Magnetic properties of irradiated austenitic stainless steel

Journal of Nuclear Materials 85 & 86 0 North-Holland Publishing Company

MAGNETIC PROPERTIES J.

T.

(1979) 787-791

OF IRRADIATED

AUSTENITIC

STAINLESS

STANLEY

College

of

Engineering

& Applied

Sciences,

Arizona

State

This report presents results of measurements on the irradiated austenitic steel. It is shown that exposure neutrons produces significant changes in the magnetic phenomenon of time dependent magnetic hysteresis into measurements below room temperature.

1.

STEEL*

I NTRODUCTI ON

Table

Magnetic

*Research

sponsored

made using

by U.S.D.O.E.

787

were hr.

given a preirradiation at 1323 K. In addition

heat the

Composition

316-l (wt %I

316-4 (wt %I

Ni Cr

13.63 17.33

14.4 17.5 2.8

Ti co CU Mn P S Si C N

a

I

Element

MO

PROCEDURES were

USA

I

Chemical

hysteresis is important for energy dissipation in cases where field fluctuates with time.

measurements

85281,

magnetic properties of neutron to high fluences of energetic permeability and introduces the the behavior of these steels for

Both alloys treatment of

The magnetic properties that are of interest from the standpoint of the operation of magnetic confinement reactors are magnetic permeability (B/H), magnetization (M), and magnetic hysteresis. The magnetic permeability determines what portion of the magnetic flux is carried by the wall of the reactor vessel. Large increases in magnetic permeability during irradiation could affect the magnetic confinement fields. The magnetization is important in determining the body forces by;; H. magnetic field gradients since FGr:#

EXPERIMENTAL

Arizona

The stainless steel specimens used for this study were supplied by Oak Ridge National Laboratory and Atomics International. The compositions of the two heats of stainless steel to be discussed in this article are given. in Table 1. The alloy designated 316-1 was a commercial heat of stainless steel while 316-4 was a special high purity heat of stainless produced for ORNL. Stanley and Garr [2] previously reported magnetic measurements on the 316-l s ecimens. Leitnaker, Bloom and Stiegler [3 5 reported that the high purity stainless had higher swelling rates than the commercial type 316.

produces significant increases in the magnetization of type 316 and 321 austenitic stainless steel. The purpose of the present article is to present additional information concerning the magnetic properties of neutron irradiated type 316 stainless steels.

2.

Tempe,

vibrating reed magnetometer [I], to measure the magnetization of small specimens (-4 mg) as a function of applied field. A superconducting solenoid was used for measurements at fields up to 3.3 T and a copper wire wound solenoid immersed in liquid nitrogen for fields up to 0.1 T.

The magnetic properties of any material used for the first wall of a magnetic confinement reactor should be of particular interest. However, relatively little information is available on how exposure to energetic neutrons generated in these reactors might affect the magnetic properties of the materials used for the first wall. Austenitic stainless steel is a particularly interesting material in this regard as it is known to be in a metastable condition with respect to transformation to a ferromagnetic phase (a-ferrite) at temperatures below about 30 K. The most extensive study to date of the effect of neutron irradiation on the magnetic properties of austenitic stainless steels is that of Stanley and Hendrickson [I]. They showed that irradiation in the range of 1 - 7.8 x 10 2~,;~~‘;;“>‘;“;“;~;,

The magnetic determining the magnetic

University,

2.32 __ 0.01

__ _-

__

0.075 1.68


0.022 0.017 0.51 0.06 __

0.006 0.002 0.01 0.005 0.0003

J. T. Stanley / Mugnetic properties

788

Table

2

Room Temperature

I tern

Al loy

I 2 z

316-4 316-4

(: 9

Magnetization

1.90

Z8

magneton

treatment

= 9.27

irradiated

x l0-24

Type

10.0

2

1.

Steel

0.0 18.2 29.9 2.64 34.3

24.9

5.2

2.6

9.7

2.6

2.7 3.0

11.5

1.31

12.2

par:icles/m3)

8.4

4.90

.f/Tesla

3

4 p&/T

Figure

2.34 8.66 4.05 8.46 .73 5.09 ‘34

0.0

Irradiation iilbe accurate to further discussion

Specimens were irradiated in EBR-Il. Data concerning irradiation temperature and fluence were provided by the two groups which supplied the specimens. Fluences are reported for neutrons of energy equal to or greater than 0.1 ratio of d a to fluence MeV . The calculated for this experiment is 6 x 10 -28 dpa m2/neut ron

I

Stainless

(IO*3

were given a preirradiation of 100 hr. at 1033 K.

0

316

(K)

723 778 873 749 802 890 773 873 698

3.50 3.50 3.30 4.50

316-l

for

Irradiation Temperature,

1.06 2.10

316-4

316-I specimens

Parameters

(1026F~~~~~~ns/m2~

316-4

*pB = Bohr

heat

ofirradiated austenitic stainless steel

!J

temperatures are estimated -25K * + See reference 3 for of irradiation conditions.

