RECENT TRENDS IN THE DEVELOPMENT OF STRUCTURAL
MATERIALS
N. P. Lyakishev
Baikov Institute of Metallurgy and Materials Science (IMET), Russian
Academy of Sciences, Russia
The applicability of structural materials is determined by the
combination of their properties including the relationship between
strength (yield strength and ultimate strength) and plasticity (relative
uniform deformation and total relative elongation to failure) as well as
fracture toughness, density, and other properties. Metallic, ceramic,
polymeric, and composite materials comprise the main fraction of
structural materials. Metallic materials exhibit the best relationship
between strength and plasticity compared with those of other structural
materials (Fig. 1) [1].
Ceramics
Normalized strength (B/E)
10-1
Composites
Polymers
10-3
Metals
10-5
100
102
104
Normalized toughness (G/E a)
Fig. 1. Relationship between the normalized toughness and normalized strength
for various materials [1], where B – ultimate strength, G – fracture toughness,
E – Young’s modulus, a – average atomic radii in molecule
133
A disadvantage of ceramic materials is their low plasticity as
against metallic materials (Fig. 2).
Fracture toughness,
(KIc), MPa m1/2
200
Steels with strain-induced
martensite
Low-alloy steels
150
Maraging steels
Aluminum
alloys
100
50
Titanium alloys
Ceramics
Polymers
0
0
0
1000
Yield strength, MPa
2000
3000
Fig. 2. Relationship between strength and fracture toughness for structural materials
Composites occupy an intermediate position between ceramics
and polymers in the specific characteristics of strength and plasticity.
Due to the indicated advantages of metallic structural materials, the
fraction of steel in the total amount of structural materials exceeds 90
%. By the end of the twentieth century, the world production of steels
continuously grew and achieved about 800 million tones per year (Fig.
3) [2].
Some deceleration of the steel production growth rate at the end
of the century is caused by the satisfaction of needs due to the
improvement of the mechanical properties of conventional carbon steels
and high-strength steels (Table 1). In Russia, about 2000 steel grades
were designed by now, and about 15 million metal items including
metal of mass consumption, high-strength steels and alloys, hightemperature alloys, cold-resistant steels, corrosion-resistant steels and
alloys, wear-resistant steels, radiation-resistant steel and alloys, cast
irons, etc. are produced.
An urgent problem is the corrosion of ordinary steels because
about 50 million tonnes of steels in the world and about 20 millions
tonnes in Russia are lost annually.
134
120
0
Quality tonnes
Million tonnes
100
0
800
Physical
tonnes
600
400
200
0
1900
1920
1940
1960
1980
2000
2020
Years
Fig. 3. World steel production in 20 th century.
Table 1
Mechanical properties of conventional carbon steels
and high-strength steels
Properties
Ultimate strength, MPa
Yield strength, MPa
Fatigue strength, MPa (on the
basis of 107 cycles)
Relative elongation, %
Impact toughness, J/cm2
Carbon steels
400 - 450
200-250
120-150
High-strength steel
2500-2800
1700-2800
-
20
45-50
9-14
35-45
In recent years, a new direction aimed at the design of new
steels with nitrogen is developed in Russia (for example, in IMET) and
other countries, promising great advantages. Nitrogen and carbon
differently affect the properties of iron alloys. Compared with carbon,
nitrogen is characterized by smaller atomic and ionic radii and more
uniformly distributed in iron. The changes in the austenite lattice
parameter due to changing nitrogen content increases the strength of the
Kh18AG20F-type steels (Fig. 4).
135
1500
0.2
900
90
600
60

300
30
Relative elongation  , %
Yield strength 0.2 , MPa
1200
0
0
3.60
3.61
3.62
Lattice parameter (a), Å
Fig.4. Strength and plasticity of the Kh18AG20F-type steel as a function of
austenite lattice parameter, which is changed under the effect of nitrogen
content.
The relative elongation somewhat decreases, but remains high
enough for plastic working. The comparison of the mechanical properties of
stainless steels containing carbon and nitrogen (by the example of a steel
Kh16N4) shows that, compared with the carbon-containing steel, the
nitrogen-containing steel has a higher strength and plasticity. All parameters
of the steel with nitrogen are better than those of the steel with carbon. The
cyclic strength of the corrosion-resistant high-nitrogen steel Kh16ANB5 is
higher than that of titanium and aluminum alloys (Fig. 5). The abrasive
resistance of a high-nitrogen steel 0Х18А is higher than that of the Hadfield
steel (Fig. 6). The nitrogen-containing stainless steels surpass the carboncontaining stainless steels in the following characteristics: ultimate strength,
yield strength, fatigue strength, relative elongation and relative reduction in
area, impact toughness, work hardening, abrasive resistance, and corrosion
resistance. In addition, the nitrogen-containing steels have a finer structure,
which is free from coarse grain-boundary carbides.
