Influence of the impact sintering temperature on the structure and properties of samples from the different iron powders

Advanced Powder Technology xxx (2016) xxx–xxx

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Influence of the impact sintering temperature on the structure and properties of samples from the different iron powders Anatolii Laptiev a,⇑, Barbara Romelczyk b, Oleksandr Tolochyn a, Tomasz Brynk b, Zbigniew Pakiela b a b

Frantsevich Institute for Problems of Materials Science of NASU, 3 Krzhizhanovskoho St., 03680 Kyiv, Ukraine Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska 141, 02-507 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 31 May 2016 Received in revised form 4 October 2016 Accepted 8 October 2016 Available online xxxx Keywords: Fe-powders Impact sintering Density Tensile strength Plasticity Fracture surface

a b s t r a c t The studies of the consolidation, structure and mechanical properties of samples from two types of iron powder are carried out. The coarse and less pure PZH3M2 as well as fine and purer DIAFE5000 powders were used. The samples are obtained by means of impact sintering method in the temperatures range of 500–1100 °C. The impact energy was 1200 J/cm3, and the initial deformation velocity - 6.5 m/s. Samples are obtained in the form of disks with a diameter of 25–27 mm and 9–10 mm high. For carrying out different mechanical tests the bars were cut out from disks. The tensile, compression, three-point bend of notched samples tests were carried out, as well as the Brinell hardness was measured after the corresponding processing of the bars. The characteristics of strength and plasticity of samples depending on the impact sintering temperature are determined. The polished surface of different samples and the fracture surface are investigated. It is established that the high density of samples is reached at a temperature of 600 and 700 °C respectively for fine and coarse powders. The samples obtained at these impact sintering temperatures possess rather low electrical resistivity, high strength, hardness, but the lowered plasticity. Namely, the samples from the PZH3M2 and DIAFE5000 powders sintered at the temperature of 700 °C have respectively: ultimate tensile strength - 406 and 336 MPa, yield stress - 353 and 190 MPa, contraction ratio - 26 and 78%, limit stress (at the fracture) - 501 and 933 MPa, the maximum crack tip stress – 738 and 876 MPa, the fracture energy at a bend of the notched samples - 4.8 and 51.2 J/ cm3 and also Brinell hardness - 1467 and 847 MPa. The increase of the samples impact sintering temperature leads to grain growth, decrease of the samples strength and increase of their plasticity. At the same time the structure of samples from the DIAFE5000 powder is more fine-grained than at samples from the PZH3M2 powder. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Iron is one of the main elements in creation of different materials. Iron is also a basis of the strongest materials [1]. At the same time, the combination of high strength and hardness in a material is reached by introduction of ultrafine hard particles to an iron basis and obtaining so-called composite materials [2–4]. The highest characteristics of the strength and hardness in a material are realized in case of keeping of ultrafine-grained or nanocrystalline structure in it [5–7]. Such composite materials can be obtained by using powder metallurgy methods and the impact sintering method, in particular, which is based on the process of impact ⇑ Corresponding author.

consolidation of powder in vacuum at a specified temperature [8–11]. High level of pressure and a certain extent of shear deformation allow obtaining of high density and strength samples at lowered temperatures. The decrease in the consolidation temperature of powders allows to keep more fine-grained structure and to provide higher strength of samples when using fine powders. This research was interesting in studying of possibility to obtain qualitative samples from iron powders of different granularity and purity at rather low temperatures. Therefore the purpose of this work was investigation of the structure and properties of samples from coarse and fine powders of iron obtained by impact sintering method in the wide range of temperatures and the establishment of the minimum of the consolidation temperature providing strong samples.

E-mail addresses: [email protected] (A. Laptiev), barbara.romelczyk@inmat. pw.edu.pl (B. Romelczyk), [email protected] (O. Tolochyn), [email protected] (T. Brynk), [email protected] (Z. Pakiela). http://dx.doi.org/10.1016/j.apt.2016.10.007 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: A. Laptiev et al., Influence of the impact sintering temperature on the structure and properties of samples from the different iron powders, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.10.007