RESULTS

3. 3.1

High

Field

Magnetic

Measurements

Figure 1 shows results of magnetization versus field measurements for the 316-4 specimens for

6

7

8

9

IO

II

I2

, mi lli Tesla/*K

Magnetization vs. Temperature Reduced Field. Pteasurement Temperature 300 K. A, item j’, Table 2; B, Item 1, Table C, Item 3, Table 2.

2;

J. T. Stanley /Magnetic

properties

three different irradiation cond’tion 26 3. The specimen irradiated to 1.3 x 10 n/m ar773K shows a magnetization versus field curve that is characteristic of a paramagnetic substance that has about twice the suceptibiiity of an unirThe specimen irradiated to ta;i;t;c#2gpeciF. n/m at 773 K shows a magnetization curve that is characteristic of so called superparamagnetic substances. The solid curve A in Fig. 1 is the Langevin function, M = np[coth($

-

where n is the number of unit volume and p is the particle.

l/@]

magnetic magnetic

particles per moment per

The Langevin function gives a reasonable fit to the experimental data for those specimans irradiated to the higher fluences, but for specimens irradiated to lower fluences the high field data are fitted better with a straight line of slope, K and intercept on the ordinate, MO. These lines are shown as dashed lines for A and B in Fig. 1. MO and K are given in Table 2 for the various specimens. For those specimens that fit a Langevin function the values of p and n are given. Magnetization made at various

versus field temperatures

measurements were between 82 K and

MEASUREMENT

Figure

2.

Parameters irradiated

Mo and to 1.8

789

of irradiated austenitic stainless steel

317 K for the 316-i alloy. Both M. and K were found to increase as the temperature decreased. The variation of these two parameters with temperature is shown in Fig. 2. 3.2

Low Field

Measurements

Measurements of magnetization versus field ware made at low fields for the 316-l alloy to determine the hysteresis loop of this material. At room temperature the residual magnstlzation is zero and the hysteresis loop is closed. At temperatures below room temperature the residual magnetization is time dependent and thus the area enclosed within the hysteresis loop will depend on the time required for one cycle of the applied field. The time dependence of the residual magnetization at 172 K is shown in Fig. 3 for the 316-l alloy. 4.

Dl5CUSSlON

The room temperature magnetic properties of neutron irradiated type 316 austenitic stainless steel can be determined from the data given In Table 2 for the irradiation temperatures and The unirradiated neutron fluences shown. austenitic stainless is a paramagnetlc substance with a volume susceptibility of about 2.5 mT/T. The initial part of a Langevin function is 1 lnear with field such that

TEMPERATURE,

K

K vs. measurement temperature x 1026 n/m2 at 698 K.

for

alloy

316-t

. I

J. T. Sranley / Magnetic properlies of irradhred auslenitic stainless sreei

790

8

1

t

t

I

-1.20

-I.30 :

-1.40

-I.!50

8

0

Figure

3.

Logarithm irradiated

t,

min.

(M/mT) vs. t’ e after turning off to 1.8 x I&’ n/m2 at 698 K.

M = n$H/3kT

Thus, the data for the unirradiated specimens and the low fluence irradiated specimens at 778 I< and 873 K can be said to fit the Langevin function. It is kEOt-th noting that the magnetic mment per atom required to give a susceptibility of 2.5 mT/T is 1.3 u The two Y state theory of iron-nickel-c developed R’romium alloys by Miodownik j-51 gives a value of 0.64 ~8 for the magnetic moment per atom in an alloy with the ccmposition of 316-4, Not much change is noted in magnetic properties2until the neutron fluence exceeds 1 x fluence of 2.1 x IO2 bo~Bu:~~n;m2F~~daa~::~~iation temperature of 778 K the steel still acts as a normal paramagnet but the volume susceptibili ty has more than tripled that af the unirradiated steel. As the neutron f luence increases the ~gnetization versus field plots show noticeable curvature. It is no longer possible to fit the magnetization versus field The initial part of with a Langevin function. the magnetization curve increases rapidly with field and then bends over into a linear portion above magnetic fields of about 0.6 T. We have chosen to characterize the magnetization curves of these specimens by giving the slope of the linear portion and its intercept on the It’would undoubtedly be possible ordinate axis. to fit the magnetization curves with a sum of Langevin functions with a distributioh of magnetic moments but we have not attempted this. At a given irradiation temperature the intercept as the neutron fluence increases, Fb¶ increases but it appears that the slope of the 1inear portion first increases and then decreases as

saturating

field

for

alloy

316-l

the fluence increases. Finally, as higher fluences are reached the magnetization curves can again be fit to a Langevin function but with a magnetic mOment per particle of several thousand Bohr magnetons. These are the so called superparamagnetlc particles. It appears that in this regime the number of particles par unit volume is constant regardless of the irradiation temperature or fluence while the magnetic moment per particle increases with fluence and decreases with increased irradiation temperature. The magnetic permeabilities of the various specimens at any field can be calculated from the data given by using the fundamental equaB = &Ii + M. tion, The magnetic hysteresis is essentially zero at room temperature for low frequency magnetic fields. However, some hysteresis will be observed for high frequency magnetic fields or for low temperatures. Previously, it was assumad that ferrite was the magnetic phase precipitating in the neutron irradiated specimens [1,2). However, the data presented here favor another interpretation, i .e., the ferromagnetic phase is an iron and/or nickel rich cluster in the austenitic matrix. Consider first the possibility that the ferromagnetic phase is ferrite. It can be formed either by a martensitic reaction or by nucleation and growth. In either case we know that its magnetic moment will be close to 2 “6 per atom and we can calculate the approximate amount of the ferrite phase from the saturation magnetization of the magnetic particles. The