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1200
Stress, MPa
1000
High-nitrogen
stainless steel
800
Titanium alloy
600
400
Aluminum alloy
200
104
105
106
107
108
Cycles to failure
Fig.5. Fatigue strength of the Kh16AN5B steel, titanium alloy, and aluminum alloy.
∆m/S*103 mg/mm2
95Kh1
1.2
Versions of the 0Kh18A steel
8
0.9 %N
0.8
0.4
110G13
1.1 %N
1.3 %N
0
30-34 HRC
60 HRC
Fig.6. Abrasive resistance of high-nitrogen steels, Hadfield steel, and the
95Kh18 steel, where ∆m/S*10 3 mg/mm2 is the weight loss related to the contact
surface area.
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The high-temperature alloys are developed generally as the
blade materials for aviation gas-turbine engines. Tungsten alloys
compared with the materials based on nickel, iron, chromium, etc. are
characterized by the maximum 100-h strength at high temperatures.
Among new structural materials for space technology,
intermetallic materials have the maximum specific strength at high
temperatures. An increase in the grain-shape factor (the ratio between
the grain length and diameter) increases the high-temperature strength
of such alloys. A new cast single-crystal super high-temperature alloy
ZhS-47 for turbine rotor blades of gas-turbine engines of the fifth and
sixth generations was designed in All-Russian Institute of Aviation
Materials (VIAM) and IMET. This alloy contains 9% rhenium and at
1000С is characterized by a record high 100-h strength (more than 350
MPa).
The intermetallics strengthened by oxides belong to the more
advantageous composites for gas-turbine engines of the sixth
generation. For example, the ultimate strength of the Ni3Al intermetallic
(designed in VIAM) strengthened by aluminum oxides Al2O3 at a
temperature of 1300С exceeds 200 MPa.
The cold-resistant steels used for gas pipeline systems operate
at a temperature of minus 20-30 degrees. The steel microstructure after
controlled rolling is textured; therefore, the properties of the steel differ
in different orientations. Since the microstructure is inhomogeneous and
non-equilibrium, some alloying elements are gradually precipitated at
grain boundaries, decreasing the strength of such steels. For example,
the impact toughness of the gas-pipeline steel developed earlier in
Bardin Central Research Institute for the Iron and Steel Industry
decreases by a factor of 4-5 within ten years of application. About 90%
of the failures of pipeline systems are caused by stress-corrosion
cracking. More uniform and equilibrium structure of the steel can be
obtained after quenching and tempering. In the nearest time, the largest
rolling mill for steel production by the technology proposed by IMET
will be built in Nizhni Tagil.
The neutron irradiation decreases plasticity, although
somewhat increases strength. The disposal of radioactive materials is
very important because a substantial quantity of such materials is
accumulated by now. The smallest induced radioactivity is
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characteristic of the isotope iron-57 and pure vanadium.
The application of deformable structural ceramics is
determined by its following characteristics: high service temperature,
hardness, strength, crack resistance, creep resistance, corrosion
resistance, stability to wear and erosion, antifriction and friction
properties. A weak point of the ceramic material is their low crack
resistance and low plasticity. Compared with ceramics, the metalceramic materials are characterized by a higher crack resistance.
Intermetallics occupy a medium position between metals and
ceramics. Their plasticity and fracture toughness are higher than those
of ceramics. The relative elongation of intermetallics substantially
increases with decreasing grain size. Their properties such as the
melting temperature, Young's modulus and the relation of Young's
modulus to density are higher than those of the corresponding metals
(Fig. 7).
E,
GPa
E/
200
Tm,
C
2000
100
1500
50
1000
0
0
Ti
TiAl
Ti3Al
Al
Melting temperature (Tm)
Young’s modulus (E)
Ratio E/, where  is the density of
material
Fig.7. Properties of titanium, aluminum, and intermetallics based on them.
139
The grain refinement provides an opportunity to deform
ceramics at rather high strain rates (about 10-2 s-1).
