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Structures and microhardness of nanostructured Cr–Ni-sputtered alloy
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Vacuum 80 (2006) 1316–1320 www.elsevier.com/locate/vacuum
Structures and microhardness of nanostructured Cr–Ni-sputtered alloy M. Nakaa,, T. Shibayanagia, M. Maedaa, H. Morib, M. Moria a
Joining and Welding Research Institute, Osaka University 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Research Center for High Voltage Electron Microscopy, Suita, Osaka 565, Japan
Abstract Nanostructured Cr–xNi alloys were prepared by magnetron sputtering in low pressure argon gas, where x varied from 0 to 20 at%. The structure and microhardness of the alloys were observed by TEM (microscopy) and Vickers hardness testing, respectively. The crystal structure of the sputtered alloys was BCC. The addition of nickel to chromium decreases the grain size of the Cr–Ni alloys and the grain sizes of the alloys changed from 88 nm for Cr to 30 nm for Cr-20 at% Ni alloys. The microhardness of the Cr–Ni alloys showed the maximum around 5 at% Ni. The increase in nickel content above 5 at% lowers the microhardness of the alloys, although the grain size of the Cr–Ni decreases. The abnormal tendency against grain size change was discussed in terms of the characteristics of the grain boundaries in the alloys. r 2006 Elsevier Ltd. All rights reserved. Keywords: Nanostructure; Microhardness; Chromium; Nickel; Film; Sputtering
1. Introduction
2. Experimental procedure
Nanostructured alloys have attracted much interest in a variety of materials engineering fields, because of their outstanding physical and chemical properties [1–3]. The mechanical properties of materials are definitely controlled by grain size, and the high strength of materials is realized by reducing the grain size to the nanometer range [3]. The sputtering method is one of the easier ones among a variety of methods to produce nanostructured alloys. Mori et al. [4] have shown that Cr–B alloys produced by sputtering have nanometer-sized grain and high hardness. Microhardness of the alloys decreases with decreasing grain size from critical values of grain size. This phenomena is called inverse Hall–Petch relation, and the abnormal tendency against the grain size is still open to the discussion with respect to the dislocation mechanism in nanostructured materials. In this work, mechanical properties of Cr–Ni alloys are investigated by measuring microhardness and correlated with grain size by observing the microstructure.
The nanostructured alloys were prepared by using a magnetron-type sputtering method. Sputtering was performed using composite targets composed of Cr and Ni metals in low-pressure argon gas. The sputtering condition and sputtering time were 600 W and 13.56 MHz, and 14.4 ks. The thickness of Cr–Ni alloys sputtered was 30 mm. The hardness measurement and microstructure investigation were performed by using Vickers hardness tester and X-ray diffactometer and electron microscope, respectively. The samples for the TEM observation were made by Ar ion thinning methods.
Corresponding author. Tel./fax: +816 6879 8649.
E-mail address: [email protected] (M. Naka). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.035
3. Results and discussion Fig. 1 shows XRD patterns of Cr and Cr–xNi alloys, where x changes from 2 to 22 at%. The Cr–Ni alloys containing Ni content up to 20 at% show the chromium– nickel solid solution with BCC structure with (1 0 0), (2 0 0), (2 1 1) and (2 2 0) peaks. Ni-22 at% Cr alloy has two phases of BCC structure and g phase since the XRD pattern shows (1 1 0) peak of gCr–Ni phase. TEM observations and the electron diffraction patterns of Cr-2 at%Ni, Cr-6 at% Ni and Cr-20 at% Ni are shown
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in Fig. 2. The observed planes are parallel to the sputtered films. The grain sizes of the sputtered alloys were 83, 69 and 31 nm for Cr-2 at% Ni, Cr-6 at% Ni and Cr-20 at% Ni alloys, respectively. The solid solutions of chromium
dissolved with nickel showed the structures with grains of nanometer size. Addition of nickel to chromium effectively reduces the grain size of chromium. Fig. 3 shows the compositional range of crystalline structures of Cr–Ni alloys prepared by sputtering, which is expressed on the corresponding Cr–Ni phase diagram [4]. The argon ions in the plasma sputter the atoms of the targets and induce the formation of metastable phases on the watercooled substrate. Although, Cr can dissolve a few percent of nickel at room temperature [4] as shown in Fig. 3, Cr-sputtered alloys possess the solubility of nickel up to 20 at%. The microhardness of Cr–Ni-sputtered alloys was measured by using Vickers tester. Fig. 4 shows the Ni content dependence of microhardness of Cr–Ni alloys. Cr–5 at% Ni alloy shows the maximum value of 10.5 GPa. The excess content of Ni reduces the hardness to 8 GPa of Cr-20 at%Ni alloy. Although the addition of Ni to Cr reduces the grain size of the Cr–Ni alloys, hardness of the alloy shows the maximum around 5 at% Ni, and decreases with the higher Ni content. In general, the decrease in the grain size increases the hardness of alloys. This tendency of hardness against the grain size is called the Hall–Petch relation in strengthening materials. Nieh et al. [5] proposed the critical grain size below which the edge dislocation cannot move. The Hall–Petch relation may not appear below the critical grain size. The critical grain size [5], lc, is green as,
Fig. 1. XRD diffraction pattern of pure Cr and Cr–Ni alloys sputtered.
