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Microstructure and Mechanical Properties of a 2000 MPa

Co-Free Maraging Steel after Aging at 753 K
Y. HE, K. YANG, W. SHA, and D. J. CLELAND

The precipitation kinetics at the aging temperature of 753 K in a 2000 MPa grade Co-free maraging
steel (Fe-18.9Ni-4.1Mo-1.9Ti, wt pct) has been studied. Under the peak-aged condition at 753 K, Ni3Ti
precipitates of moderate size were uniformly distributed in the martensite matrix, leading to optimal
combination of strength (2000 MPa of yield strength) and fracture toughness (70 MPa 1m). The ultra-high
strength of the maraging steel subjected to long time aging at 753 K is attributed to the high resistance
to coarsening of precipitates. The orientation relationship between martensite and Ni3Ti was observed
as (011)M//(0001)-Ni3Ti and [111]M//[1120]-Ni3Ti. The Orowan mechanism is the dominant strength-
ening mechanism.

I. INTRODUCTION AND EXPERIMENTAL (6) The formation of a great amount of extremely fine precipi-
PROCEDURE tates leads to the rapid increase of the yield strength to
1700 MPa at the initial stages of aging at the low tem-
THIS article reports systematic work on the microstructure perature of 713 K. With the growth of the precipitates,
and its relation to mechanical properties in a 2000 MPa marag- the yield strength gradually increases to 2000 MPa, accom-
ing steel after aging at 753 K. Strengthening and toughening panied by a reduction in ductility. The KIC value reduces
mechanisms and other controversial issues are also discussed. from 73.7 MPa 1m gradually to 62 MPa 1m. The
This article follows previous research[1] and concentrates on extremely fine precipitates at the early stages of aging are
aging at an intermediate, or normal, temperature, 753 K, of the cause of the slightly low fracture toughness.
the 2000 MPa grade Co-free maraging steel. The steel com- (7) The strengthening mechanism at the early stages of low-
position is Fe-18.9Ni-4.1Mo-1.9Ti (wt pct). The subject of temperature aging is through the shearing and looping
study was introduced in Reference 1. The main conclusions of high-density dislocations passing the precipitates.
from that work can be summarized in the following points. At later stages, the Orowan looping mechanism operates.
(1) The phase transformation temperatures of the 2000 MPa The material under study was described in Reference 1.
grade maraging steel studied in the present work are The steelmaking was in a double-vacuum melting furnace
Ms  408 K, Mf  321 K, As  783 K, and Af  993 K. with an ingot of 25 kg in weight and 100 mm in diameter.
Cooling to room temperature can achieve a single-phase The steel was homogenized at 1473 K for 24 hours, then
martensitic structure. forged and rolled at 1123 K to a 25-mm-thick plate followed
(2) There is rapid age hardening across the aging tempera- by air cooling. Samples were cut along the rolling direction
ture range of 713 to 813 K in the Co-free maraging steel. and solution treated at 1073 K for 1 hour, water quenched,
(3) In the early stages of aging at 713 K, the Co-free marag- and aged for 3, 6, 12, 20, or 50 hours.
ing steel has extremely fine precipitates with average Tensile tests used standard sample size, with a gage diameter
diameter 2 to 3 nm. Ni3Ti precipitates are identified, but of 3 mm and gage length 5 times the gage diameter (15 mm).
the crystallography of another family of spherical precipi- They were conducted in an AG-5000A MTS machine at a
tates has not been identified. Ni3Ti is the main precipi- displacement rate of 0.2 mm/min. The KIC tests were conducted
tate, growing and coalescing into its normal needle or rod with an INSTRON* Schenck 100 kN resonance hydraulic
shape along the 111 direction with increasing aging
time. This type of precipitate has strong resistance to *INSTRON is a trademark of Instron Corp., Canton, MA.
coarsening at this aging temperature.
(4) The spherical precipitates exist in a stable manner after fatigue-testing machine and standard three-point bending sam-
low-temperature aging, with strong resistance to coarsening. ples were used, with two different dimensions. Samples mea-
(5) When aged at 713 K, there is no reverted austenite. suring 8  16  70 mm were used when the tensile strength
was lower than 1800 MPa, while 5  10  55 mm samples
were used for higher strength levels. All tests were conducted
at room temperature.
Y. HE, formerly Ph.D. Student, with the Institute of Metal Research, Specimens for microstructural characterization were cut from
Chinese Academy of Sciences, Shenyang 110016, China, is now a Research the KIC samples (from unstrained parts) and investigated in a
Associate with the Laboratory for Materials Science, Delft University of
Technology, 2628 AL Delft, The Netherlands. K. YANG, Professor, is with
PHILIPS* CM12 transmission electron microscope (TEM).
the Institute of Metal Research, Chinese Academy of Sciences, Shenyang
110016, China. W. SHA, University Reader, and D. J. CLELAND, Professor, *PHILIPS is a trademark of Philips Electronic Instruments Corp.,
are with the Metals Research Group, School of Civil Engineering, The Queen’s Mahwah, NJ.
University of Belfast, Belfast BT7 1NN, United Kingdom. Contact e-mail:
w.sha@qub.ac.uk The foils for TEM were prepared by electropolishing in 10 pct
Manuscript submitted December 15, 2003. perchloric acid and 90 pct methanol solution at 233 K and