The strength properties of structural materials in recent decades
were increased due to the development of the alloys with new chemical
and phase compositions. However, in recent years, new trends in the
improvement of the properties of structural materials at the expense of
the formation of micro- and nanocrystalline structure are outlined.
The materials with microstructural fragments ranging from 1 to
100 nm in size are called nanocrystalline [3]. The structural
nanocrystalline materials are manufactured mainly by the methods of
powder metallurgy, crystallization from amorphous state, and severe
plastic deformation. The structural features of the nanocrystalline
materials (grain size, substantial fraction of grain boundaries and their
condition, porosity and other structure defects) are determined by the
preparation methods and strongly affect their properties.
The compacting of fine powders is considerably determined by
the particle size distribution, impurity content, surface condition, shape
of the particles, and the consolidation technique. It was shown that the
cold pressing methods such as uniaxial (static, dynamic and vibrational)
pressing and uniform (hydrostatic, gasostatic) pressing as well as the
methods of severe plastic deformation do not allow one to obtain porefree compacted materials from nanopowders. The sintering of the
nanopowders without pressure also does not allow one to obtain porefree nanostructural compacted materials since, at high temperatures, the
samples become more dense, but their grain size increases. The methods
of pressure sintering are generally used to obtain bulk nanomaterials.
With increasing pressure, the sintering temperature that provides the
absence of porosity decreases. In this case, the grain size of the sintered
compacted materials also decreases. For example, as pressure of the
sintering of iron nanopowders increases to 400 MPa, the temperature
providing the absence of porosity decreases from 700 to 350C [4]. The
grain size of sintered compacted materials thus decreases more than by
an order of magnitude, i.e., from 1.2 to 0.08 m.
The methods of a hot isostatic pressing and high-temperature
gas extrusion (HTGE) are successfully used to obtain bulk equaldensity compacted materials with homogeneous nanostructure. The
HTGE of Ni powders provides the preparation of compacted materials
140
characterized by a grain size of 100 nm and improved combination of
mechanical properties: UTS = 700 MPa and  = 15 % [5, 6].
It is also possible to obtain nanostructural materials from
amorphous alloys by low-temperature annealing. The nanostructure can
consist only of crystallites or a mixture of nanosize crystals and
amorphous phase. The obtained nanocrystalline materials have
increased strength and improved magnetic properties. Nanostructural
alloys can be also obtained by the methods of thermomechanical
treatment of pressed billets from amorphous powders [7,8].
The method of severe plastic deformation (SPD), which
includes the deformation to a high degree at a relatively low
temperature (below 0.3-0.4 Тm) under the conditions of a high applied
stress, allows one to obtain bulk pore-free nanocrystalline metals and
alloys. The nonconventional methods (torsion under hydrostatic
pressure, equal-channel angular pressing, multiple multiaxial
deformation, alternating bending, accumulated rolling with binding,
equal-channel screw pressing, etc.) allow one to deform billets without
changing final cross-section and shape and to reach very high degrees
of deformation, which is necessary for the formation of nano- and
submicrocrystalline structure.
The mechanical properties of structural nanomaterials
substantially differ from the properties of conventional coarse-grained
materials. For example, the yield strength of an austenitic steel
12Kh18N10T with a grain size of about 100 nm is increased from 250
MPa to 1340 MPa at a relative elongation of 27% [9], and the yield
strength of a plain carbon steel St3 is increased from 295 MPa to 840
MPa at a relative elongation of 10% [10]. As compared with a widely
used alloy Ti-6Al-4V, the nanostructural pure Ti obtained by the SPD
method has a higher strength (UTS = 1100 MPa) and the same plasticity
( = 10%) [11]. In this case, the cyclic strength increases in the regions
of both high-cycle and low-cycle fatigue.
The wear resistance of the nanostructural metal materials is
much higher than that of coarse-grained alloys. For example, a decrease
in the grain size of nickel from 10 m to 10 nm decreases its wear rate
from 1330 to 7,9 m3/m [12].
Figure 8 presents the mechanical properties of nanocrystalline
steels compared with coarse-grained analogs [13-14]. The plasticity of
141
nanocrystalline materials can be increased both due to the formation of
the structure promoting the activation of the dislocation mechanisms of
intercrystalline sliding and due to a decreased concentration of
technological defects (pores, microcracks and inclusions).
Ultimate strength σB, GPa
3
High-nitrogen steel
Kh15AN4
2
NC
High-strength steels
1
Low-carbon steels
0
0
10
20
30
Relative elongation , %
40
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