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2. Experimental procedure Two iron powders from different producers were chosen for carrying out the researches – first one was coarse with particle size 50–150 lm (PZH3M2, Ukraine) and the second one - fine with particles of 3–10 lm (DIAFE5000, Germany). The chemical composition and particle size distribution of powders are presented in Table 1 and the photographs of the chosen powders are shown in Fig. 1. Before carrying out of impact sintering process there were researches conducted on volume shrinkage of previously pressed briquettes. Briquettes were obtained at three levels of pressure 300, 500 and 700 MPa. Conventional sintering of the briquettes was carried out at temperatures of 900 and 1100 °C within 20 min. Results on the volume shrinkage of briquettes are presented in the Table 2. Apparently from Table 2, sintering of briquettes leads to different densification degree of coarse and fine powders at rather high temperature. Coarse powder is hardly densified as the greatest volume shrinkage makes 0.82% at the temperature of 1100 °C for briquettes that were obtained at the minimum pressure and fine powder is densified at a considerable volume which depends on the densification pressure of briquettes. The shrinkage is maximum and equals 27% at the low pressure (300 MPa) and the shrinkage is minimum at the level of 15% at the high pressure (700 MPa). The mentioned data allows us to estimate a condition (the sizes and relative density) of a briquette (green samples) before the subsequent impact sintering. More detailed researches on conventional sintering of the chosen iron samples weren’t conducted. Green samples (briquettes) were obtained with the same pressure equal to 500 MPa for the subsequent impact sintering. Such pressure provided the relative density of briquettes from the powder PZh3M2 at the level of 80–82%, and from the powder DIAFe5000 - 74–75%. The briquettes were loaded into the vacuum camera, heated to the set temperature and after isothermal holding within 20 min were densified by the effect of impact loading. The impact energy was 1200 J/cm3, and the initial deformation velocity – 6.5 m/s. The impact sintering was carried out at temperatures of 600, 700, 800, 900, 1000 and 1100 °C and for the coarse powder impact sintering was carried out also at the temperature of 500 °C. The specimens were obtained in the form of disks with a diameter of 25–27 mm and 9–10 mm high resulting from the impact sintering of briquettes at different temperatures. Fig. 2 shows the briquettes after cold pressing and also ready specimens after the subsequent impact sintering. Varisized rectangular bars were cut out by means of electro discharge machining (EDM) for carrying out mechanical tests from the disks obtained after impact sintering, Fig. 3. The bars were grinded by abrasive disc before tests. The density of polished bars was measured by the Archimedes method. The electrical resistivity was determined by comparing the voltage drop on the reference resistor and the studied samples. There were determined the strength and the plasticity of samples under tensile (diameter of a neck of 2.5 mm, length of a neck of 10–12 mm) and compression (4
4
8 mm), there were

measured Brinell hardness for an assessment of mechanical properties. Besides, the three-point bend of notched specimens was executed on the samples intended for a bend (4
4
25 mm, distance between supports of 20 mm) for an assessment of fracture toughness. The notch of 1.2–1.5 mm in depth was executed in the electro spark way using a brass wire with a diameter of 0.1 mm. The microstructure of specimens was examined on the polished surfaces by means of the ZEISS EVO 50XVP of ZEISS AG firm (Germany) scanning electron microscopes. The fracture surfaces analyses were carried out using the JSM-6490-LV microscope of JEOL firm (Japan). 3. Experimental results and discussion One of the main issues, which arise at consolidation of powders especially when using external pressure, low temperatures and vacuum, is density level. At the same time ensuring of high level of density is necessary, but not sufficient for powder samples. Also, extent of interpartial interaction or strength of interpartial boundaries are of great importance at low-temperature consolidation. The preliminary estimate of quality of interpartial interaction can be obtained by means of such physical characteristic as specific electrical resistivity of samples. First of all, not without interest are results of measurement of density and electrical resistivity of iron samples from coarse and fine powders depending on the impact sintering temperature, Fig. 4. From the data on Fig. 4a the attention is deserved, first, by higher level of density of samples from the powder DIAFE5000, and, secondly, almost constant value of density of the samples from the powder PZh3M2 obtained at 700 °C and above. Besides, the reference data of the compact iron density is slightly lower than density of samples from fine powder and slightly higher than density from coarse powder. Such situation can be connected not so much with porosity existence, as with a chemical composition of iron powder. For example, the raised content of impurity can reduce the absolute density of iron and increase its electrical resistivity. Really, resistivity of samples from coarse powder is higher than resistivity of samples from fine powder and more than resistivity of compact iron purity of 99.90%, Fig. 4b. The electrical resistivity of samples from fine powder is lower than resistivity of compact iron with the specified purity, but higher than resistivity of compact iron purity of 99,95%. It is necessary to pay attention also to the fact that the absolute value of the electrical resistivity of samples from fine powder practically doesn’t change in all temperature interval of consolidation. Resistivity of samples from coarse powder becomes low at the densification temperature of 700 °C and at higher densification temperatures shows a tendency even to some growth. It is still difficult to explain such tendency. Thus, results of measurement of density and electrical resistivity of samples testify that samples have the high density and low resistivity in the range of the impact sintering temperatures of 700–1100 °C. In connection with this circumstance interest is caused by mechanical properties of samples from the coarse and fine powders obtained at different temperatures.