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40 V. Fractography observations of tensile and fracture tough- (0111)Ni3Ti spot (labeled 1 in Figure 1(d)) in the diffraction pat-
ness tested samples were made in a Cambridge S360 scan- tern under the [011]M zone, i.e., with the incident electron beam
ning electron microscope machine to evaluate the failure mode. along this direction in the martensite lattice. The average dia-
The quantitative X-ray diffraction method was employed to meter and length of the precipitates, which are dispersed in
measure the amount of retained austenite. All results shown the martensite laths, are about 5 and 15 nm, respectively. In
in this article are for aging at 753 K. addition, similar to samples aged at 713 K,[1] there is extremely
fine dispersion of spherical precipitates in the lath martensite,
with an average diameter of about 7 nm, as shown in Fig-
II. RESULTS ure 1(c). The diffraction patterns from these spherical precipitates
could not originate from any common types of precipitates
A. Microstructure found before in maraging steels, as discussed in Reference 1.
A typical TEM micrograph and its associated selected area The clear and bright diffraction spots (e.g., the spot labeled 2)
diffraction (SAD) pattern are shown in Figure 1, for the Co- from this type of precipitate in Figure 1(d) enable an accurate
free maraging steel aged for 3 hours at 753 K. In the bright- determination of two of its interplanar distances, at 2.44 and
field image in Figure 1(a), needle-shape precipitates are visible 1.394 Å (the camera constant L  25.1 mm Å). These elim-
in the martensite matrix, which itself has a high dislocation inate the possibility of the common types of precipitates in
density. Figure 1(b) is a dark-field image obtained using the maraging steels Ni3Mo, Ni3Ti, Fe2Mo, and Fe2Ti as well as the

Fig. 1—TEM images and SAD pattern of the Co-free maraging steel aged at –753 K for 3 h. (a) Bright-field image showing precipitation and high density
dislocation in the martensite matrix. (b) Dark-field image taken from the (0111)Ni3Ti spot labeled 1 in (d). (c) Dark-field image of spheroidal precipitates
taken from the spot labeled 2 in (d). (d) [011]M SAD pattern.

