J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Effect of Chromium Content on Mechanical Properties of V-xCr-4Ti-0.15YAlloys
Takeshi MIYAZAWA, Takeo MUROGA and Yoshimitsu HISHINUMA
National Institute for Fusion Science, Gifu, JAPAN(Received: 16 September 2014 / Accepted: 19 January 2015)

A combined effect of purification and yttrium (Y) addition was investigated to improve impact properties ofhigh-chromium V-xCr-4Ti alloys. Tensile tests and Charpy impact tests were carried out by using miniaturized specimens.
V-10Cr-4Ti-0.15Y alloy had ultimate tensile stress (UTS) at 700 oC and yield stress (YS) approximately equal to and higher
than those of V-4Cr-4Ti alloy, respectively. V-10Cr-4Ti-0.15Y alloy had more favorable impact properties than those ofprevious V-xCr-4Ti alloys with x > 9 wt.%. It is proposed that purification and Y addition may suppress the formation of thelarge Ti-rich complexes with carbon, oxygen and nitrogen which act as crack initiation sites.
Keywords: low-activation materials, blanket structural materials, solution hardening of Cr, tensile properties,impact properties

dislocations, was suppressed, and resulting in the loss ofDSA.
Cr is known to increase the strength of V at hightemperatures. Increasing Cr content relative toV-4Cr-4Ti-0.15Y alloy is considered to be an effective
method in order to compensate for the reduction ofhigh-temperature strength due to the reduction ininterstitial impurities by the scavenging effect of Y.
However, excess Cr addition can lead to loss of ductility
and degradation of workability [10]. The influence of theinterstitial impurities on the Cr effects is not known.
In this study, V-xCr-4Ti-0.15Y alloys with high-Cr
contents are fabricated in order to improve the impact
properties of V-xCr-4Ti alloys without degrading thehigh-temperature strength. A combined effect ofpurification and Y addition is investigated by theevaluation of tensile properties and impact properties.

1. Introduction

Vanadium (V)-based alloys nominally containing
Chromium (Cr) of 4 wt.% and Titanium (Ti) of 4 wt.% areregarded as advanced blanket structural materials forfusion reactors because they potentially have low-induced
activation characteristics, high-temperature strength, andhigh thermal stress factors [1]. The upper and lower limits
for the operation temperature of V-4Cr-4Ti alloy are
determined by high-temperature strength and irradiation
embrittlement, which are currently assumed to be around
700 and 450 oC, respectively [2]. It is well known thatsolid solution and precipitation of interstitial impurities ofCarbon (C), Oxygen (O) and Nitrogen (N) strongly affectthe mechanical and irradiation properties in V-4Cr-4Ti
alloys [3, 4]. The previous study clarified that the largeand small precipitates were identified as Ti-rich andTi-C-O complexes, respectively [5]. V-4Cr-4Ti alloy isannealed at 1000 oC to remove the interstitial impurities
from the alloy matrix by the formation of these Ti
precipitates. Oxygen, however, tends to remain in thematrix compared with C and N even after the annealing,
and can enhance irradiation hardening [6]. Therefore,
yttrium (Y) with stronger chemical affinity with O than Ti
is added to remove O further from the matrix.
V-4Cr-4Ti-0.15Y alloy has exhibited a reduction ofirradiation hardening and an improvement of ductility intensile tests after neutron irradiation around 400 oC, andis expected to expand the lower temperature operation
limit [7, 8]. However, V-4Cr-4Ti-0.15Y alloy has also
exhibited a reduction of high-temperature strength due tothe loss of hardening by the dynamic strain aging (DSA)
effect [9]. With the Y addition, the formation of Cottrell
atmospheres, composed of interstitial C, N and O locking