Table 1 The chemical composition and particle size distribution of the iron powders PZH3M2 and DIAFE5000. Powder

O

Chemical composition, mas.% PZh3M2 0.32 DIAFe5000 0.31

C

P

S

Si

Mn

Ca

Ni

Cr

V

0.07 0.02

0.117 0.112

0.011 –

0.24 –

0.16 –

0.132 0.102

0.066 –

0.064 –

0.017 –

+0.056–0.071 8.1% d(50)-8.5 lm

+0.071–0.100 34.9%

Particle size distribution Powder 0.045 mm PZh3M2 25% DIAFe5000 d(10)-4.2 lm

+0.045–0.056 11.1%

+0.100–0.160 19.7% D(90)-27 lm

+0.160–0.250 1.2%

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(b)

(a)

Fig. 1. The initial iron powders of PZH3M2 (a) and DIAFE5000 (b) at different magnification.

Table 2 Change of the briquettes sizes depending on pressure of cold pressing and conventional sintering temperature. Initial dimension (mm)

Sintering temperature, °C (isotherm. holding 20 min)

Specimen number and powder type

Pressure of cold pressing (MPa)

Final dimension (mm)

Volume shrinkage (%)

Diameter

Height

Diameter

Height

4068 4074 4069 4075 4070 4076

PZH3M2

300 300 500 500 700 700

25.05 25.1 25.05 25.15 25.05 25.15

7.0 7.0 6.4 6.2 6.1 5.8

900 1100 900 1100 900 1100

25.05 25.05 25.05 25.05 25.05 25.05

7.0 6.95 6.35 6.2 6.05 5.8

0 0.82 0.78 0.8 0.81 0.8

4071 4077 4072 4078 4073 4079

DIAFE 5000

300 300 500 500 700 700

25.05 25.2 25.05 25.2 25.05 25.15

7.15 7.5 5.7 6.9 6.05 6.0

900 1100 900 1100 900 1100

22.4 22.6 23.1 23.3 23.55 23.9

6.5 6.9 5.5 6.5 5.7 5.65

27.31 26.01 17.94 19.47 16.73 14.96

Fig. 3. A general view of the specimens after electro discharge cutting on bars for mechanical tests.

Fig. 2. A general view of the cold-pressed briquettes from iron powder and the specimens after impact sintering.

The tensile tests give the most important information on the strength and plasticity of powder samples. Results of determination of an ultimate tensile strength, yield stress, plasticity (contrac-

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7.95

(a)

Density, g/cm³

7.85 7.8 PZH3M2 DIAFE5000 Fe 99,99%

7.75

El. Resistivity,

· cm

14

7.9

(b)

13 12

PZH3M2 DIAFE5000 Fe 99,90% Fe 99,95 % Fe 99,99%

11 10 9

7.7 400

600 800 1000 Densification Temperature, °C

400

1200

600 800 1000 Densification Temperature, °C

1200

Fig. 4. Density (a) and specific electrical resistivity (b) of the samples from PZH3M2 and DIAFE5000 powders depending on the impact sintering temperature.

500 450 400 350 300 250 200 150 100 50 0

PZH3M2 DIAFE5000

400

Contraction Ratio, %

90 80

Tensile Yield Stress, MPa

(a)

The reason of high plasticity of samples from fine powder that is more fine-grained samples can be connected with high true (not conditional) strength of samples. Actually the ultimate tensile strength of material determined at the tensile test is conditional magnitude and doesn’t characterize the real strength of material. True strength is shown at breakage of a sample and it is equal to the stress in the smallest section of a sample neck at the time of the breakage. Therefore the real strength of samples was determined, that is strength at the time of breakage or the limit strength which is shown in Fig. 5d. This figure shows that the true strength of fine-grained samples is almost twice higher than the true strength of coarse-grained samples. For confirmation of existence of more fine-grained samples structure from fine powder, the flat polished surface was investigated which is presented in Fig. 6. From photos presented in Fig. 6 it is evident that the impact sintered samples from fine powder are more fine-grained in relation to samples from coarse powder. At the same time the impact sintering temperature increase leads to size increase of grain in samples, both from coarse and fine powder, especially in consolidation cases at temperatures over 700 °C. Despite it, the structure of

600 800 1000 Densification Temperature, °C

(c)

70 60 50 40 30 20

PZH3M2

10

DIAFE5000

0 400

600 800 1000 Densification Temperature, °C

1200

500 450 400 350 300 250 200 150 100 50 0

(b)

