8. NANOPHASE ALLOYS

N.I. Noskova
Institute of Metal Physics, Ural Division, Russian Academy of Sciences
18 Kovalevskaya Str., GSP-170
Ekaterinburg, 620219, Russia
fax: (3432) 745 244
e-mail: noskova@imp.uran.ru

We were interested in studying the strength and plasticity and the structure of nanograins and their boundaries for nanocrystalline polyphase alloys. We have investigated nanocrystalline Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 , Fe 73 Ni 0.5 Nb 3 Cu 1 Si 13.5 B 9 , Fe 5 Co 70 Si 15 B 10 , Pd 77.5 Cu 6 Si 16.5 , and Pd 81 Cu 7 Si 12 ribbons produced by superfast quenching from the melt followed by fast heating to 723-923 K in a vacuum. The annealing times were from 10 seconds to 1 hr. The alloy Pd 81 Cu 7 Si 12 was produced by a rapid quenching of the melt and was crystallized during creep tests in the temperature range between 623 and 823 K and at stresses between 39 and 0.7 MPa. Under creep at 723 K, at a stress of 2.1 MPa, the resulting alloy had a nanocrystalline structure with a grain size of <10 nm. Under these conditions, the alloy exhibited an elevated plasticity.

The phase composition and the microstructure of the alloys were studied using the transmission electron microscope. The microstructure of the nanocrystalline Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 alloy was studied in situ at different stages of crystallization of the amorphous ribbons in the column of an electron microscope. The specific features of the structure of nanometer-sized grains and of the phase composition of the alloy have been established depending on the schedule of the crystallization annealings. High-resolution transmission electron microscopy (HREM) was used to study the structure of nanophase crystals and their interfaces in nanophase alloys. It was shown that the interfaces between chemically similar nanophases may have different structures: they may represent a crystalline junction (with a transition region of no more than 0.2 nm in width) between nanocrystals whose lattices are misoriented by 2-70 ° ; they may be twin boundaries; they may have a more complex structure with dislocations; or they may even have an amorphous structure. Defect stability is different in nanocrystal and nanophase interfaces and is determined by the way the nanocrystalline structure is produced.

The tensile strength of the alloys was determined by stretching ribbon specimens to failure at a rate of 1.6 x 10-5 - 7 x 10-5 s-1 at 293-723 K. Rapid crystallization of metallic glass at an elevated temperature under rapid heating and cooling was shown to result in a significant gain in the strength of the alloy Fe5Co70Si15B10 at 300 K, of the alloy Pd81Cu7Si12 at 573 K, and of the alloy Fe73.5Cu1Nb3Si13.5B9 at 673 K.

Production of the Nanophase Alloys

Figure 8.1 shows the various ways the alloys are transformed from the amorphous state to the nanophase state. Included are Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Pd 81 Cu 7 Si 12 alloys (Noskova et al. 1992, Glazer et al. 1992, Noskova et al. 1993b, Noskova et al. 1994, Kuznetsov et al. 1996, Noskova et al. 1996a). This process applies to Fe 5 Co 70 Si 15 B 10 (Noskova et al. 1995, Glazer et al. 1993). Figure 8.2 shows electron micrographs taken after crystallization from the amorphous state upon in situ heating of the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy without deformation and after deformation (Noskova et al. 1996b and Noskova et al. 1997b). Data on the phase composition of the alloys tested by TEM and HREM in various regimes and properties are presented in Table 8.1 (Noskova et al. 1992, Noskova et al. 1993b, Noskova et al. 1995, Noskova et al. 1997c, and Noskova 1997).


Fig. 8.1. The various ways of transformation from amorphous state to nanophase state of Fe- and Pd-based alloys: a-f are Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 [1-3] and g, i, k are Pd 81 Cu 7 Si 12 [4-6].


Fig. 8.2. Electron micrographs taken after crystallization from amorphous state during in situ heating of the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy without deformation and after deformation (a) and a schematic of the crystallization alloy without deformation and after deformation (b).

