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On structural sensibility of work function
Vacuum 63 (2001) 367}370
On structural sensibility of work function V.V. Levitin *, O.L. Garin, V.K. Yatsenko , S.V. Loskutov Department of Physics, Zaporozhye Technical University, Zaporozhye 69063, Ukraine Plant **Motor-Sich++, Zaporozhye 69068, Ukraine
Abstract Work function of metals decreases under the in#uence of plastic strain and fatigue tests. The higher the degree of strain, the larger the work function drops. The phenomenon is related to motion of dislocations on the surface. Creation of charged steps on a surface is a precursor to crack nucleation. 2001 Elsevier Science Ltd. All rights reserved. PACS: 62.20.Mk; 79.90.#b; 61.72.Hh Keywords: Alloy; Work function; Strain; Fatigue; Dislocation; Step; Dipole
1. Introduction Stresses applied to metals have an e!ect on ionic lattice as well as on the free electron gas. It was discovered that the work function (WF) of metals changes as a result of strain [1}4]. However, the structural sensibility of WF has not been su$ciently studied. The aim of this work was the investigation of the work function dependence upon plastic strain and upon fatigue tests of metals and alloys.
2. Experimental Materials under investigation were Al, heat-resistant steel and Ti-based alloy. Specimens and blades of gas-turbine engine were objects of study.
* Corresponding author.. E-mail address: [email protected] (V.V. Levitin).
Heat-resistant Fe-based steel contained (wt%) 0.15 C, 15.8 Cr, 2.1 Ni, 1.0 Mo and 0.07 N. Ti-based alloy contained 6.1 Al and 3.3 Mo. Dumbbell specimens with gauge dimensions of 20;10;0.2 mm were cut from an aluminium foil. These specimens were used for investigation which consisted of straining at a constant rate and in measuring the WF at selected points on the surface. Tensile strain rate was 5.67;10\ m/s. At various stages of deformations straining was stopped and the distribution in WF was measured along selected lines of the surface of the specimen. WF was measured in steps of 1 mm along three or six longitudinal lines, which were parallel to the specimen axis. Flat specimens of Al and heat-resistant steel of dimensions 101;11.5;3 mm were used for fatigue tests. These specimens had the shape of a single shovel. The test frequency was equal to 5881 rad/s for Al and 2338 rad/s for steel. Contact potential di!erence was measured by Kelvin capacitor method [5]. As it is known, the
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 1 5 - 9
368
V.V. Levitin et al. / Vacuum 63 (2001) 367}370
surface of specimen and an oscillating electrode form the capacitor of alternating voltage. The signal of unbalance should be compensated. Quantities of WF were calculated from these data. Experimental set-up includes a portable piezoelectrical source of mechanical vibrations and set of measuring equipment. Details of experimental techniques have been reported [6]. Standard electrode was made of gold. Diameter of electrode was equal to 1.4 mm. The accuracy of WF measurement was $2.0 meV. Real gas-turbine compressor blades of Ti-alloy were examined. The length of blades under investigation was 80 mm. Several blades were annealed at 923 K in 3 h. Strengthening treatment of blades was carried out by means of special installation with steel balls as a working tool. Balls were put in motion by chamber walls vibrating under ultrasonic "eld with the frequency of 17.2 kHz and the amplitude of 45 m [7]. WF was measured along the back and along edges of blades. Experimental points were located at intervals of 1 mm.
3. Results and discussion In Fig. 1, the typical dependences of stresses and WF upon strain are presented. The region of elastic strain is limited to "0.005. In the region of plastic deformation, WF drops sharply, decreasing of 180 meV. It is important to note the existence of a limiting value of the WF. Starting from a certain elongation ("0.055) no signi"cant decrease in WF is observed. However, after unloading WF increases. Evidently, that is caused by a relaxation process. There occurs a rapid rise of WF (by about 20 meV) without loading and then slow relaxations, which is realised in 12}15 h. The transition to the stage of plastic strain at "0.005 causes a characteristic decrease in WF along lines of measuring. The greater the degree of strain in the gauge part of the specimen, the greater the decrease in WF. WF distribution for heat-resistant steel after fatigue tests is presented in Fig. 2. Before fatigue tests, in the initial state, a scattering of WF quantities along specimens is observed (Fig. 2a). The scattering of points is equal to about 30 meV.