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Table I. Interplanar Spacings of Precipitates (Spacing in Nanometer/Intensity)*

Ni3Mo[2] Ni3Ti[2] Fe2Mo[2] Fe2Ti[3] Fe3O4[4] Fe2O3[5]

0.484/30 0.368/30
0.296/60 0.269/100
0.236/60 0.2388/30 0.253/100 0.252/80
0.2224/20 and 0.2220/50 0.221/20 0.218/100 0.2199/100 0.221/40
0.2112/80 0.213/50 and 0.207/50 0.205/60 0.2038/100 0.209/70
0.1969/80 0.195/100 0.202/100 0.1998/100
0.1951/100 0.198/60 0.1947/30 0.1843/60
0.172/20 0.1828/30 0.1712/40 0.1697/70
0.1611/80 0.1604/30
0.1523/50 0.151/20 0.137/60 0.1341/60 0.1481/90 0.1488/50 and 0.1457/50
0.1280/50 0.1330/20 0.133/60 0.1302/60 0.1327/20 0.1313/40
0.1267/20 0.1276/50 0.129/100 0.1247/60 0.1280/40 0.1261/20
0.1191/20 0.123/60 0.1196/60 0.1211/20 0.1192/20
0.1186/50 0.1173/20 0.118/60 0.1166/20
0.1122/40 0.1143/40
0.1094/50 0.1087/50 0.109/60 0.1094/80 0.1106/40
0.1085/20 0.1068/50 0.1050/50 0.1058/50
0.1046/20 and 0.1038/20 0.104/100
0.1027/20 and 0.0819/20 0.0990/40 0.099/30
*Only the strong diffractions (I/I0 20 pct) are selected.

surface oxidation products Fe3O4, Fe2O3, etc. These interpla- cles denote double diffraction spots due to the two orienta-
nar spacings are represented in Table I and compared to make tions of Ni3Ti. The dark-field images shown in Figures 2(e)
this clearer. Therefore, they may be regarded as a new, unknown and (f) using diffraction spots labeled 3 and 4 display the same
type of precipitate. The spherical precipitates and the Ni3Ti rod or orientation, distribution, and size of the rodlike Ni3Ti preci-
needle-shape precipitates are in about the same number in this pitates as in Figures 2(d) and (c), respectively. Ni3Ti has hexa-
Co-free maraging steel in the early stage of aging at 753 K. gonal crystal structure and thus has 12 equivalent orientation
After aging for 12 hours at 753 K, the peak hardness is relationships with martensite,[6] or 12 variants.
reached.[1] The corresponding microstructure and diffraction In the [011]M zone diffraction pattern, there is also a com-
patterns are shown in Figure 2. Full sets of diffraction pat- plete set of spots from the spherical precipitates, labeled 5 in
terns from Ni3Ti and the unknown spherical precipitates are Figure 2(a) and using open triangle symbols in Figure 2(b).
shown in Figure 2(a). The spot labeled 1 in Figure 2(a), Figure 2(g) is the dark-field image using spot labeled 5 of the
belonging to the Ni3Ti family –shown using open square sym- spherical precipitates showing their morphology. Figure 2(h)
bols in Figure 2(b), is the (2240)Ni3Ti diffraction spot, in the is the dark-field image using its double diffraction spot, labeled
line
– connecting the incident beam spot and the matrix spot 6 in Figure 2(a). It can be seen that this type of precipitate is
(222)M. Dark-field image using this spot from Ni3Ti is shown evenly distributed in the matrix, slightly coarsened compared
in Figure 2(c). The orientation relationship between the -Ni- with those in the specimen aged for 3 hours, now having an
Ti precipitates and the martensite matrix is average diameter of about 9 nm. Comparing the diffraction
patterns in Figures 1(d) and 2(a), both from the [011]M zone,
(011)M//(0001)-Ni3Ti Ni3Ti spots are stronger after aging for a longer time.
– – It should be noted that in the TEM, the magnetic field of
[ 111]M//[11 20]-Ni3Ti
the steel samples deflects the electron beam, in an uncontrol-
The spot labeled 2 in Figure 2(a), belonging to the Ni3Ti lable manner, deteriorating significantly the image quality.
family shown using circle symbols in Figure 2(b), is from Ni3Ti In addition, the extremely large thickness of the “thin” foil
in relation– to an equivalent martensitic orientation. The spot is specimens (even thicker when they were tilted) compared
again (2240)Ni3Ti, this–time along the line connecting the transmis- to the size of these ultra-fine precipitates makes the precipi-
sion spot and the (2 22)M spot. Its corresponding dark-field tates appear to have a much higher number density.
image is shown in Figure 2(d). The orientation relationship After the long aging, 50 hours, Ni3Ti precipitates grow,
between this set of Ni3Ti precipitates and the martensite matrix is but they do not coarsen or dissolve into the matrix. The aver-
age diameter and length are now about 17 and 40 nm, respec-
(011)M//(0001)-Ni3Ti tively, but their distribution remains almost the same as under
–– – peak hardness aging condition. This is shown in Figure 3(a).
[ 1 11]M//[11 20]-Ni3Ti
The spherical precipitates are still present. Their average
As can be seen from Figures 2(c) and 2(d), the rodlike Ni3Ti diameter is larger, about 13 nm.
precipitates have very dense distribution in the martensite For aging until 20 hours in 753 K, no austenite was
matrix, and the two sets both evenly distribute along their observed in the TEM work. The austenite peaks in X-ray dif-
respective preferred orientations. The average diameter and fraction are not statistically significant (i.e., the calculated
length are about 10 and 35 nm, respectively. In the schematic amount of it is lower than 2 pct). Therefore, it may be con-
diffraction pattern shown in Figure 2(b), the small filled cir- cluded that there is no austenite reversion. After 50 hours of