Table 8.1
The Structure and Properties of Nanocrystalline Polyphase Alloys

Conditions produced

Conditions of tests

Structure and properties

T (K), t (h)

T(K)

s

(MPa)

e , c-1

s s (MPa)

s B (MPa)

d (%)

D (nm)

Phase

Fe75.5Cu1Nb3Si13.5B9 - uniaxial tension

rapid quench

300

-

7 10-5

2000

2105

1.8

-

amorphous

623-1.0

"

-

"

258

258

0.2

4

a -(Fe-13%Si)

723-1.0

"

-

"

280

287

0.3

10

12-8

-

a -(Fe-13%Si)

(FeNb)2B,

FeNbB

813-0.5

"

-

"

530

548

0.8

10

10

-

a -(Fe-13%Si)

(FeNb)2B,

FeNbB

923-0.003

"

-

"

80

84

0.7

6

5

-

a -(Fe-18%Si)

(FeNb)2B,

Fe3Si, FeNbB

Fe5Co70Si15B10- uniaxial tension

rapid quench

300

-

7 10-5

 

1880

0.0

-

amorphous

873-1.0

"

-

"

945

950

0.3

50-200

90-200

-

a -Co, b -Co

Co2Si,

Co2B, CoB

923-0.003

"

-

"

1880

2100

1.0

15-50

15-50

50

a -Fe, a -Co,

b -Co, Fe3Si

Co2Si (FeCo)2B

Pd77.5Cu6Si16.5- uniaxial tension

rapid quench

300

-

7 10-5

 

820

0.0

-

amorphous

572-1.0

300

-

"

 

710

0.0

4

Pd

rapid quench

573

-

"

310

550

4.3

4

Pd, Pd5Si

573-1.0

573

-

"

350

640

1.6

4

"

rapid quench

773

-

"

140

160

1.6

6

"

573-1.0

773

-

"

60

140

1.0

6

Pd, Pd5Si,

Pd9Si2

The Pd 81 Cu 7 Si 12 alloy selected for the present study has a crystallization temperature of 733 K. This was determined using the variation of the electric resistivity during heating at a rate of 20 K/min. The crystallization temperature for the amorphous alloy Pd 77.5 Cu 6 Si 16.5 was found with the calorimetric technique, which revealed the existence of two crystallization temperatures, 685 K and 720 K. To begin, we recorded the temperature dependence of the yield strength for the amorphous alloy Pd 81 Cu 7 Si 12 under conditions of uniaxial active tension at a rate of 1.6 x 10-3 s-1 . This was found to be 1870 MPa, 1250 MPa, and 870 MPa at the test temperatures of 293 K, 373 K, and 573 K, respectively. The percentage elongation in the tests varied from 0.1 to 4.3%. The yield strength at room temperature of the amorphous alloy annealed at 523 K for 30 minutes decreased to 45 MPa and the percentage elongation decreased to zero. The above data were used to choose, for the creep tests, a temperature range of 573-823 K and a variation in stress between 39 and 0.7 MPa. The tests were conducted in air. The stresses in the creep tests can be seen to be much lower than the yield strength of the amorphous alloy in the tensile tests at T = 573 K and that of the crystallized alloy in the tensile tests at T = 293 K. The structural changes in the alloy under the creep conditions were studied by transmission electron microscopy.

Figure 8.3 displays micrographs showing the structure of the alloy after annealing (as described in Noskova et al. 1993) for 10 seconds at 823 K (Fig. 8.3a), after the creep tests at a temperature below the crystallization temperature for the alloy (Fig. 8.3b), and after the creep test at a temperature above the crystallization temperature of the alloy (Figures 8.3c and 8.3d). Studies by Noskova and others (Noskova et al. 1994, Kuznetsov et al. 1996, and Noskova et al. 1996a) show the histograms of grain (phase particle) size vs. the number of grains (number of phase particles) N. These histograms present statistical results obtained from the micrographs by counting the number of fine, medium, and coarse crystal grains in the specimens subjected to various creep conditions and after the crystallization annealing.


Fig. 8.3. Electron micrographs and electron microdiffraction patterns of the Pd 81 Cu 7 Si 12 alloy after the following treatments: (a) annealing at 823 K for 10 s; (b) creep at 673 K; (c) creep at 773 K, test time = 20 min.; (d) creep at 773 K, test time = 1 h.

According to the data in Figure 8.3 and in Noskova et al. (1994), Kuznetsov et al. (1996), and Noskova et al. (1996a), the size of the grains produced in the Pd-Cu-Si alloy under creep conditions (above the crystallization temperature) differ significantly from the size of the grains in the alloy crystallized from the amorphous state under conditions of conventional annealing. In addition, the alloy undergoes incomplete crystallization during creep at a temperature of 728 K, and under such conditions the alloy has the structure of an amorphous matrix containing particles of Pd and the crystalline phase of the (Pd-Cu) lambda-solid solution (Fig. 8.3b). Conventional annealing at 723 K for 10 seconds causes complete crystallization of the alloy; the mean grain size in this case is about 30 nm (Noskova et al. 1994, Kuznetsov et al. 1996, and Noskova et al. 1996a). During creep at a temperature of 773 K and a stress of 2.1 MPa, a short-term loading (20 min) of the specimens produces nanocrystalline grains in the alloy (mean grain size 10 nm) that give rise to diffraction rings (Fig. 8.3c) consisting of numerous reflections in the microdiffraction pattern (with a selector diaphragm of 0.25 Ám). As in the case of the annealing producing the nanocrystalline state of the alloy, annealing of the alloy at 823 K for 10 seconds gives rise to crystal grains with a mean size of about 40 nm (Fig. 8.3a). The grain size increases when the time of the creep test at 773 K is increased to 1 hr (Fig. 8.3d).

