by JK Chakravartty and R. Kapoor Mechanical Metallurgy Division and AK Suri Materials Group

43 Pages·2011·1.56 MB·English

BARC/2011/E/017 B
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THERMOMECHANICAL PROCESSING OF Nb-1Zr-0.1C ALLOY FOR USE IN
COMPACT HIGH TEMPERATURE REACTORS: A FIRST REPORT
by
J.K. Chakravartty and R. Kapoor
Mechanical Metallurgy Division
and
A.K. Suri
Materials Group
2011
7 BARC/2011/E/017
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C GOVERNMENT OF INDIA
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A ATOMIC ENERGY COMMISSION
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THERMOMECHANICAL PROCESSING OF Nb-1Zr-0.1C ALLOY FOR USE IN
COMPACT HIGH TEMPERATURE REACTORS: A FIRST REPORT
by
J.K. Chakravartty and R. Kapoor
Mechanical Metallurgy Division
and
A.K. Suri
Materials Group
BHABHA ATOMIC RESEARCH CENTRE
MUMBAI, INDIA
2011
BARC/2011/E/017
BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT
(as per IS : 9400 - 1980)
01 Security classification : Unclassified
02 Distribution : External
03 Report status : New
04 Series : BARC External
05 Report type : Technical Report
06 Report No. : BARC/2011/E/017
07 Part No. or Volume No. :
08 Contract No. :
10 Title and subtitle : Thermomechanical processing of Nb-1Zr-0.1C alloy for use in
compact high temperature reactors: a first report
11 Collation : 41 p., 12 figs., 3 tabs., 1 ill.
13 Project No. :
20 Personal author(s) : 1) J.K. Chakravartty; R. Kapoor
2 ) A.K. Suri
21 Affiliation of author(s) : 1) Mechanical Metallurgy Division,
Bhabha Atomic Research Centre, Mumbai
2 ) Materials Group, Bhabha Atomic Research Centre, Mumbai
22 Corporate author(s) : Bhabha Atomic Research Centre,
Mumbai - 400 085
23 Originating unit : Mechanical Metallurgy Division,
BARC, Mumbai
24 Sponsor(s) Name : Department of Atomic Energy
Type : Government
Contd...
i
BARC/2011/E/017
30 Date of submission : July 2011
31 Publication/Issue date : August 2011
40 Publisher/Distributor : Head, Scientific Information Resource Division,
Bhabha Atomic Research Centre, Mumbai
42 Form of distribution : Hard copy
50 Language of text : English
51 Language of summary : English, Hindi
52 No. of references : 33 refs.
53 Gives data on :
60 Abstract : Nb-1Zr-0.1C is a potential material for use in high temperature nuclear reactors. Use of this
alloy in components requires appropriate thermomechanical processing to break the cast microstructure
and to obtain uniformly distributed fine stable precipitates so as to produce the desired mechanical
properties at the high operating temperatures. This report reviews the thermomechanical processing of
Nb-1Zr-0.1C alloy carried out over the years by other researchers and the high temperature creep behavior
of the alloy. The hot deformation of Nb-1Zr-0.1C alloy carried out at Mechanical Metallurgy Division is
also presented here. From this review it is evident that most primary hot working studies were carried out
between 1500 to 1700 °C. The subsequent annealing treatments, which require holding at lower
temperatures of about 1100 to 1300 °C for very long times help further transform the precipitates from
coarse orthorhombic to very fine cubic. Our studies on Nb-1Zr-0.1C alloy also confirm that optimum hot
working lies at temperatures beyond 1500 °C where dynamic recrystallization initiates, and optimally
around 1700 °C where dynamic recrystallization transforms the microstructure. Working at temperatures
lower than 1000 °C may lead to the undesirable effect of both micro as well as macro strain localization,
and should be avoided.
70 Keywords/Descriptors: NIOBIUM ALLOYS; ZIRCONIUM ALLOYS; CASTING; MICROSTRUCTURE;
RECRYSTALLIZATION; CREEP; YIELD STRENTH; EMBRITTLEMENT; GRAIN BOUNDARIES
71 INIS Subject Category : S 36
99 Supplementary elements :
ii
Table of Contents
List of Figures iv
List of Tables v
Abstract 1
1 Introduction 2
2 Precipitates in Nb alloys 7
3 Processing of cast ingots 10
4 Creep of Nb-1Zr alloys 14
5 Summary 19
6 Recommendations for hot working of Nb-1Zr-0.1C 20
Reference 22
Annexure I - Work carried out at MMD, BARC, compiled as paper 23

