a = 270 MPa

1500 Time (h)

Figure 6: Creep curves for uniaxial specimens of Alloy 617, identified in key by the magnitude of test stress, showing data at stresses of 340 MPa (diamonds), 330 MPa (crosses), 325 MPa (plus symbols), 315 (triangles), 279 MPa (squares) and 270 MPa (diamonds)

a) 1.0E+02 .

b) 1.0E+04

1.0E+00 J-

1.0E+02

1.0E-03 1.0E-04 | £ 1.0E-05 ;

£ 1.0E-06 |

J 1.0E-07 -

« 1.0E-08 ■ § 1.0E-09 .

1.0E-10 ■

1.0E-11 -

1.0E+02

Stress (MPa)

» Imperial College Data * Knezevlc Data

Stress (MPa)

■ial creep pr

d) 1.0E-03 -

1.0E-07

1.0E+02

Stress (MPa)

► Imperial College Data ' Knezevic Data

Figure 7: Uniaxial creep properties for Alloy 617: a) Failure strain (f) against stress (a), b) Time to rupture (tr) against stress c) Minimum creep rate against stress, d) Average creep rate ;; Imperial College data (diamonds) as been plotted alongside Knezevic data[6] for comparison

t = 0 h

t = 97 h

t = 135 h

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.4

£y (%)

reep strain field evolution of specimen N-1: visualisation of the strain field using rlaid on the image of the specimen at three points in time during the test

t = 0 h

t = 180 h

1.2 2.4 3.6 4.8 6.0 7.2 8.4 9.6 10.4

£y (%)

Figure 9: Creep strain field evolution of specimen N-2: visualisation of the strain field using DIC overlaid on the image of the specimen at three points in time during the test; strain accumulation is much more localised in comparison to specimen N-1

0 100 200 300 400 500 600 t(h)

Figure 10: Normalised resistance ratio (|) against time obtained using ACPD probes for a) Specimen N-1 with results shown at the centreline (solid line) and at distances of 24 mm (circles), 18 mm (diamonds), 12 mm (triangles)and 6 mm (squares) from it and b) Specimen N-2 with results shown at the centreline (solid line) and 20 mm away from it (squares)

(/) Q. V

3.5% 3.0% i i ■ N-1 (t = 97 h) - N-2 (t = 97 h)

2.5% i □ N-1 (t= 135 h)

2.0% 1.5% * ï ii i 4 Î & N-2 (t = 135 h)

i 1 * **

1.0% 0.5% 0.0% ï lté if £ I

0.0 0.1 0.2 0.3 0.4

0.5 r/a

0.6 0.7 0.8 0.9 1.0

Figure 11: Creep Strain measured using DIC along throat of notched Alloy 617 specimens at 97 h (solid squares for specimen N-1 and solid triangles for specimen N-2) and 135 h (hollow squares for specimen N-1 and hollow triangles for specimen N-2) into the test

Creep Strain (%)

Creep Strain (%)

Creep Strain (%) _

Creep Strain (%)

Ol O Ol

secondary/minimum creep rate data is significantly different, although there is overlap with the average creep rate plot. Furthermore, the Knezevic data showed much weaker stress dependency on failure strain. This could be due to differences in prior heat treatment between the material concerned. Knezevi _ et a. —d two ffnt annealing treatments whereas the materia, use within this research had been subject to a different annealing cycle before bei: thermally aged for 17000 hours. As mentioned earlier, alloy 617 shows an improvement in tensile properties after exposure to temperature for an extended period of time [5], while for other materials the effect of thermal ageing is detri-295 mental to creep resistance [21]. Hence, it may be that the ageing process has affected the creep properties.

Notched specimens were tested to visualise>creep deformation at elevated temperatures under multiaxial loading conditions as well as compare with FE models. The first thing to note is that both specimens deform in a similar 300 manner, with the strain along the notch throat being similar in magnitude 97 and 135 hours into the tests (see figure 11). Nevertheless, it is noted that although promise has been shown, more specimens must be tested before concrete conclusions can be drawn.

Comparisons with FE modelling were performed. The FE combined an 305 average creep strain rate analysis with a calibrated Cocks-Ashby damage model and results show a reasonable amount of agreement with experimental results. Discrepancies may be either due to inherent material variability, as suggested by the difference in data sets shown by figure 7c and d, or a requirement for more uniaxial creep tests data to be obtained and analysed in order to further 310 refine the material properties.

The ACPD sensor shows that the strain is highly localised. This makes sense there is little plastic strain within the specimen further away from the notch, which means that it is unlikely to have undergone creep here as this region does not experience significant enough loading. This observation is supported 315 by strain fields in figure 8 and figure 9. Nevertheless, probes centred around a point 6 mm from the defect have detected acceleration showing some sensitivity

of the sensor in this configuration when a stress raiser is in its vicinity. 8. Conclusions

Tests have been performed to characterise Alloy 617 at 700°C. Tensile prop-320 erties stated are consistent with previous literature values in[4]. Six creep tests have been performed at various stresses to provide creep data, which has been listed in table 5. Experiments are detailed that use DIC to map the strain fields of a specimen undergoing creep. Notched specimens have been tested to investigate the field evolution with respect to multiaxiality. Finite Element 325 models using average creep rate properties from this data set have shown good agreement with experiments, with the quality of agreement increasing as time progresses.

The ACPD sensor shows promise for operation on specimens subject to 700° C. The presence of a defect may be detected from up to 6 mm away, and 330 may be increased if the current sensor electronics or configuration is adjusted to suit.

Testing is ongoing in order to better evaluate the behaviour of the alloy over a greater range of test stresses and lifetimes. However, values stated here are used to provide a reasonable characterisation of its material properties. In 335 future, it will be necessary to extend the work on to other aspects of creep behaviour including its response to multiaxial stress states (i.e. using notched bars) and with regards to creep crack growth (CCG), for example using compact tensions (CT) specimens.

ledgements

This research was supported by EU project COMTES 700 [RFC-CP-04003]; and the European Community's Research Fund for Coal and Steel (7FP) under the MACPLUS project [ENER/FP7EN/249809/MACPLUS]. Dr Joseph Corcoran and Dr Keith Tarnowski are thanked for their advice in configuring and troubleshooting the ACPD sensor. In addition, Professor Martyn Pavier and

345 Dr Mahmoud Mostafavi have provided invaluable support in proofreading this paper.

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[3] A. C. F. Cocks, M. F. Ashby, On creep fracture by void growth, Prog. Mater. Sci. 27 (27) (1982) 189-244.

355 [4] Inconel alloy 617 (2005. Accessed 16/11/2013).

URL http://www.specialmetals.com/assets/smc/documents/alloys/ inconel/inconel-alloy-617.pdf

[5] W. L. Mankins, J. C. Hosier, T. H. Bassford, Microstructure and Phase Stability of INCONEL Alloy 617, Metall. Trans. 5 (12) (1974) 2579-2590.

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