The resulting grain size depends on the stress applied. For instance, when the stress was decreased from 2.1 to 1.5 MPa, differences in grain sizes developed: some grains were as small as 4 nm and some were as large as 40 nm. The decrease of the stress in the creep tests from 1.5 to 0.7 MPa produced an increase in the grain size as a whole, but the differences in the grain size were preserved (Table 8.2). In addition to the grain size and phase-particle size, the data in the table include the plastic, physical, and strength-related properties of the crystalline alloys produced in the experiments. It can be seen from these data that the alloy exhibits the highest plasticity when creep occurs at temperatures of 673 and 728 K and at relatively high stresses (39 and 3.9 MPa, respectively).

In these tests, the time before the specimen failed did not exceed 1 or 2 min, and the alloy had a mixed amorphous-crystalline structure. After the creep tests under such conditions, the microhardness of the alloy was 5.3 GPa and the resistivity was 82 µcap_omegacm. The alloys after crystallization under creep conditions virtually always prove to be sufficiently long-lived and have a fairly high plasticity. For these alloys, Table 8.2 gives the strains after the tests rather than at failure. The crystallized specimens never failed in the tests. It should be noted that the alloy specimen with the 10 nm grain size proved to have both high plasticity and sufficiently high strength. After being subjected to the stress of 2.1 MPa at 773 K for 1 hr, not only did the specimen not fail but even its deformation did not increase. It can be supposed that it was easy to deform this specimen until the grain size increased to about 40 nm. The microhardness of this specimen was 7.3 GPa, whereas the resistivity decreased. Apparently, the plasticity of the Pd-Cu-Si alloy depends on the structural, temperature, and loading conditions. For instance, the plasticity of this alloy is high if the alloy has an amorphous crystalline structure and if the grain size does not exceed 10 nm and, possibly, this is why the alloy exhibits high creep rates. When the test time is increased (>1 hr), the alloy deformation increases with increasing grain size but the increase does not seem to be very significant.

Effect of Ultrasonic Waves on the Structure of the Nanophase Alloys

The structure of the nanocrystalline Cu before and after focused ultrasonic wave (UW) treatment is shown in Figures 8.4a and 8.4b. The electron microscope photographs were taken from different parts of the same specimen after irradiation (Noskova et al. 1997c, Noskova et al. 1993a, Klychin et al. 1991, and Noskova et al. 1997a). By investigating the regions in which the focus of the UW beam was situated, we were able to obtain the parameters of the dislocation structure of the ultrasonically irradiated materials (see Fig. 8.4b).

The structural parameters of the unirradiated material were obtained from the electron microscopic photographs of regions that were a long way from the focus (Fig. 8.4a). Exposure to a UW beam focused on the surface of the deformation disk did not produce any particular rearrangement of the dislocation structure of the copper, although the cell boundaries did "break up" (see Fig. 8.4b). Electron diffraction from this region indicates a reduction in misorientation of the substructural formations, suggesting that relaxation processes have taken place. In the copper, it appears that the randomly distributed dislocations are partially annihilated and new boundaries formed for the structural elements under the effect of a UW beam. The microhardness of nanocrystalline Cu before and after UW treatment varied from 0.95 GPa to 0.55 GPa. Note that no marked changes in the structure of the nanocrystalline FeCuNbSiB alloy occur after the ultrasonic treatment (Noskova et al. 1997c). Within the accuracy of measurements, its microhardness measured before and after the ultrasonic treatment was found to be between 12.2 and 12.3 GPa. Ultradispersed (3-5 nm in size) phases arising from UW treatment do affect Hc of the metallic glass. Coercivity of metallic glasses after ultrasonic treatment varied from 0.4 Am-1 to 0.6 Am-1 in Fe5Co70Si15B10 and from 13.5 Am-1 to 6.8 Am-1 in Fe81Si7B12 (Noskova et al. 1993a).