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aging at this temperature, the amount of austenite becomes B. Mechanical Properties

significant, 9 pct. Reverted austenite is also clearly visible After aging at 753 K, the tensile properties as a function
between martensite laths, as shown in Figures 4(a) and (b). of aging time are shown in Figures 5(a) and 5(b). With only
There is no precipitation within the austenite phase. From 3 hours of aging, the tensile strength (
b) and the yield
the selected area diffraction pattern shown in Figure 4(c), the strength (
0.2) reached 1960 and 1880 MPa, respectively.
orientation relationship between austenite and martensite is After 12 hours of aging, these have achieved their maximum
of N-W type, i.e., (111)A//(110)M, [110]A//[100]M. value, at 2135 and 2078 MPa, respectively. This is consis-

Fig. 2—TEM images and SAD pattern of the Co-free maraging steel aged at 753 K for 12 h. (a) [011]M SAD pattern. (b) Indexing of (a), the large filled circles
– –
denoting martensite–spots,– –the open squares (one labelled 1) denoting Ni3Ti: (0001)//(011)M, [1120]//[111]M, the open circles (one labelled 2) denoting Ni3Ti:
(0001)//(011)M, [1120]//[1 11]M, the small –filled circles referring to double diffraction spots, and the open triangles denoting spheroidal precipitates. (c) through
( f ) Dark-field images taken from the (1120)Ni3Ti spots labeled 1, 2, 3, and 4 in (a), respectively. (g) and (h) Dark-field images of spheroidal precipitates taken
from the spots labeled 5 and 6 in (a), respectively. Note that the spot labels are not always closest to the corresponding spots due to the close proximity of sev-
eral fine spots adjacent to each other. (a) and (b) should be examined in conjunction. Not all sets of precipitate patterns are included in the indexing pattern (b).

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Fig. 2—(Continued). TEM images and SAD pattern of the Co-free maraging steel aged at 753 K for 12 h. (a) [011] – M–SAD pattern. (b) Indexing of (a), the
large filled circles denoting martensite– spots,
– –the open squares (one labelled 1) denoting Ni3Ti: (0001)//(011)M, [11 20]//[111]M, the open circles (one labelled 2)
denoting Ni3Ti: (0001)//(011)M, [11 20]//[1 11]M, the small filled circles referring to double diffraction spots, and the open triangles denoting spheroidal
–
precipitates. (c) through ( f ) Dark-field images taken from the (11 20)Ni3Ti spots labeled 1, 2, 3, and 4 in (a), respectively. (g) and (h) Dark-field images of
spheroidal precipitates taken from the spots labeled 5 and 6 in (a), respectively. Note that the spot labels are not always closest to the corresponding spots
due to the close proximity of several fine spots adjacent to each other. (a) and (b) should be examined in conjunction. Not all sets of precipitate patterns
are included in the indexing pattern (b).