lc ¼ 3Gb=pð1 vÞH,
Fig. 2. TEM observations of Cr–Ni alloys sputtered alloys.
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Fig. 3. Phase formation range in the sputtered alloys expressed on the corresponding Cr–Ni phase diagram.
Fig. 4. Effect of Ni content on microhardness of nanostructured Cr–Ni alloys.
Table 1 Shear modulus and estimated critical grain size for Cr–Ni sputtered alloys
Cr-2 at% Ni Cr-5 at% Ni Cr-12 at% Ni Cr-20 at% Ni
Young’s modulus, S(Gpa)
Critical grain size, dc(nm)
124 90 131 123
5.5 4.0 1.5 6.6
where G, b, n and H are shear modulus, Burgers vector, Poisson ratio and hardness of alloys, respectively. The G value of Cr–Ni-sputtered alloys in Table 1 were obtained by measuring load–depth curves of Fig. 5
with dynamic hardness tester. From the dynamic hardness measurements, the G obtained was 100–130 GPa which is 30% of pure nanostructured Cr [6]. The calculated lc from Eq. (1) using G and H observed is from 2 to 7 nm. The observed grain size of the Cr–Ni-sputtered alloys is larger than the calculated lc below which the edge dislocation cannot move. This suggests that the Cr–Ni-sputtered alloys possess the mobile dislocations in the films. Inspite of decreasing the grain size of the sputtered alloys, the decrease in microhardness is more clearly shown in Fig. 6, where microhardness of Cr alloys is plotted against the d1/2. d is the grain size of alloys. In Fig. 5 microhardness of the present Cr–Ni-sputtered alloys and Cr–B alloys [4] are also plotted with the inverse of square root of grain size d. In both alloys microhardness of alloys decreases with decreasing grain size below critical value of grain size. These phenomena are called inverse Hall–Petch relation. Although, alloys have mobile dislocations as discussed above, the inverse Hall–Petch relation are present. Mechanism for the inverse Hall–Petch relation in Cr–Ni alloys is presented as follows. The characteristics of the grain may become different from that for the smaller grain below the critical grain size as shown in Fig. 6. The other type of deformation mode such as slip motion of grains in the Cr–Ni alloys may occur instead of preventing the dislocation motion. This change in deformation mode may be proposed by estimating the change in volume fraction of grain boundary with grain size in the Cr–Ni alloys. Fig. 7 gives the change in volume fraction of grain boundaries with solute concentration for Cr–Ni alloys, where volume fraction of grain boundaries was calculated by Muetschele model [6] with the 1 mm of thickness of grain boundaries. In Fig. 7 calculated values of Cr–B are also included [7]. Increase in solute concentration increases the volume fraction of grain boundaries for both alloys. The increase in the volume fraction of grain boundaries brings about the inverse Hall–Petch relation. The characteristics of grain boundaries can be changed by annealing Cr–Ni alloys. Annealing at 773 K for 104 s increases microhardness from 8 to 12 GPa without changing grain size. This result indicates that the characteristic of the grain boundaries of sputtered alloys may change below the critical grain size. The change in the characteristic of grain boundaries was also reported in Cr–B sputtered alloys [6]. In the Cr–B-sputtered alloys the inverse Hall–Petch relation was observed and the microhardness also decreased with decreasing grain size below a critical value of grain size by the formation of amorphous phase at grain boundaries in Cr–B alloys. The change in characteristic of grain boundaries of Cr–B alloys is caused by the formation of amorphous phase [6]. Since formation of amorphous phase in Cr–Ni alloys was not observed, the characteristic of grain boundaries may change with decreasing grain size.