J. T. Stanley /Magnetic properties of irradiated austenitic stainless steel

specimens showing the largest magnetic effects should have only about 2% of the ferrite phase. In these specimens we would still expect to see the magnetic contribution of the austenite phase at high fields. In other words we should expect to see a magnetization curve which increases rapidly with magnetic field at low fields and then bends over into a linear rise of magnetization with field as the ferrite particles become saturated. The linear portion of the high field curve should have about the same slope as the unirradiated steel. instead we see that the curve bends over and becomes almost horizontal at high fields [Fig. l., Curve This is an indication that the austenite A]. matrix has undergone a considerable change in composition. Such a large change in composition cannot be accounted for by the segregation of alloying constituents required to form 2% ferrite by a nucleation and growth mechanism. Formation of carbides during irradiation can also be eliminated as a means of changing the austenite compositions since the carbon content of these high purity alloys is very low. Another feature of the data that is inconsistent with ferrite phase being the ferromagnetic phase in these alloy is the change in the parameter M. with decreasing temperature shown in Fig. 2. The Curie temperature of any ferrite phase that could form in these alloys is much higher than room temperature. Measurements made below room temperature should show little or no change in the saturation magnetization of an individual particle of ferrite. Thus, the magnetization versus temperature reduced field plots for different measurement temperatures should all superimpose. This should be true even if the assembly contains a distribution of particle sizes. The fact that MO increases with decreasing temperature means that the magnetic particles have variations in composition and that the Curie temperature versus composition must be below room temperature over part of the expected range of compositions [6]. Now consider that the ferromagnetic phase is an Fe and Ni rich cluster of atoms in the austenite matrix. The Curie temperature of a 50% Ni binary Fe - Ni alloy is 800 K and it decreases as the amount of iron increases [7]. For a 28 at .% Ni alloy with the face-centered cubic structure the Curie temperature is 373 K. For pure nickel the Curie temperature is 626 K and it decreases rapidly as Cr is added reaching 273 K at 9 at % Cr. Addition of MO to the Ni also decreases the Curie temperature rapidly. It is possible that a miscibility gap exists in the Cr - Fe - Mo - Ni quantenary system such that the alloy separates into two or more face centered cubic phases and that one of these phases is ferromagnetic. It is not possible to determine the compositions of these phases from the present data. It

is

known

that

Ni

segregates

to

free

791

surfaces in Fe - Cr - Ni alloys during irradiation [8]. Nickel segregation to void surfaces is a possible source of the magnetization increases. However, the void density in th 316-4 alloy is known to be around 3.6 x 10 26 voids/m3 while the magnetic particle density is about three orders of magnitude greater. Also, the surface segregation is thought to be a non-equilibrium effect driven by the defect migration to the voids. One would expect the solute segregation to decrease with time upon post irradiation annealing but it has been shown that post irradiation annealing can increase the magnetic effects [1,2]. CONCLUSIONS

5.

The magnetization of type 316- austenitic stainless steel is significantly increased by irradiation in the temperature range 698-873 K at neutron fluences greater than I x 1026 n/m2 [E > 0.1 MeV]. The magnetization at large magnetic fields can be determined from two parameters given in tabular form for various irradiation temperatures and fluences. The changes in magnetic properties with irradiation result from clustering of iron and/ or nickel atoms on the austenite matrix rather than from formation of the ferrite phase.

References [I]

J. T. Stanley be published

and L. E. in J. Nucl.

[2]

J. T. Stanley 6A II19751 p.

531.

[3]

and

K.

R.

to

Hendrickson, Mat. Garr,

Met.

Trans.

J. M. Leitnaker, E. E. Bloom, and J. Stiegler, J. Nucl. Mat. 49 [1973-741

0.

p. 57. [4]

D. p.

G. Doran, 207.

[5]

A. p.

P. Miodownik, 541.

[6]

E. Kneller in Magnetism and Metallurgy Vol . 1, A. E. Berkowitz and E. Kneller ed. Academic Press, N. Y., NY, 1969 pp. 365-471.

[7]

M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill Inc., Y ., NY, 1958.

[8]

P. R. Mat.,

Okamoto

Trans.

Acta

and

53 [1974]

A.N.S.,

H.

17 [I9731

Met.,

Widersich,

p. 336.

I8

[I9701

J.

N.

Nucl.