reduction in cross section area after fracture. After prolonged

aging, 50 hours, there is slight reduction in strength, the ten-
sile strength being 2019 MPa and the yield strength being
1986 MPa, accompanied by marginal increases in elonga-
tion and reduction in area. Over the entire aging range, the
ratio between yield and tensile strength is within 0.96 to 0.98.
Although there is a significant increase in strength (Figure 5(a))
(100 to 200 MPa) after overaging compared to underaging,
the ductility parameters, viz. elongation and reduction in area,
remained at reasonable levels (Figure 5(b)), i.e., in the ranges
of 9 to 11 pct and 50 to 55 pct, respectively.
Figure 5(c) shows the variation of the fracture toughness
KIC after different aging times at 753 K. The KIC value is
73 MPa m after 3 hours of aging, and gradually decreases
with increasing aging time. At peak hardness, i.e., after
12 hours of aging, KIC reaches its minimum, 69.3 MPa 1m.
However, this is similar to the fracture toughness of the 18Ni
(300) Co-containing maraging steel with similar strength
level.[7] With further increase in aging time, i.e., decrease in
strength, KIC recovers gradually, reaching 75 MPa 1m after
50 hours. In general, it may be concluded that under peak
hardness condition, there is a good combination of strength
and fracture toughness.

C. Fractography
After aging at 753 K, the Co-free maraging steel demon-
strates good and stable tensile ductility, with a typical cone-
shaped fracture surface. This naked-eye scale cone-shaped
fracture surface cannot be shown in the fractomicrographs
below due to their high magnifications, but was based on
Fig. 3—TEM dark-field images and SAD patterns of the Co-free maraging visual examinations of the fractured sample. Figures 6(a),
–
steel aged at 753 K for 50 h. (a) Ni3Ti precipitates taken from the (1120) through (c) show the microscopical fractography of the tensile
spot in [011]M SAD pattern. (b) Spheroidal precipitates. specimens after underaging, peak aging, and overaging,
respectively. It can be seen that under all aging conditions,
tent with the development of hardness over time.[1] At peak a dimple structure is apparent. The fracture process was
strength, the Co-free maraging steel can still maintain its through the nucleation, growth, and coalescence of micro-
good ductility, with 9 pct elongation at fracture and 51 pct voids, resulting in intracrystalline (transgranular) fracture.

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(a)

(b)

(c)

Fig. 5—Effect of aging time on mechanical properties of the Co-free marag-

ing steel, after aging at 753 K: (a) yield (0.2 pct proof stress) and tensile
strength; (b) elongation at failure and reduction in area after failure; and
(c) fracture toughness, KIC.
Fig. 4—Reverted austenite TEM images and SAD pattern in the Co-free
maraging steel aged at 753 K for 50 h. (a) Bright-field image of the interlath
reverted austenite. (b) Dark-field image of the interlath reverted austenite. Figures 6(d) through (f) show fractography micrographs
(c) [001]M SAD and [011]A SAD, where the subscripts M and A refer to of KIC specimens after underaging, peak aging, and over-
martensite and austenite, respectively. aging, respectively. In essentially the same way as for tensile
fractured specimens (Figures 6(a) through (f)), these speci-
In underaging condition (3 hours), the dimples are relatively mens exhibit ductile dimple structure under all aging con-
small and shallow (Figure 6(a)), indicating relatively poor ditions. Though the difference between aging conditions,
plastic deformability. In peak aging (12 hours, Figure 6(b)) specifically aging time, is not significant, careful examina-
and overaging (50 hours, Figure 6(c)) conditions, the dimples tion reveals that there are almost no large dimples under
are relatively big and deep, showing good plastic deforma- peak aging. A uniform distribution of relatively shallow
bility of the materials. This is consistent with the observed dimples is exhibited in this condition, after aging for 12 hours
slight increase of tensile ductility over increasing aging time. (Figures 6(b) and (e)). In comparison, there are relatively