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Fig. 5. Load–depth curves of Cr–Ni sputtered alloys.
Fig. 7. Volume fraction of grain boundaries for Cr alloys plotted with solute content. Fig. 6. Microhardness of Cr-sputtered alloys plotted against d1/2.
4. Conclusions The nanostructured Cr–Ni alloys were prepared by sputtering at low argon pressure. The microstructure and microhardness were investigated by X-ray diffractometry,
transmission electron microscopy and Vickers hardness testing. The Cr–Ni sputtered alloys with Ni content up to 20 at% possess the nanostructured grains. The grain size of alloys decreases with increasing Ni content. Microhardness of
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Cr–Ni alloys shows the peak at Ni content of 5 at%, and microhardness of alloys decreases with further increase in Ni content up to 20 at%. Below the critical grain size, microhardness of alloys decreases with decreasing grain size. This inverse Hall– Petch in alloys is explained by the change in characteristics of grain boundaries. The observed grain size of Cr–Nisputtered alloys is larger than the calculated lc below which the edge dislocation cannot move. Volume fraction at grain boundaries estimated in alloys increases by reducing the grain size. This increase in grain boundaries may cause inverse Hall–Petch relation in Cr–Ni sputtered alloys.
References [1] Jose-Yacaman M. Met Mater Trans 1998;29A:125. [2] Roco M, Williams R, Alivisatos P. In: Siegel R, Roco M, editors. Nanotechnology Research Directions. Switzerland: Kulwer; 2003. p. 3. [3] Koch C. In: Siegel R, Roco M, editors. Nanostructure Science and Technology. Switzerland: Kulwer; 2000. p. 93. [4] Baker H, editor. Alloy Phase Diagram in ASM Handbook, vol. 3. USA: ASM International; 1992. p. 155. [5] Nieh TG, Wadsworth J. Scripta Met 1991;25:955. [6] Muetschele T, Kircheim R. Scripta Met 1987;21:1101. [7] Mori M, Shibayanagi T, Maeda M, Naka M. Scripta Met 2001; 44:2035.
Vacuum 80 (2006) 1316–1320 www.elsevier.com/locate/vacuum
Structures and microhardness of nanostructured Cr–Ni-sputtered alloy M. Nakaa,, T. Shibayanagia, M. Maedaa, H. Morib, M. Moria a
Joining and Welding Research Institute, Osaka University 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Research Center for High Voltage Electron Microscopy, Suita, Osaka 565, Japan
Abstract Nanostructured Cr–xNi alloys were prepared by magnetron sputtering in low pressure argon gas, where x varied from 0 to 20 at%. The structure and microhardness of the alloys were observed by TEM (microscopy) and Vickers hardness testing, respectively. The crystal structure of the sputtered alloys was BCC. The addition of nickel to chromium decreases the grain size of the Cr–Ni alloys and the grain sizes of the alloys changed from 88 nm for Cr to 30 nm for Cr-20 at% Ni alloys. The microhardness of the Cr–Ni alloys showed the maximum around 5 at% Ni. The increase in nickel content above 5 at% lowers the microhardness of the alloys, although the grain size of the Cr–Ni decreases. The abnormal tendency against grain size change was discussed in terms of the characteristics of the grain boundaries in the alloys. r 2006 Elsevier Ltd. All rights reserved. Keywords: Nanostructure; Microhardness; Chromium; Nickel; Film; Sputtering
1. Introduction
2. Experimental procedure
Nanostructured alloys have attracted much interest in a variety of materials engineering fields, because of their outstanding physical and chemical properties [1–3]. The mechanical properties of materials are definitely controlled by grain size, and the high strength of materials is realized by reducing the grain size to the nanometer range [3]. The sputtering method is one of the easier ones among a variety of methods to produce nanostructured alloys. Mori et al. [4] have shown that Cr–B alloys produced by sputtering have nanometer-sized grain and high hardness. Microhardness of the alloys decreases with decreasing grain size from critical values of grain size. This phenomena is called inverse Hall–Petch relation, and the abnormal tendency against the grain size is still open to the discussion with respect to the dislocation mechanism in nanostructured materials. In this work, mechanical properties of Cr–Ni alloys are investigated by measuring microhardness and correlated with grain size by observing the microstructure.