2. Experimental

Results of chemical analysis for V alloys used in thisstudy are shown in Table 1. V-4Cr-4Ti alloy is thereference alloy, so-called NIFS-HEAT-2, which wasfabricated by electron beam melting and vacuum arc
re-melting in 166 kg-scale [11, 12]. Three alloys ofV-xCr-4Ti-0.15Y (x = 4, 6, 10 wt.%) were fabricated by 15
kg-scale levitation melting at Daido Steel Co. Ltd. [13, 14].
Figure 1 shows fabrication process of V-xCr-4Ti-0.15Y (x= 4, 6, 10 wt.%) alloys. These ingots were cut andmachined to remove a cavity formed at the bottom and theperiphery. After the cutting and machining, the size ofthese ingots was 140 mm in diameter and 45 mm in height.
Small blocks with 30 [ 30 [ 20 mm3 were cut from
V-10Cr-4Ti-0.15Y ingot in order to investigate theauthor’s e-mail: miyazawa.takeshi@LHD.nifs.ac.jp

89©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

T. Miyazawa et al., Effect of Chromium Content on Mechanical Properties of V-xCr-4Ti-0.15Y Alloys

Table 1 Chemical composition of the alloys (wt.%)
CodeVCr
V-4Cr-4Tibal.
4.02
V-4Cr-4Ti-0.15Ybal.
4.23
V-6Cr-4Ti-0.15Ybal.
6.21
V-10Cr-4Ti-0.15Y
bal.
9.93

Ti
4.15
4.17
4.16
3.90

workability. Each ingot was canned into a 304 typestainless steel container by electron-beam welding. The
canned ingot was hot-forged in the range of 900-1150 oC.
After forging, cold rolling was performed to obtain various
thickness of plates. V-4Cr-4Ti-0.15Y and V-6Cr-4Ti-0.15Y
alloys had such a favorable workability that nointermediate annealing was required through the several
steps of cold rolling. However, V-10Cr-4Ti-0.15Y alloy
had an unfavorable workability since the intermediate
annealing (1000 oC for 3.6 ks) was required in order toobtain plates by cold rolling.
The sheet specimens for Vickers hardness tests werecut from the plates with 1 mm in thickness. The sheet
specimens were mechanically and electrolyticallypolished, followed by annealing at the range of 600-1100
oC for 3.6 ks in a vacuum better than 1 x 10-4 Pa. The
hardness tests were carried out at 500 gf for 30 s at room
temperature (RT). The hardness was calculated byaveraging the hardness obtained from ten indents.
Miniature tensile specimens with a gauge size of 1.2 x 5 x0.25 mm3, called SSJ specimens, were punched from theplates with 0.25 mm in thickness. Miniaturized Charpy
specimens were machined from plates with 4 mm inthickness for V-4Cr-4Ti-0.15Y and V-6Cr-4Ti-0.15Y
alloys, and from plates with 2 mm in thickness for
V-10Cr-4Ti-0.15Y alloy. The Charpy specimens had
dimensions of 1.5 x 1.5 x 20 mm3, a notch angle of 30o,and a notch depth of 0.3 mm, so that ligament size was1.2 mm. Charpy and tensile specimens were annealed at1000 oC for 7.2 ks for V-4Cr-4Ti alloy, and at 950 oC for
3.6 ks for V-xCr-4Ti-0.15Y alloys. Tensile tests wereperformed from RT to 800 oC in a vacuum better than 1 x
10-4 Pa. The initial strain rate was 6.67 x 10-4 s-1. Charpy
impact tests were conducted from -196 oC to RT using theinstrumented Charpy impact testers at International
Research Center for Nuclear Materials Science, Tohoku
University and NIFS. The crosshead speed of Charpy
impact tests was 5 m/s. Fracture surfaces were analyzed
using scanning electron microscopy (SEM).

Y<0.01
0.11
0.08
0.09

C
0.012
0.011
0.013
0.011

N
0.009
0.009
0.011
0.014

O
0.012
0.009
0.00950.007

Fig. 1. Fabrication process of V-xCr-4Ti-0.15Y (x = 4, 6,10 wt.%) alloys.

3. Results

Figure 2 shows hardness recovery as a function of thefinal annealing temperature after cold rolling. Every
hardness is the average of ten indents corresponding
sample at different surface position. The hardness has aminimum between 900 and 1000 oC. The minimum values
increased with increasing Cr content.