PZH3M2 DIAFE5000

400

1200

Limit Stress, MPa

UltimateTensile Strength, MPa

tion ratio) and the limit (rupture) true stress of samples are presented in Fig. 5. Apparently in Fig. 5a, high values of tensile strength have the samples sintered at the temperature of 700 and 600 °C respectively when using coarse and fine powders. Thus, the ultimate tensile strength (UTS) of samples from coarse powder is higher than UTS of samples from fine powder at the densification temperatures of 700 °C and above. Excess makes about 75 MPa and it is almost constantly for all the subsequent temperatures above 700 °C. The yield stress (YS) of samples from coarse powder is also almost twice more than the YS of samples from fine powder, Fig. 5b. At the same time plastic characteristics and, in particular, contraction ratio is significantly higher at samples from fine powder. And the high plasticity level of samples from fine powder is observed even in case of consolidation at a low temperature – 600 °C. For comparison of the properties of powder samples with properties of usual iron we will present reference data for armko-iron from [12]: UTS – 412 MPa, YS – 157 MPa, contraction ratio - 61%. Thus it is important to note that the YS and plasticity of samples from the powder DIAFE5000 obtained at 700 °C exceed similar characteristics of usual iron.

1000 900 800 700 600 500 400 300 200 100 0

600 800 1000 Densification Temperature, °C

1200

(d)

PZH3M2 DIAFE5000

400

600 800 1000 Densification Temperature, °C

1200

Fig. 5. Ultimate tensile strength (a), yield stress (b), plasticity (contraction ratio) (c) and the limit (rupture) stress (d) of samples from coarse and fine powders depending on the impact sintering temperature.

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Fig. 6. Structure of samples from the powder PZH3M2 (the left row) and the powder DIAFE5000 (the right row) obtained at the temperatures of 700 (a, b), 800 (c, d), 900 (e, f), 1000 (g) and 1100 °C (h).

samples from the powder DIAFE5000 remains smaller, than structure of samples from the powder PZH3M2. Samples from various powders differ not only in grain size, but also chemical composition, see Table 1. In comparison with the DIAFE5000 powder the increased amount of impurity in the PZH3M2 powder leads to different behavior of samples at the tensile test. It is shown not only on characteristics of strength and plasticity, but also on charts of samples test which are shown in Fig. 7. Fundamental difference of the charts presented in Fig. 7 is that a so-called sharp yield point appears at the tensile of the powder PZH3M2 samples obtained at temperatures of 700 °C and above, while it is absent on the powder DIAFE5000 samples. Emergence of the sharp yield point testifies to the known fact of behavior of bcc-metals with impurity or Cottrell’s atmospheres which hold dislocations and demand the increased stress to start process of a sample deformation [13]. It

is important to pay attention to the fact that the stress (under which a sharp yield point appears) and physical limit of flow decrease, while increasing of the consolidation temperature of powder samples. It is coordinated with the fact that the structure coarsening promotes decrease in a yield stress of plastic material according to the Hall-Petch mechanism [14,15]. The general view of the samples obtained at different temperatures and from different powders after tensile test is shown in Fig. 8. Samples plasticity can be also qualitatively estimated by general view of the samples presented in this figure. Thus, the tensile test of samples from the coarse and fine powders obtained by impact sintering at the different temperatures shows that high strength is reached on the samples impact sintered at the temperature of 700 °C using PZH3M2 powder and 600 °C using purer DIAFE5000 powder. At the same time plastic

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Engineering Stress, MPa

450

(a)

400 350

5

2

300 250 200 150

1

100 50

3

4

6

7

0 0

10 20 Relative Deformation, %

30

Engineering Stress, MPa

6

500 450 400 350 300 250 200 150 100 50 0

(b) 4

2 0

3

7

6

10 20 30 Relative Deformation, %

5 40

Fig. 7. Tensile test charts of the samples from the powders PZH3M2 (a) and DIAFE5000 (b) obtained at different impact sintering temperatures, °C: 1–500, 2–600, 3–700, 4– 800, 5–900, 6–1000 and 7–1100.

Fig. 8. The general view of the specimens from the powder PZH3M2 (a) and DIAFE5000 (b) after tensile test. Specimens are obtained by impact sintering temperatures, °C: 1– 500, 2–600, 3–700, 4–800, 5–900, 6–1000 and 7–1100.