The nanostructured alloys were prepared by using a magnetron-type sputtering method. Sputtering was performed using composite targets composed of Cr and Ni metals in low-pressure argon gas. The sputtering condition and sputtering time were 600 W and 13.56 MHz, and 14.4 ks. The thickness of Cr–Ni alloys sputtered was 30 mm. The hardness measurement and microstructure investigation were performed by using Vickers hardness tester and X-ray diffactometer and electron microscope, respectively. The samples for the TEM observation were made by Ar ion thinning methods.
Corresponding author. Tel./fax: +816 6879 8649.
E-mail address: [email protected] (M. Naka). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.035
3. Results and discussion Fig. 1 shows XRD patterns of Cr and Cr–xNi alloys, where x changes from 2 to 22 at%. The Cr–Ni alloys containing Ni content up to 20 at% show the chromium– nickel solid solution with BCC structure with (1 0 0), (2 0 0), (2 1 1) and (2 2 0) peaks. Ni-22 at% Cr alloy has two phases of BCC structure and g phase since the XRD pattern shows (1 1 0) peak of gCr–Ni phase. TEM observations and the electron diffraction patterns of Cr-2 at%Ni, Cr-6 at% Ni and Cr-20 at% Ni are shown
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in Fig. 2. The observed planes are parallel to the sputtered films. The grain sizes of the sputtered alloys were 83, 69 and 31 nm for Cr-2 at% Ni, Cr-6 at% Ni and Cr-20 at% Ni alloys, respectively. The solid solutions of chromium
dissolved with nickel showed the structures with grains of nanometer size. Addition of nickel to chromium effectively reduces the grain size of chromium. Fig. 3 shows the compositional range of crystalline structures of Cr–Ni alloys prepared by sputtering, which is expressed on the corresponding Cr–Ni phase diagram [4]. The argon ions in the plasma sputter the atoms of the targets and induce the formation of metastable phases on the watercooled substrate. Although, Cr can dissolve a few percent of nickel at room temperature [4] as shown in Fig. 3, Cr-sputtered alloys possess the solubility of nickel up to 20 at%. The microhardness of Cr–Ni-sputtered alloys was measured by using Vickers tester. Fig. 4 shows the Ni content dependence of microhardness of Cr–Ni alloys. Cr–5 at% Ni alloy shows the maximum value of 10.5 GPa. The excess content of Ni reduces the hardness to 8 GPa of Cr-20 at%Ni alloy. Although the addition of Ni to Cr reduces the grain size of the Cr–Ni alloys, hardness of the alloy shows the maximum around 5 at% Ni, and decreases with the higher Ni content. In general, the decrease in the grain size increases the hardness of alloys. This tendency of hardness against the grain size is called the Hall–Petch relation in strengthening materials. Nieh et al. [5] proposed the critical grain size below which the edge dislocation cannot move. The Hall–Petch relation may not appear below the critical grain size. The critical grain size [5], lc, is green as,
Fig. 1. XRD diffraction pattern of pure Cr and Cr–Ni alloys sputtered.
lc ¼ 3Gb=pð1 vÞH,
Fig. 2. TEM observations of Cr–Ni alloys sputtered alloys.
(1)
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Fig. 3. Phase formation range in the sputtered alloys expressed on the corresponding Cr–Ni phase diagram.
Fig. 4. Effect of Ni content on microhardness of nanostructured Cr–Ni alloys.