Fig. 2. Recovery of hardness as a function of final
annealing after cold rolling. Open squares represent
hardness for V-4Cr-4Ti alloy [5].

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T. Miyazawa et al., Effect of Chromium Content on Mechanical Properties of V-xCr-4Ti-0.15Y Alloys

Figures 3 and 4 show the dependence of tensile
strength and elongation on test temperatures, respectively.
Tensile strength for V-xCr-4Ti-0.15Y alloys increased withincreasing Cr content. It must be noted that there was adifference in the dependence of ultimate tensile strength(UTS) and total elongation (TE) on test temperature
between V-4Cr-4Ti alloy and V-xCr-4Ti-0.15Y alloys.
Between 600 to 700 oC, UTS for V-4Cr-4Ti alloy remained
almost constant, while that for V-xCr-4Ti-0.15Y alloys
decreased. V-10Cr-4Ti-0.15Y alloy has UTS at 700 oC
approximately equal to that of V-4Cr-4Ti alloy. TE forV-xCr-4Ti-0.15Y alloys increased remarkably above 600oC. Figure 5 shows tensile stress-strain curves for thealloys at 700 oC. Tensile tests for V-4Cr-4Ti alloy at 700 oC
were carried out twice in order to confirm thereproducibility of serrations. The deformation behavior ofV-4Cr-4Ti alloy during tensile tests typically withintemperatures ranging 300 to 750 oC was shown to exhibit
DSA, which is manifested by oscillations in the flow stressas serrations on the stress-strain curves [9]. Serrations forV-xCr-4Ti-0.15Y alloys disappeared at 700 oC as shown inFig. 5. The decrease in strength and increase in elongation
at 700 oC for V-xCr-4Ti-0.15Y alloys are likely tocorrespond to the loss of hardening due to DSA.
Figure 6 shows absorbed energies from -196 oC toRT. In the present study, ductile-to-brittle transition
temperature (DBTT) is defined as the temperature wherethe absorbed energy is half of the upper-shelf energy(USE). Table 2 summarizes USEs and DBTTs for thealloys tested in the present study. DBTTs for V-4Cr-4Ti
and V-4Cr-4Ti-0.15Y alloys were below -196 oC. DBTTsfor V-6Cr-4Ti-0.15Y and V-10Cr-4Ti-0.15Y alloys wereestimated as -194 and -130 oC, respectively.

Fig. 4. Dependence of uniform elongation (UE) and totalelongation (TE) on test temperatures.

Fig. 5. Tensile stress-strain curves for the alloys at 700 oC.
Tensile tests for V-4Cr-4Ti alloy were carried out twice inorder to confirm the reproducibility of serrations.

Fig. 6. Absorbed energies in the Charpy impact tests.
These energies are normalized by specimen width (B =1.5 mm) and ligament size (b = 1.2 mm).

Fig. 3. Dependence of yield stress (YS) and ultimate
tensile strength (UTS) on test temperatures.

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T. Miyazawa et al., Effect of Chromium Content on Mechanical Properties of V-xCr-4Ti-0.15Y Alloys(a)(b)(c)

500 nmFig. 7. SEM images for fracture surface tested at -196 oC. (a) V-4Cr-4Ti-0.15Y, (b) V-6Cr-4Ti-0.15Y, (c) V-10Cr-4Ti-0.15Y.
initiation of crack formation due to the large Ti-richprecipitates (about 500-1000 nm). The contamination of
gaseous impurities such as O may result in the formationof the large Ti-rich precipitates to act as crack initiationsites. Therefore, the reasons for favorable impactproperties of V-10Cr-4Ti-0.15Y alloy are considered to be
as follows: (1) The purification could suppress theformation of the large Ti-rich precipitates because
V-xCr-4Ti-0.15Y alloys examined in this study have lowlevel of C, O and N content. (2) The formation of Y2O3
inclusions during the melting process [9, 14] could further
reduce O content in the matrix, and then suppress theformation of the large Ti-rich precipitates. Further studyof microstructural examinations is necessary to determine
the size, number density and distribution morphology ofprecipitates in V-10Cr-4Ti-0.15Y alloy.