characteristics of samples from different powders become higher at the increased impact sintering temperatures that is connected, obviously, with formation of more coarse-grained structure. Equally with tensile test, the crack propagation resistance tests of samples are important as well. True values of crack resistance can be obtained on rather bulky samples for plastic materials due to the need of providing a uniform stressed state. If available small-sized samples, it is advisable to obtain an approximate estimate of crack resistance or to determine the fracture work of the notched sample. The fracture work or fracture energy of a sample considers at the same time two most important characteristics of material – strength and plasticity. The fracture work on physical essence corresponds to J-integral and allows estimating the crack propagation resistance of material also. Therefore, the threepoint bend tests of notched samples were carried out. The results

of these tests are presented in Fig. 9. Apparently from Fig. 9 the samples from fine and purer DIAFE5000 powder possess higher characteristics of strength and fracture work in comparison with the samples from the PZH3M2 powder. The maximum strength, to be exact maximum crack tip stress, after which the sample fracture begins, is realized on the fine-grained samples sintered at 600 °C and on the coarse-grained samples sintered at 700 °C. More high powder consolidation temperatures lead to small decrease in critical (maximum) crack tip stress. The impact sintering temperatures of 500 and 600 °C are low for the PZH3M2 powder and don’t provide strong interaction between powder particles, though the strength at the level of 300 MPa exceeds a yield stress of samples with the coarse grain structure. Nevertheless, this strength level doesn’t provide emergence of plastic deformation and, as a result, the fracture work of samples is almost equal to zero. In this case it

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1200

80 Fracture Energy, J/cm²

(a)

Crack Tip Stress, MPa

1000 800 600 400 PZh3M2

200

70 60 50

DIAFE5000

30 20 10 0

0 600 800 1000 Densification Temperature , °C

PZH3M2

40

DIAFE5000

400

(b)

400

1200

600 800 1000 Densification Temperature, °C

1200

Fig. 9. The crack tip stress (a) and fracture energy (b) of samples from the PZH3M2 and DIAFE5000 powders obtained by impact sintering at the different temperatures.

crack through sample. From the presented test charts of samples it is visible that the impact sintering temperature increase from 700 °C leads to insignificant decreasing of the crack tip stress; and to increase in plastic deformation prior to fracture of a sample; and to less rate of the loading decrease at the fracture of a sample. The existence of sharp yield point, which appears at a smaller stress with the impact sintering temperature increase, is characteristic for samples from the PZH3M2 powder. Such situation is observed at the tensile test. The fact of embrittlement of the PZH3M2 samples sintered at a temperature of 1100 °C is interesting and demands additional studying. The embrittlement was shown in rapid load decrease at the initial stage of a sample fracture (see Fig. 10a, curve 7). The highest crack tip stress is realized on the most fine-grained DIAFE5000 samples obtained by impact sintering at 600 °C. However the maximum fracture work is

is necessary to understand that the plasticity of powder material significantly depends on boundaries strength between particles. It is necessary to pay attention that the fracture work of dense iron samples can differ several times depending on the size and purity of powder. For example, the fracture energy at a bend of the notched samples from the DIAFE5000 powder sintered in the range of temperatures of 700–1000 °C is seven times more than the fracture energy of the PZH3M2 samples sintered at the same temperatures, Fig. 9b. Important information on the stresseddeformational behavior of the notched samples at their bend is given by charts of test, which are submitted in Fig. 10. The test charts of the notched samples allow estimating three basic characteristics: the maximum crack tip stress, the degree of plastic deformation prior to fracture of a sample and rate of the loading decrease at the fracture, that is, at the propagation of a

1200

(a)

700

Crack Tip Stress, MPa

Crack Tip Stress, MPa

800

600 500 400

2

300 200 1

7

100

3

4

5

(b)

1000 800 600

6

400

7 5

200

6

2

4 3

0

0 0

1

2 3 4 Deflection, mm

5

6

0

2

4 6 8 Deflection, mm

10

12

Fig. 10. The test charts of the notched samples at a three-point bend. Samples are obtained from the PZH3M2 (a) and DIAFE5000 (b) powders at the impact sintering temperatures, °C: 1–500, 2–600, 3–700, 4–800, 5–900, 6–1000 and 7–1100.

Fig. 11. The general view of the notched samples obtained from the PZH3M2 (a), DIAFE5000 (b) powders and tested at a three-point bend. Samples are obtained at the impact sintering temperatures, °C: 1–500, 2–600, 3–700, 4–800, 5–900, 6–1000 and 7–1100.