Fig. 1. Dependences of WF and stress upon strain for aluminium: (solid line) stress; (z) work function.
Area of 10 mm length of the specimen is located near a clamp of the vibration source. One should compare WF quantities in the loaded area (15}25 mm from the left) and WF in unloaded part (more than 25 mm). The largest level of applied cycling stresses is located at 17 mm from the left edge of the specimen. In Fig. 2b one can see that a minimum of WF appears after 50% of the specimen durability. At this place the WF drop equals to 75 meV. It is two times more than the scattering of points. During subsequent fatigue tests, the minimum erodes. The di!erence in WF between unloaded and loaded areas of specimen achieves 100 meV. Thus, WF was found to decrease at the very place of the specimen where the fatigue crack would be formed, long before its origination. Observed falling of WF re#ects some structural changes during stress cycling, especially nearby the surface of the specimen. WF that has been measured along the blade axis drops extremely owing to strengthening with steel balls, which are vibrating in ultrasonic "eld (Fig. 3). There is a great di!erence in WF between annealed and strengthened blades. The increment of is equal to 400 meV. Evidently, the surface deformation as well as the appearance of residual stresses result in the decrease of WF. The variation in WF between strengthened blades is equal to about 100 meV. If a metal specimen is subjected to stresses, the generation of dislocations is known to occur. There exists a threshold stress for this process to begin.
V.V. Levitin et al. / Vacuum 63 (2001) 367}370
369
Fig. 2. Distribution of WF along fatigue tested specimens of heat-resistant steel: (a) initial quantities (number of cycles N"0), three kinds of points conform to various series of measures; (b) curves from 1 to 3: N was changed form 52 to 69;10 cycles; (c) curves from 1 to 3: N was changed from 72 to 84;10 cycles; (d) curves from 1 to 3: N was changed from 89 to 97;10 cycles.
Fig. 3. Distribution of WF along the blade backs: (*) annealed blade; "ve kinds of other points conform to "ve blades after strengthening.
Dislocations, which have been generated, move along intersected slip planes. Generation of dislocations nearby a surface, as well as their breaking through, results in the occurrence of steps on the surface. These steps are known to have electric dipole moments [8]. Negative charges are accumulated between surface atoms. Dipoles contribution tends to decrease the work function. Stepped surfaces of Au and Pt single crystals have been studied with the direct microscopic method [8]. Steps were made by spark erosion machining the surfaces. Authors discovered WF drop of 300 meV and the corresponding step density of 10}10 steps/cm. WF decreased linearly with increasing step densities. Our results of measurements are of the same order of magnitude.
370
V.V. Levitin et al. / Vacuum 63 (2001) 367}370
References [1] Gray PP. Phys Rev Lett 1969;22:700. [2] Lang ND, Kohn W. Phys Rev 1971;B3:1215. [3] Levitin VV, Loskutov SV, Pogosov VV. Fizika Metallov i Metallovedenie 1990;9:73 (in Russian). [4] Levitin VV, Loskutov SV, Pravda MI, Serpetsky BA. Solid State Commun 1994;92:973.
[5] Woodru! DP, Delchar TA. Modern techniques of surface science. Cambridge: Cambridge University Press, 1986. [6] Loskutov SV, Levitin VV, Man'ko VK, Serpetsky BA, Pavlyutkin YS. Zavod Lab 1999;65:43 (in Russian). [7] Kulemin AB, Kononov VV, Stebelkov IA. Strength problems. 1981;1:70 (in Russian). [8] Besocke K, Krahl-Urban B, Wagner H. Surf Sci 1977; 68:39.