Table 8.2
Properties of Pd81Cu7Si12 and Pd 77.5Cu6Si16.5 After Crystallization under Creep Conditions

CREEP CONDITIONS

STRUCTURE AND PROPERTIES

T (K)

s (MPa)

t (h)

e (mm/min)

d (nm)

Phase

d (%)

H (GPa)

r (m W cm)

Pd81Cu7Si12

623

39

1.00

0.01

-

amorphous

80

3.4

-

673

39

0.03

12.00

-

2

amorphous,

Pd, g -(Pd-Cu)

69

4.5

-

728

3.9

0.015

20.00

-

2

amorphous

Pd, g -(Pd-Cu)

53

5.3

82

773

2.1

0.30

0.40

10

Pd, g -(Pd-Cu),

Pd5Si, Pd9Si12

>15

8.8

-

773

2.5

0.30

1.00

10

Pd, ,g -(Pd-Cu),

Pd5Si, Pd9Si12

50

7.4

-

773

2.1

1.00

0.10

40

Pd, ,g -(Pd-Cu),

Pd5Si, Pd9Si12

15*

7.3

50

773

1.5

1.00

0.06

4;40

Pd, ,g -(Pd-Cu),

Pd5Si, Pd9Si12

>10

7.4

-

773

0.7

1.00

0.01

5;50

Pd, ,g -(Pd-Cu),

Pd5Si, Pd9Si12

>1*

3.5

39

823

2.1

0.15

0.30

30

Pd, ,g -(Pd-Cu),

Pd5Si, Pd9Si12

10*

6.4

-

823

-

0.003

-

25

Pd, ,g -(Pd-Cu),

Pd5Si, Pd9Si12

1.5

6.4

-

Pd77.5Cu6Si16.5

573

39

1.00

0.01

-

2

amorphous,

Pd

80

-

-

773

2.5

0.30

1.00

10

Pd, Pd5Si

50

-

-

773

1.5

1.00

0.06

6

Pd, Pd5Si

Pd9Si12

>10

-

-


Fig. 8.4. The structure of the nanocrystalline Cu before (a) and after (b) UW treatment.

Conclusion

The investigation of the structure and phase composition of the nanocrystalline multiphase Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 5 Co 70 Si 15 B 10 alloys has shown that the alloys crystallized from the amorphous state at 773-933 K for certain holding times have a nanophase structure.

The crystallized alloy PdCuSi may exhibit a sufficiently high plasticity under creep conditions if it has an amorphous-crystalline structure or if the grain size does not exceed 10 nm.

The experimental results show conclusively that a focused UW beam does, in fact, have a local effect on the structure of the deformation disk when it is in a highly stressed state.

References

Glazer, A., V. Lukshina, A. Potapov, and N. Noskova. 1992. Phys. Met. and Metallogr. 74:163-166.

Glazer, A., N. Noskova, E. Ponomareva, V. Lukshina, and A. Potapov. 1993. Phys. Met. and Metallogr. 76:222-224.

Klychin, V., V. Nikolaev, N. Noskova, and E. Ponomareva. 1991. Phys. Met. and Metallogr. 71:188-198.

Kuznetsov, V., E. Ponomareva, and N. Noskova. 1996. J. of Non-Crystalline Solids 205-207:829-832.

Noskova, N. 1997. Nanostructured Materials 9:505-508.

Noskova, N., V. Serikov, A. Glazer, N. Kleinerman, and A. Potapov. 1992. Phys. Met. and Metallogr 74:52-57.

Noskova, N., V. Klyachin, E. Ponomareva, and V. Boltachev. 1993. Phys. Met. and Metallogr. 75:64-69.

Noskova, N., E. Ponomareva, A. Glazer, V. Lukshina, and A. Potapov. 1993. Phys. Met. and Metallogr. 76:171-173.

Noskova, N., E. Ponomareva, V. Kuznetsov, A. Glazer, V. Lukshina, and A. Potapov. 1994. Phys. Met. and Metallogr. 77:509-512.

Noskova, N., E. Ponomareva, V. Lukshina, and A. Potapov. 1995. Nanostructured Materials 6:969-972.

Noskova, N., E. Ponomareva, I. Pereturina, and V. Kuznetsov. 1996a. Phys. Met. and Metallogr. 81:110-115.

Noskova, N., and E. Ponomareva. 1996b. Phys. Met. and Metallogr. 82:542-548 and 630-633.

_____. 1997a. Nanostructured Materials 9:141-144.

_____. 1997b. Nanostructured Materials 9:379-382.

Noskova, N., E. Ponomareva, and M. Myshlyaev. 1997c. Phys. Met. and Metallogr. 83:511-515.


Published: August 1997; WTEC Hyper-Librarian