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reached on the samples obtained at 1000 °C. In this case it is determined by high plasticity of samples. The general view of the notched samples after test on the bend, Fig. 11, also testifies to their different plasticity. High plasticity and fracture energy of samples from the powder DIAFE5000 cause high crack resistance of material. In the work [16] cyclic crack resistance of samples from different powders was investigated and is shown that samples from the powder DIAFE5000 are characterized by slower growth of a crack at cyclic loading in comparison with samples from PZH3M2 powder. The carried-out mechanical tests of powder samples shows the possibility to obtain strength samples at rather low temperatures, and when using pure powder – to reach high plastic characteris-

tics. The fracture surface of samples, Fig. 12, was of interest due to the influence of the consolidation temperature of samples on their properties. Apparently in Fig. 12, the fracture surfaces of samples from the PZH3M2 and DIAFE5000 powders significantly differ. All samples from the DIAFE5000 powder have a classical dimple fracture, and the sizes of dimples increase with the increase of impact sintering temperature. The fracture surface of the samples from the PZH3M2 powder sintered at the temperature of 700 °C and above is characterized by big parts with very small dimples and big cavities like ‘‘craters”. The samples sintered at 500 and 600 °C are characterized by the interpartial fracture testifying to lack of strong bond between particles. The fracture of samples from the PZH3M2 powder, which propagate through a body of particles

Fig. 12. The fracture surface of samples from the PZH3M2 (a, c, e, g, i, k) and DIAFE5000 (b, d, f, h, j, l) powders obtained at temperatures of 600 (a, b), 700 (c, d), 800 (e, f), 900 (g, h), 1000 (i, j) and 1100 °C (k, l).

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and grains, is characterized by formation of very small dimples. It is connected, probably, with impossibility of realization of big plastic deformation of grains. The difficulty with realization of big plastic deformation of grains can be caused by influence of the raised impurity content in the PZH3M2 powder. Thus, impurity of this iron powder brakes or blocks the movement of dislocations that leads to emergence, on the one hand, of sharp yield point and, on the other hand, to decrease in the material plasticity level. The fracture surface of the samples obtained at the increased temperatures demonstrates existence of inclusions in them. These inclusions are located at the bottom of some dimples, Fig. 12. The spectral analysis of the inclusions in the PZH3M2 samples shows availability of manganese, chrome and silicon in them and only iron in DIAFE5000samples, Fig. 13. Absence of any elements except iron (spectrum 4) in the inclusion shown in Fig. 13b causes a question of the nature of this inclusion. It is quite possible that these inclusions are fine oxides of iron. Further, it is necessary to find out true nature of the specified inclusions.

The samples obtained by impact sintering from different powders were also tested for compression. The compression tests for plastic materials are practically not carried out, but such tests are of interest in connection with existence of different structure and quality of the used powders and also in connection with smaller ‘‘sensitivity” of samples at compression to porosity and quality of interpartial borders. Besides, there was an attempt to consider friction forces between end of samples and a surface of the compressing plates at the strength and plasticity when processing compression charts of the studied samples. The technique of the consideration of the maximum friction forces at compression of samples is described in work [17]. This consideration relies on the manifestation of the maximum friction forces and lie in the fact that the maximum friction stress can’t exceed a half of a yield stress of material – sr 6 0,5 rs [18]. According to this technique the stress in a sample due to action of friction forces is counted according to the formula:

r ¼ p=½1 þ ð1=6Þ  ðd=hÞ;

ð1Þ

Fig. 13. The spectrum analysis of the inclusions in the PZH3M2 (a) and DIAFE5000 (b) samples.

1400 1200

1

1000 800

2

600 3

400

(b)

1200 Stress, MPa

Stress, MPa

1400

(a)

1

1000 800 2

600 400

3

200

200

0

0

0

0.2

0.4 0.6 Deformation

0.8

0

0.2

0.4 0.6 Deformation

0.8

1

Fig. 14. The typical compression charts of samples from the PZH3M2 (a) and DIAFE5000 (b) powders obtained as a result of calculation of the charts ‘‘effort-time”: 1 – ‘‘engineering stress – relative deformation”, 2 – ‘‘true stress without friction forces – true deformation”, 3 – ‘‘true stress with consideration of friction forces – true deformation”.

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A. Laptiev et al. / Advanced Powder Technology xxx (2016) xxx–xxx

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properties are shown in Fig. 15. As follows from this drawing the yield stress and the maximum compression stress of samples from fine and purer DIAFE5000 powder are less than the similar properties of samples from coarse with the raised impurity content powder of PZH3M2. At the same time the plasticity of samples from the DIAFE5000 powder is higher than the plasticity of samples from the PZH3M2 powder. The increase of the impact sintering temperature leads to decrease of a yield stress and, to a lesser extent, to decrease in the maximum stress and also to increase of plasticity samples. The carried-out compression tests of PZH3M2 samples showed that the greatest strength is observed on the samples densified at the lowest temperature of 500 °C while these samples have the lowest strength at tensile. It testifies that small porosity and the weakened borders between particles slightly influence on the compression strength. However the plasticity significantly depends on strength of borders and considerably decreases when weakening borders with decreasing of the impact sintering temperature. Influence of powders consolidation temperature on behavior of the obtained