Table 1 Shear modulus and estimated critical grain size for Cr–Ni sputtered alloys
Cr-2 at% Ni Cr-5 at% Ni Cr-12 at% Ni Cr-20 at% Ni
Young’s modulus, S(Gpa)
Critical grain size, dc(nm)
124 90 131 123
5.5 4.0 1.5 6.6
where G, b, n and H are shear modulus, Burgers vector, Poisson ratio and hardness of alloys, respectively. The G value of Cr–Ni-sputtered alloys in Table 1 were obtained by measuring load–depth curves of Fig. 5
with dynamic hardness tester. From the dynamic hardness measurements, the G obtained was 100–130 GPa which is 30% of pure nanostructured Cr [6]. The calculated lc from Eq. (1) using G and H observed is from 2 to 7 nm. The observed grain size of the Cr–Ni-sputtered alloys is larger than the calculated lc below which the edge dislocation cannot move. This suggests that the Cr–Ni-sputtered alloys possess the mobile dislocations in the films. Inspite of decreasing the grain size of the sputtered alloys, the decrease in microhardness is more clearly shown in Fig. 6, where microhardness of Cr alloys is plotted against the d1/2. d is the grain size of alloys. In Fig. 5 microhardness of the present Cr–Ni-sputtered alloys and Cr–B alloys [4] are also plotted with the inverse of square root of grain size d. In both alloys microhardness of alloys decreases with decreasing grain size below critical value of grain size. These phenomena are called inverse Hall–Petch relation. Although, alloys have mobile dislocations as discussed above, the inverse Hall–Petch relation are present. Mechanism for the inverse Hall–Petch relation in Cr–Ni alloys is presented as follows. The characteristics of the grain may become different from that for the smaller grain below the critical grain size as shown in Fig. 6. The other type of deformation mode such as slip motion of grains in the Cr–Ni alloys may occur instead of preventing the dislocation motion. This change in deformation mode may be proposed by estimating the change in volume fraction of grain boundary with grain size in the Cr–Ni alloys. Fig. 7 gives the change in volume fraction of grain boundaries with solute concentration for Cr–Ni alloys, where volume fraction of grain boundaries was calculated by Muetschele model [6] with the 1 mm of thickness of grain boundaries. In Fig. 7 calculated values of Cr–B are also included [7]. Increase in solute concentration increases the volume fraction of grain boundaries for both alloys. The increase in the volume fraction of grain boundaries brings about the inverse Hall–Petch relation. The characteristics of grain boundaries can be changed by annealing Cr–Ni alloys. Annealing at 773 K for 104 s increases microhardness from 8 to 12 GPa without changing grain size. This result indicates that the characteristic of the grain boundaries of sputtered alloys may change below the critical grain size. The change in the characteristic of grain boundaries was also reported in Cr–B sputtered alloys [6]. In the Cr–B-sputtered alloys the inverse Hall–Petch relation was observed and the microhardness also decreased with decreasing grain size below a critical value of grain size by the formation of amorphous phase at grain boundaries in Cr–B alloys. The change in characteristic of grain boundaries of Cr–B alloys is caused by the formation of amorphous phase [6]. Since formation of amorphous phase in Cr–Ni alloys was not observed, the characteristic of grain boundaries may change with decreasing grain size.
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Fig. 5. Load–depth curves of Cr–Ni sputtered alloys.
Fig. 7. Volume fraction of grain boundaries for Cr alloys plotted with solute content. Fig. 6. Microhardness of Cr-sputtered alloys plotted against d1/2.
4. Conclusions The nanostructured Cr–Ni alloys were prepared by sputtering at low argon pressure. The microstructure and microhardness were investigated by X-ray diffractometry,
transmission electron microscopy and Vickers hardness testing. The Cr–Ni sputtered alloys with Ni content up to 20 at% possess the nanostructured grains. The grain size of alloys decreases with increasing Ni content. Microhardness of
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Cr–Ni alloys shows the peak at Ni content of 5 at%, and microhardness of alloys decreases with further increase in Ni content up to 20 at%. Below the critical grain size, microhardness of alloys decreases with decreasing grain size. This inverse Hall– Petch in alloys is explained by the change in characteristics of grain boundaries. The observed grain size of Cr–Nisputtered alloys is larger than the calculated lc below which the edge dislocation cannot move. Volume fraction at grain boundaries estimated in alloys increases by reducing the grain size. This increase in grain boundaries may cause inverse Hall–Petch relation in Cr–Ni sputtered alloys.
References [1] Jose-Yacaman M. Met Mater Trans 1998;29A:125. [2] Roco M, Williams R, Alivisatos P. In: Siegel R, Roco M, editors. Nanotechnology Research Directions. Switzerland: Kulwer; 2003. p. 3. [3] Koch C. In: Siegel R, Roco M, editors. Nanostructure Science and Technology. Switzerland: Kulwer; 2000. p. 93. [4] Baker H, editor. Alloy Phase Diagram in ASM Handbook, vol. 3. USA: ASM International; 1992. p. 155. [5] Nieh TG, Wadsworth J. Scripta Met 1991;25:955. [6] Muetschele T, Kircheim R. Scripta Met 1987;21:1101. [7] Mori M, Shibayanagi T, Maeda M, Naka M. Scripta Met 2001; 44:2035.