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Fig. 6—Fracture morphologies of Co-free maraging steel aged at 753 K. (a) through (c) Tensile specimens aged for 3 h, 12 h, and 50 h, respectively. (d) through
( f ) KIC specimens aged for 3 h, 12 h, and 50 h, respectively.

large and deep dimples in underaged (Figures 6(a) and (d))

-FeMo,[8] -Fe7Mo6,[8,13] FeTi,[8,11] Fe2Ti,[8] ,[2,16,17] dis-
and overaged (Figures 6(c) and 6(f)) specimens. Compared persion austenite,[15] etc. Though research has been less
to the specimens aged at 713 K,[1] the large and deep dim- concentrated on Co-free grades, there are already, sometimes
ples in these specimens aged at 753 K are indicative of a conflicting, reports on dispersion austenite,[18] Ni3Ti,[6,18–20]
larger amount of energy absorption through the relatively Fe7Mo6,[19] and Fe2Mo, FeMo, or FexTi multiphase large par-
large plastic deformation during crack growth. The crack ticles.[20] Many possible precipitation phases have similar struc-
tips are easily blunted, with a large fracture surface energy. tures, interplanar distances, which are further complicated
This agrees with the generally high fracture toughness after by double diffraction and overlapping of diffraction from mul-
aging at this temperature. tiple phases. In addition, variations in alloy composition, aging
temperature, and time also affect the type and morphology of
precipitation phases. Nanometer-sized ultra-fine precipitates
III. DISCUSSION are difficult to analyze for many if not all available techniques.
The present research has revealed the precipitation of highly
Though the precipitation behavior and hardening mechan- dispersed needle- or rod-shape Ni3Ti at both low (713 K)[1] and
isms in maraging steels have been widely studied, there are medium (753 K) temperatures in the Co-free maraging steel,
differing opinions on the nature of precipitation phases. For with strong resistance to coarsening. Ni3Ti has a hexagonal
example, in the 18Ni series steels, there have been reports structure, with lattice constants a  0.5101 nm and c  0.8307 nm.
on many different types of precipitation phases including Vasudevan et al. using computer simulation of diffraction
-Ni3Mo,[8–12] -Ni3Ti,[6,8–10,12–16] Laves-Fe2Mo,[2,8–10,12] patterns of precipitates combined with composition analysis