Compressive Strength, MPa

Compress. Yield Stress, MPa

where p – the external pressure upon a sample, d = 2(F/p)0,5– the equivalent diameter of a sample, F – the area of cross section, h – sample height. The size F and h change at the sample compression. It is necessary to pay attention that the specified formula gives the reduced value of stress in a sample as far as it considers action of the maximum friction forces. Actually, it is the lower limit of stress from action of friction forces, and the top limit of stress is equal to the external pressure p. The consideration of friction forces shows that there is stress maximum at compression of this plastic material on the counted chart ‘‘stress-deformation”. The stress maximum is absent on the chart without friction forces, so false idea of continuous hardening of material is created. The typical chart of samples compression by ‘‘Instron 8802” test machine and also the counted charts of compression ‘‘stress – deformation” with and without consideration of friction forces are shown in Fig. 14. Proceeding from results of calculation of all compression charts, the yield stress, the maximum compression stress and the plastic deformation corresponding to the maximum stress are determined. Values of these

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Fig. 15. The yield stress (a), the maximum compression stress (b) and the plastic deformation corresponding to the maximum stress (c), and also Brinell hardness (d) of samples from the PZH3M2 and DIAFE5000 powders obtained at different temperatures.

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Fig. 16. The compression charts of samples from the PZH3M2 (a) and DIAFE5000 (b) powders. Samples are obtained at the impact sintering temperatures, °C: 1–500, 2–600, 3–700, 4–800, 5–900, 6–1000, 7–1100.

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Fig. 17. Appearance of the initial samples and compression tested samples from the PZH3M2 (a) and DIAFE5000 (b) powders. Samples are obtained at the impact sintering temperatures, °C: 1–500, 2–600, 3–700, 4–800, 5–900, 6–1000 and 7–1100.

samples is well shown on the typical compression charts of the samples, Fig. 16. The provided charts demonstrate that the sintering temperature increase leads to decrease in the maximum strength of samples and to increase in extent of deformation of samples until realization of the maximum strength. The behavior of samples can be qualitatively estimated at compression if considering the appearance of samples after test that is presented in Fig. 17. Along with the given properties of powder iron it is also necessary to show the level of samples hardness depending on the impact sintering temperature. Results of measurement of the Brinell hardness at load of 1000 N and diameter of a ball of 2,5 mm are shown in Fig. 15d. High level of hardness of the samples obtained at low sintering temperatures attracts attention. In this case, as well as in case of samples compression, small porosity and less strong bond between particles have no great influence on hardness. Therefore values of hardness were the greatest for ‘‘low-temperature” samples, that is for samples with more finegrained structure for the same powder. However, comparison of hardness of the samples obtained from different powders shows, that not only the size of grains influences hardness. Samples from the fine DIAFE5000 powder have finer structure, than samples from the coarse PZh3M2 powder (see Fig. 6), but the hardness at the first samples is less, than at the second. In this case the impurity of different elements in iron powder have essential influence on hardness, we should admit that the content of these elements in the PZH3M2 powder is raised. Therefore, the increased impurity amount in iron leads to increase of a yield stress of material and its hardness. It is known that scientific and technical literature describes rather often that the hardness of material is compared with its yield stress; and convinces in many cases in justice of the ratio HB  3r0.2, that is offered by Tabor [19]. Really, and in our case correlation between the hardness of samples and their yield stress is observed (compare Fig. 15a and d). It is also necessary to pay attention that hardness decrease as well as yield stress decrease along with increase in sintering temperature of samples happen to be with different intensity – fast decrease in hardness occurs at the beginning, and then – slow decrease. The reason of such behavior can be connected with the different speed of structure evolution. Higher growth of grains is replaced by the slow growth of the grains size with increase in impact sintering temperature of samples.

4. Conclusions Thus, on the basis of results of the conducted researches on impact sintering of iron powders of different granularity and purity it is possible to draw the most general and main conclusion that the impact sintering in vacuum method is rather effective for obtaining of high-quality samples from iron powder at rather low temperatures; and more concrete conclusions are as follows.

&

&

&

&

&

Obtaining of high density samples from the coarse PZh3M2 and the fine DIAFE5000 powders at the impact sintering in vacuum can be reached at the temperature of 600 °C. The increase of samples impact sintering temperature from 500 up to 1100 °C leads to essential coarsening of structure. The high strength of iron powder samples, equal or exceeding the strength of traditional iron, is reached in case of powder consolidation at the temperature of 700 °C and above. The maximum values of plasticity and fracture energy of the notched samples are realized on the samples impact sintered at the temperature of 1000 °C due to formation of coarsegrained structure. The increased concentration of impurity in iron powder results in higher values of yield stress and hardness, however in significantly lower consumption of fracture energy of material.