On structural sensibility of work function V.V. Levitin *, O.L. Garin, V.K. Yatsenko , S.V. Loskutov Department of Physics, Zaporozhye Technical University, Zaporozhye 69063, Ukraine Plant **Motor-Sich++, Zaporozhye 69068, Ukraine
Abstract Work function of metals decreases under the in#uence of plastic strain and fatigue tests. The higher the degree of strain, the larger the work function drops. The phenomenon is related to motion of dislocations on the surface. Creation of charged steps on a surface is a precursor to crack nucleation. 2001 Elsevier Science Ltd. All rights reserved. PACS: 62.20.Mk; 79.90.#b; 61.72.Hh Keywords: Alloy; Work function; Strain; Fatigue; Dislocation; Step; Dipole
1. Introduction Stresses applied to metals have an e!ect on ionic lattice as well as on the free electron gas. It was discovered that the work function (WF) of metals changes as a result of strain [1}4]. However, the structural sensibility of WF has not been su$ciently studied. The aim of this work was the investigation of the work function dependence upon plastic strain and upon fatigue tests of metals and alloys.
2. Experimental Materials under investigation were Al, heat-resistant steel and Ti-based alloy. Specimens and blades of gas-turbine engine were objects of study.
* Corresponding author.. E-mail address: [email protected] (V.V. Levitin).
Heat-resistant Fe-based steel contained (wt%) 0.15 C, 15.8 Cr, 2.1 Ni, 1.0 Mo and 0.07 N. Ti-based alloy contained 6.1 Al and 3.3 Mo. Dumbbell specimens with gauge dimensions of 20;10;0.2 mm were cut from an aluminium foil. These specimens were used for investigation which consisted of straining at a constant rate and in measuring the WF at selected points on the surface. Tensile strain rate was 5.67;10\ m/s. At various stages of deformations straining was stopped and the distribution in WF was measured along selected lines of the surface of the specimen. WF was measured in steps of 1 mm along three or six longitudinal lines, which were parallel to the specimen axis. Flat specimens of Al and heat-resistant steel of dimensions 101;11.5;3 mm were used for fatigue tests. These specimens had the shape of a single shovel. The test frequency was equal to 5881 rad/s for Al and 2338 rad/s for steel. Contact potential di!erence was measured by Kelvin capacitor method [5]. As it is known, the
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 1 5 - 9
368
V.V. Levitin et al. / Vacuum 63 (2001) 367}370
surface of specimen and an oscillating electrode form the capacitor of alternating voltage. The signal of unbalance should be compensated. Quantities of WF were calculated from these data. Experimental set-up includes a portable piezoelectrical source of mechanical vibrations and set of measuring equipment. Details of experimental techniques have been reported [6]. Standard electrode was made of gold. Diameter of electrode was equal to 1.4 mm. The accuracy of WF measurement was $2.0 meV. Real gas-turbine compressor blades of Ti-alloy were examined. The length of blades under investigation was 80 mm. Several blades were annealed at 923 K in 3 h. Strengthening treatment of blades was carried out by means of special installation with steel balls as a working tool. Balls were put in motion by chamber walls vibrating under ultrasonic "eld with the frequency of 17.2 kHz and the amplitude of 45 m [7]. WF was measured along the back and along edges of blades. Experimental points were located at intervals of 1 mm.
3. Results and discussion In Fig. 1, the typical dependences of stresses and WF upon strain are presented. The region of elastic strain is limited to "0.005. In the region of plastic deformation, WF drops sharply, decreasing of 180 meV. It is important to note the existence of a limiting value of the WF. Starting from a certain elongation ("0.055) no signi"cant decrease in WF is observed. However, after unloading WF increases. Evidently, that is caused by a relaxation process. There occurs a rapid rise of WF (by about 20 meV) without loading and then slow relaxations, which is realised in 12}15 h. The transition to the stage of plastic strain at "0.005 causes a characteristic decrease in WF along lines of measuring. The greater the degree of strain in the gauge part of the specimen, the greater the decrease in WF. WF distribution for heat-resistant steel after fatigue tests is presented in Fig. 2. Before fatigue tests, in the initial state, a scattering of WF quantities along specimens is observed (Fig. 2a). The scattering of points is equal to about 30 meV.