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eliminated the possibility of Ni3Mo in a T250 Co-free maraging the matrix is minimized. Although the precipitate dispersion
steel.[6] Sha et al. used atom-probe field-ion microscopy to makes the dislocation movement difficult, when the disloca-
study the composition of the rod- or needle-shape precipitates tions start to move, their uniform movement in the matrix
in a T300 steel (Fe-18 Ni-2.4 Mo-2.2 Ti-0.2 Al-0.2 Si-0.1 C, can be sustained over short distances.[22] Stress concentration
at. pct).[19,21] The results confirm that the precipitate is Ni3Ti is more easily formed around impurity inclusions such as
rather than Ni3Mo, but with the replacement of a fraction of Ti(C,N,S), leading to void formation, coalescence, and enlarg-
Ni and Ti in the precipitate by Fe and Al. This is in agreement ing.[23] In addition, the use of Mo in this Co-free maraging
with the result from the present study. Of course, the compo- steel minimizes the precipitation at prior austenite grain bound-
sition differences between different maraging steels will affect aries,[24] thereby avoiding cracking along the grain bound-
the precipitation phases. Therefore, accurate identification and aries. This results in the improvement of fracture toughness
repeated confirmation are always necessary. also. The combination of these effects is the main reason why
The TEM experiments have also revealed the presence of this Co-free maraging steel can sustain good ductility and
extremely fine spherical precipitates in the Co-free maraging toughness at strength levels above 2000 MPa. The testing
steel after low and medium temperature aging. This type of results show that aging at the intermediate temperature can
precipitate, though yet unidentified in terms of crystal struc- achieve the best combination of strength and toughness, com-
ture, also has strong resistance to coarsening. Figure 1(d) gives pared to aging at low[1] and high[25] temperatures.
a complete and clear set of diffraction patterns from this spher- After a very long time aging, a small amount of reverted
ical precipitate, exhibiting typical hexagonal arrangement with austenite forms along the martensite lath boundaries. In the
sixfold symmetry. Based on the lattice constants of possible overaging condition, slight precipitate coarsening combined
precipitation phases of hexagonal, cubic, and orthorhombic with the small amount of reverted austenite causes a limited
systems, the interplanar distances and indexing from this pat- reduction in strength accompanied by an increase in tough-
tern, and other patterns from this precipitate, no match with ness and ductility.
any type of precipitate mentioned previously could be made.
In addition, attempts at matching other compounds Fe3Mo,
Ni4Mo, Fe2Mo3, Fe0.54Mo0.73, NiTi, and Ni3Fe, as well as pos- IV. CONCLUSIONS
sible surface oxidation products Fe2O3, Fe3O4, and FeO, have
all failed. Therefore, it is likely that this spherical precipitate 1. The orientation relationship between -Ni3Ti and– marten-
is a new, unknown type of phase or a complex of multiple site matrix is (011)M//(0001)-Ni3Ti, [111]M//[11 20]-Ni3Ti.
intermetallic compounds. Further investigation is necessary, At peak hardness after aging at the intermediate tempera-
but this is outside the scope of this article. ture of 753 K, the rod-shape Ni3Ti precipitates have an
Gerold and Haberkorn believed that when the radius of average diameter of 10 nm and length of 35 nm. This type
the precipitates is smaller than 15b (b is the Burger’s vec- of precipitate has a strong resistance to coarsening at this
tor), dislocations shear through precipitates.[22] Otherwise, a aging temperature.
looping mechanism operates. The critical radius (15b) in the 2. The crystallographically unidentified spherical precipi-
current system would be about 3.8 nm. After aging for 12 hours tates exist after aging at 753 K. They reveal strong resist-
at 753 K, the Ni3Ti precipitates have an average diameter ance to coarsening.
of 10 nm and length of 35 nm, much larger than this critical 3. At the later stages of aging at 753 K, reverted austenite
size. Therefore, the hardening mechanism may be interpreted starts to form along martensite lath boundaries.
using the Ashby–Orowan relationship:[6] 4. After aging at the intermediate temperature of 753 K, the
precipitates have a suitable and effective size, uniform dis-
11/(1  v) l d
a b ln a b
Gb persion distribution, giving a yield strength of 2000 MPa
s0.2  s0 
2p(l  d) 2 2b together with an elongation and reduction in area of over 9
and 50 pct, respectively. The KIC value is over 70 MPa 1m.
where
0.2 is the yield strength of the maraging steel in aged The combination of strength and toughness is optimal, com-
condition,
0 is the yield strength in solid solution condition, pared to aging at 713 and 813 K. The combined properties
G is the shear modulus of the lath martensite matrix, b is reach the level for 18Ni (300) Co-containing maraging steels.
Burgers vector, v is Poisson’s ratio,  is particle spacing
between precipitates, and d is diameter of precipitates.
The type of orientation relationship between the precipitates ACKNOWLEDGMENTS
and the matrix in maraging steels keeps the precipitates coher-
ent with the matrix for significant aging periods, limiting the The work was partly supported by the Young Scientist
growth and coarsening of them to larger, less-effective sizes. Research Fund in Liaoning Province, China, Fund Number
This contributes to the ultra-high strength of the material. 20031006.
In maraging steels, a high dislocation density is produced
during the phase transformation in the Fe-Ni matrix. The very
low impurity inclusion levels (Table I in Reference 1) ensure REFERENCES
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