iii
List of Figures
Fig. 1. Comparison of yield stress of some refractory alloys. T-111: Ta-8.7W-0.04Zr;
TZM: Mo-0.5Ti-0.1Zr; C103: Nb-10Hf-1Ti-0.5Zr; PWC-11: Nb-1Zr-0.1C
from [5], and Nb-5W-2Mo-1Zr from [6]. 3
Fig. 2. Comparison of the UTS of some refractory alloys. Adapted from [7]. 3
Fig. 3. Recommended stress – temperature design domain of Nb-1Zr based on
radiation embrittlement, recrystallization, creep and the ultimate tensile
strength [5]. 5
Fig. 4. Operating temperature window for V, Nb, Ta, Mo and W refractory alloys, Fe-
9%Cr ferritic-martensitic steel, Fe 13%Cr oxide dispersion strengthened
ferritic steel, 316 austenitic stainless steel, solutionized and aged Cu–2%Ni–
0.3%Be, and SiC/SiC composites. The light shaded areas represent
uncertainties in the minimum and maximum temperature limits [11]. 6
Fig. 5. Room temperature strength (YS and UTS) vs. ductility plot for different Nb
alloys; data extracted from Ref. [10]. 7
Fig. 6. The ratio of Zr/Nb in precipitates plotted against time of aging at 1077 °C
comparing two studies. Prior to aging the samples were double annealed at
1482 °C for 1 h followed by 1202 °C for 2 h. 9
Fig. 7. TEM micrograph and diffraction pattern of extracted carbide particles from
Nb-1Zr-0.1C alloy after extrusion at 1600 °C, cold work of 90% followed by
two part anneal of 1 h at 1482 °C + 2 h at 1204 °C [21]. 10
Fig. 8. Back scattered SEM image of Nb-1Zr annealed at 1560 °C and aged for 1100
h at (a) 975 °C and (b) 1125 °C showing Zr O precipitates both at grain
2
boundaries and within grains [23]. 10
Fig. 9. Creep rate vs. creep strain for Nb-1Zr and PWC-11 in different conditions,
tested at 1077 °C and 40 MPa. The annealed condition for Nb-1Zr is 1482 °C
for 1 h. The annealed condition for PWC-11 is 1482 °C for 1 h followed by
1202 °C for 2 h. Two aging conditions after annealing for PWC-11 are
shown: aging at 1077 °C for 1000 h and 1127 °C for 1000 h. Data taken from
Titran [22]. 15
Fig. 10. Plot of pseudo strain rate vs. test stress for Nb alloys in different conditions.
Nb-1Zr was annealed at 1202 °C for 1 h. PWC-11 was annealed at 1482 °C
for 1 h followed by 1202 °C for 2 h, the aging condition was 1077 °C for 1000
h. C103 was annealed at 1337 °C for 1 h. Taken from from Refs. [22, 30]. 16
Fig. 11. Steady state strain rate vs. applied stress for pure Nb, Nb-1Zr, and Nb-1Zr-
0.1C at 1027 °C (taken from [21]). The stress exponent for all of them is
about 6. 17
Fig. 12. Comparison of the creep rate vs. strain plot for Nb-1Zr and PWC-11 taken
from Titran [22, 29] and Nb-1Zr taken from [31]. For the sake of comparison
the data of Davidson is converted to same stress and temperature as described
in text. Note that the creep strain axis has a break from 0.02 to 0.3. 18
iv
List of Tables
Table 1: Commercially available Nb alloys for use at high temperatures [10]. 5
Table 2: Precipitation reaction in Nb-10W-1Zr-0.1C. M(ss) represents the solid
solution, M’ is solid solution in metastable equilibrium with M C, and
2
M is solid solution in equilibrium with monocarbide MC [14]. 8
Table 3. Summary of the hot working conditions to break the cast microstructure
of Zr-1Nb-0.1C of the materials used by researchers. The source of the
material is written in brackets. 13

v
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Thermomechanical processing of Nb­1Zr­0.1C alloy for use in 
compact high temperature reactors: a first report 
J.K. Chakravarttya, R. Kapoora, A.K. Suri 
a Mechanical Metallurgy Division, 
Materials Group, Bhabha Atomic Research Centre, Mumbai. 