Table 2 Summary of Charpy impact properties.
Code
USE (J/mm3)
DBTT (oC)
V-4Cr-4Ti
0.39
Below -196
V-4Cr-4Ti-0.15Y
0.35
Below -196
V-6Cr-4Ti-0.15Y
0.38
-194
V-10Cr-4Ti-0.15Y
0.40
-130
Figure 7 presents SEM images of the fracturesurfaces of V-xCr-4Ti-0.15Y alloys tested at -196 oC. The
fracture surface of V-4Cr-4Ti-0.15Y alloy exhibited goodductility. The fracture surface of V-6Cr-4Ti-0.15Y alloy
was a mixture of ductile fractures with dimples and brittle
fractures with secondary cracks. The fracture surface ofV-10Cr-4Ti-0.15Y alloy indicated fully brittle fracture
characterized by cleavage and secondary crack withrandom directions.

4. Discussions

V-10Cr-4Ti-0.15Y alloy has UTS at 700 oC and YS
approximately equal to and higher than those of V-4Cr-4Ti
alloy, respectively. The Cr addition provides strengthening
as a substitutional solute in vanadium alloys. In order tocompensate for the decrease of high-temperature strength
due to the loss of hardening by DSA, the Cr addition is aneffective method, as shown in Fig. 3. Because it is known
that the change in UTS at high-temperature has a close
relation with thermal creep properties, it is necessary toinvestigate the creep properties of V-10Cr-4Ti-0.15Yalloy.
Figure 8 summarizes the dependence of DBTT forV-xCr-(4-5)Ti and V-xCr-4Ti-0.15Y alloys on Cr content[10, 15]. Higher Cr content increases DBTT ofV-xCr-(4-5)Ti
alloyssignificantly.
However,
V-10Cr-4Ti-0.15Y alloy has lower DBTT than those ofV-xCr-(4-5)Ti alloys with x > 9 wt.%. It seems that thefactors contributing to DBTT are not only the solution
hardening of Cr but also the difference in precipitation
behavior. Sakai et al. [10] clarified that high DBTTs ofV-xCr-4Ti alloys with x > 10 wt.% were caused by highflow stress due to solution hardening of Cr, and by

Fig. 8. Dependence of DBTT for V-xCr-(4-5)Ti andV-xCr-4Ti-0.15Y alloys on Cr content. Open circles andopen triangles represent V-xCr-4Ti alloys annealed at heat
treat conditions to obtain a mean grain size of 17 Pm, andat 950 oC [10]. Open squares with cross represent
V-xCr-5Ti alloys annealed at 1125 oC [15]. Closed squares
represent V-xCr-4Ti-0.15Y alloys annealed at 950 oC (thisstudy).

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T. Miyazawa et al., Effect of Chromium Content on Mechanical Properties of V-xCr-4Ti-0.15Y Alloys

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High-temperaturetensileproperties
andlow-temperature
impactproperties
ofhigh-Cr
V-xCr-4Ti-0.15Y alloys (x = 6, 10 wt.%) were examined.
1) V-10Cr-4Ti-0.15Y alloy had UTS at 700 oC and YS
approximately equal to and higher than those of
V-4Cr-4Ti alloy, respectively. High Cr addition isshown to compensate for the decrease in thehigh-temperature strength by purification and Yaddition.
2) V-10Cr-4Ti-0.15Y alloy has more favorable impact
properties than those of previous V-xCr-(4-5)Ti alloys
with x > 9 wt.%. It is proposed that purification and Y
addition may suppress the formation of the large
Ti-rich precipitates to act as crack initiation sites.

Acknowledgement

The authors are grateful to Prof. K. Fukumoto ofFukui Univ. for the support in the preparation of thespecimens and the fruitful discussions. This work wassupported by Grant-in-Aid for Scientific Research (C),(2013-2015) 25420889.

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