Fig. 1. Dependences of WF and stress upon strain for aluminium: (solid line) stress; (z) work function.
Area of 10 mm length of the specimen is located near a clamp of the vibration source. One should compare WF quantities in the loaded area (15}25 mm from the left) and WF in unloaded part (more than 25 mm). The largest level of applied cycling stresses is located at 17 mm from the left edge of the specimen. In Fig. 2b one can see that a minimum of WF appears after 50% of the specimen durability. At this place the WF drop equals to 75 meV. It is two times more than the scattering of points. During subsequent fatigue tests, the minimum erodes. The di!erence in WF between unloaded and loaded areas of specimen achieves 100 meV. Thus, WF was found to decrease at the very place of the specimen where the fatigue crack would be formed, long before its origination. Observed falling of WF re#ects some structural changes during stress cycling, especially nearby the surface of the specimen. WF that has been measured along the blade axis drops extremely owing to strengthening with steel balls, which are vibrating in ultrasonic "eld (Fig. 3). There is a great di!erence in WF between annealed and strengthened blades. The increment of is equal to 400 meV. Evidently, the surface deformation as well as the appearance of residual stresses result in the decrease of WF. The variation in WF between strengthened blades is equal to about 100 meV. If a metal specimen is subjected to stresses, the generation of dislocations is known to occur. There exists a threshold stress for this process to begin.
V.V. Levitin et al. / Vacuum 63 (2001) 367}370
369
Fig. 2. Distribution of WF along fatigue tested specimens of heat-resistant steel: (a) initial quantities (number of cycles N"0), three kinds of points conform to various series of measures; (b) curves from 1 to 3: N was changed form 52 to 69;10 cycles; (c) curves from 1 to 3: N was changed from 72 to 84;10 cycles; (d) curves from 1 to 3: N was changed from 89 to 97;10 cycles.
Fig. 3. Distribution of WF along the blade backs: (*) annealed blade; "ve kinds of other points conform to "ve blades after strengthening.
Dislocations, which have been generated, move along intersected slip planes. Generation of dislocations nearby a surface, as well as their breaking through, results in the occurrence of steps on the surface. These steps are known to have electric dipole moments [8]. Negative charges are accumulated between surface atoms. Dipoles contribution tends to decrease the work function. Stepped surfaces of Au and Pt single crystals have been studied with the direct microscopic method [8]. Steps were made by spark erosion machining the surfaces. Authors discovered WF drop of 300 meV and the corresponding step density of 10}10 steps/cm. WF decreased linearly with increasing step densities. Our results of measurements are of the same order of magnitude.
370
V.V. Levitin et al. / Vacuum 63 (2001) 367}370
References [1] Gray PP. Phys Rev Lett 1969;22:700. [2] Lang ND, Kohn W. Phys Rev 1971;B3:1215. [3] Levitin VV, Loskutov SV, Pogosov VV. Fizika Metallov i Metallovedenie 1990;9:73 (in Russian). [4] Levitin VV, Loskutov SV, Pravda MI, Serpetsky BA. Solid State Commun 1994;92:973.
[5] Woodru! DP, Delchar TA. Modern techniques of surface science. Cambridge: Cambridge University Press, 1986. [6] Loskutov SV, Levitin VV, Man'ko VK, Serpetsky BA, Pavlyutkin YS. Zavod Lab 1999;65:43 (in Russian). [7] Kulemin AB, Kononov VV, Stebelkov IA. Strength problems. 1981;1:70 (in Russian). [8] Besocke K, Krahl-Urban B, Wagner H. Surf Sci 1977; 68:39.