Abstract 
Nb‐1Zr‐0.1C is a potential material for use in high temperature nuclear reactors.  
Use of this alloy in components requires appropriate thermomechanical processing 
to break the cast microstructure and to obtain uniformly distributed fine stable 
precipitates  so  as  to  produce  the  desired  mechanical  properties  at  the  high 
operating temperatures.  This report reviews the thermomechanical processing of 
Nb‐1Zr‐0.1C alloy carried out over the years by other researchers and the high 
temperature creep behavior of the alloy.  The hot deformation of Nb­1Zr­0.1C alloy 
carried  out  at  Mechanical  Metallurgy  Division  is  also  presented  here.    From  this 
review it is evident that most primary hot working studies were carried out between 
1500 to 1700 °C.  The subsequent annealing treatments, which require holding at 
lower temperatures of about 1100 to 1300 °C for very long times help further 
transform the precipitates from coarse orthorhombic to very fine cubic.  Our studies 
on Nb‐1Zr‐0.1C alloy also confirm that optimum hot working lies at temperatures 
beyond 1500 °C where dynamic recrystallization initiates, and optimally around 
1700 °C where dynamic recrystallization transforms the microstructure.  Working at 
temperatures lower than 1000 °C may lead to the undesirable effect of both micro as 
well as macro strain localization, and should be avoided. 
Keywords: Nb‐1Zr alloys, high temperature reactors, thermomechanical processing, 
creep. 

1
1 Introduction 
The development of Nb alloys for use as structural materials in nuclear reactors 
was started in the early 1960’s by NASA for the specific application of nuclear 
powered reactors in space vehicles referred to as Space Nuclear Power Systems (for 
example see Ref. [1‐3]).  These are low powered systems of 10’s to 100’s kW  with 
e
an operating life of around 5 to 10 years (see Ref. [4] for overview of these systems).  
The research in refractory alloys for nuclear applications did halt for a decade after 
which the 1980’s saw a renewed interest in the use of refractory alloys in nuclear 
fission reactors in space.  The specific requirements for alloys to be used in Space 
Nuclear Powered Systems were their ability to retain strength at relatively high 
temperatures (1000 to 1200 °C), long term thermal creep strength (less than 1% 
strain in 7 years was the benchmark for space reactors), good compatibility with 
liquid alkali metals, and low parasitic neutron cross‐section [5].  Nb alloys satisfy 
these requirements and are easier to fabricate as compared to Mo and W alloys.  Ta, 
which has a comparable fabricability, has the disadvantage of a higher density and a 
higher neutron cross‐section as compared to Nb.  For the special application of space 
nuclear power systems, the lowest density of refractory metals would be preferred, 
which in this case are the Nb alloys (Nb‐1Zr, PWC‐11 and C‐103).  However, the 
drawback  of  Nb  alloys  is  that  these  have  the  lowest  yield  strength  at  high 
temperatures as shown in Fig. 1.  At high temperatures (1000 to 1200 ºC) the 
strength of all Nb alloys fall in the same band.  Mo alloys have higher strength values 
and  Mo  based  TZM  and  Ta  alloy  (T‐111)  and  have  the  highest  strengths  [5].  
Although the drop in strength with increasing temperature is higher for the T‐111 
and TZM, they are still stronger than Nb alloys at 1300 °C.  One of the alloys Nb‐5W‐
2Mo‐1Zr [6] shows dynamic strain aging between 600 to 1000 °C, a region in which 
no hot working should be carried out.  The UTS vs. temperature behavior of some Nb 
alloys (Fig. 2) shows that for most alloys in the intermediate temperature regime 
UTS increases with increasing temperature signifying dynamic strain aging. 
2