Acknowledgment The work was financed by Polish National Science Centre, the Contract no UMO-2014/15/N/ST8/03388. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2016.10.007. References [1] N.P. Ljakishev, G.V. Scherbedinsky, Problem of creation high-quality steels, Vestnik Akademii Nayk SSSR 12 (1991) 50–60 (in Russian). [2] E.P. Elsukov, V.V. Ivanov, S.F. Lomaeva, G.N. Konygin, S.F. Zfjats, A.S. Kaygorodov, Hard nanocomposite on the basis of iron and a cementite, Perspect. Mater. 6 (2006) 59–63 (in Russian). [3] J.R. Rieken, I.E. Anderson, M.J. Kramer, G.R. Odette, E. Stergar, E. Haney, Reactive gas atomization processing for Fe-based ODS alloys, J. Nucl. Mater. 428 (2012) 65–75. [4] T.J. Goodwin, S.H. Yoo, P. Matteazzi, J.R. Groza, Cementite-iron nanocomposite, Nanostruct. Mater. 8 (1997) 559–566. [5] R. Tejedor, R. Rodriguez-Baracaldo, J.A. Benito, J.M. Cabrera, J.M. Prado, Plastic deformation of a nanostructured and ultra-fine grained Fe-1%Cr with a bimodal grain size distribution, Int. J. Mater. Form. 1 (2008) 487–490. [6] S.V. Matrenin, A.P. Il’in, A.I. Slosman, L.O. Tolbanova, Sintering of nanodispersed iron powder, Perspect. Mater. 4 (2008) 81–87 (in Russian). [7] V.F. Terent’ev, A.G. Kolmakov, D.V. Prosvirnin, Fatigue durability of submicro – and nanocrystalline of iron, titan and nickel alloys, Deform. Fract. Mater. 9 (2007) 2–11 (in Russian). [8] A.V. Laptiev, A.I. Tolochin, V.V. Kovyliaev, D.G. Verbilo, E.A. Kondrjakov, Impact sintering of heat-resistant stainless steel powder X17H2. Part II. Mechanical properties of specimens and estimation of diffusion coefficient by isothermal duration and by impact densification, Metallophysika i noveyshie tekhnologii 34 (2012) 521–540 (in Russian). [9] A.V. Laptiev, L.A. Krjachko, A.I. Tolochin, D.G. Verbilo, M.E. Golovkova, Comparison the structure and mechanical properties of the convetional and ultrafine Ag-30Ni composities obtained by the impact sintering method, Metallophysika i noveyshie tekhnologii 34 (2012) 1001–1018 (in Russian). [10] A.V. Laptiev, A.I. Tolochin, E.V. Homenko, Influence of impact pressing temperature on density, structure and properties powdered copper, in: Proc.

Please cite this article in press as: A. Laptiev et al., Influence of the impact sintering temperature on the structure and properties of samples from the different iron powders, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.10.007

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[11]

[12] [13] [14]

A. Laptiev et al. / Advanced Powder Technology xxx (2016) xxx–xxx IPM NASU Electric contacts and electrodes, Kiev, Ukraine, 2012, pp. 117–124 (in Russian). A.I. Tolochin, A.V. Laptiev, I.Yu. Okun, Impact densification of tungsten powder in wide range of temperatures. Part 2. Mechanical properties, Metallophysika i noveyshie tekhnologii 36 (2014) 217–228 (in Russian). G.V. Samsonov (Ed.), Physical and Chemical Properties of Elements, Handbook, Kiev, Naukova dumka, 1965 (in Russian). A.H. Cottrell, B.A. Bilby, Dislocation theory of yielding and strain ageing of iron, Proc. Phys. Soc. A 62 (1949) 49–53. E.O. Hall, The deformation and ageing of mild steel, Proc. Phys. Soc. 64 (1951) 747–753.

[15] N.J. Petch, The cleavage strength of polycrystalline, J. Iron Steel Inst. 173 (1953) 25–28. [16] T. Brynk, B. Romelczyk, A. Laptiev, O. Tolochyn, Z. Pakiela, Fatigue crack growth in Fe mini-samples consolidated by means of impact sintering, Key Eng. Mater. 577–578 (2014) 245–248. [17] A.V. Laptev, A.I. Tolochin, D.G. Verbilo, I.Yu. Okun, Structure and properties of Kh20N80 alloy powders produced by impact sintering at different temperatures, Powder Metall. Met. Ceram. 54 (2015) 416–427. [18] M.I. Storozhev, E.A. Popov, Theory of Metals Forming, Mashinostroenie, Moskow, 1977 (in Russian). [19] D. Tabor (Ed.), Hardness of Metals, Clarendon Press, London, 1951.

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