Aging of Copper-Titanium Dilute Alloys in Hydrogen Atmosphere: Influence of Prior-Deformation on Strength and Electrical Conductivity*

Transcription

1 Materials Transactions #2011 Journal of Japan Research Institute for Advanced Copper-Base Materials and Technologies Aging of Copper-Titanium Dilute Alloys in Hydrogen Atmosphere: Influence of Prior-Deformation on Strength and Electrical Conductivity* Satoshi Semboshi 1, Shin-ichi Orimo 1, Hisashi Suda 2, Weilin Gao 2 and Akira Sugawara 2 1 Institute for Materials Research, Tohoku University, Sendai , Japan 2 DOWA METALTECH Co., Ltd., Iwata , Japan The influence of prior-deformation on the mechanical and electrical properties of Cu-4.2 mol% Ti alloys aged in a hydrogen pressure of 0.8 MPa was examined. This follows from the results of aging solution-treated Cu-Ti alloys in a hydrogen atmosphere, which significantly improved their electrical conductivity over alloys conventionally aged in vacuum, without degradation of the mechanical strength. The maximum-strength was enhanced in the prior-deformed specimen, and the strengthening and increase in electrical conductivity were accelerated during aging in a hydrogen atmosphere, compared to that for the non-deformed specimen. As a result, the balance between the strength and the conductivity is improved within shorter aging time for specimens that are more severely deformed and then aged in a hydrogen atmosphere. The strengthening is mainly due to age-hardening by the growth of finely dispersed precipitates of Cu 4 Ti and TiH 2, which are preferentially nucleated at lattice defects such as dislocations and nano-sized deformation twins. The improved conductivity is closely related to reduction of the solute Ti concentration in the copper matrix, which is attributed to the precipitation of TiH 2 and Cu 4 Ti. Thus, prior-deformation assists to render a good combination of strength and electrical conductivity for Cu-Ti dilute alloys during aging in a hydrogen atmosphere. [doi: /matertrans.m ] (Received August 1, 2011; Accepted September 27, 2011; Published xxxx yy, zzzz) Keywords: copper-titanium alloy, hydrogen, aging, prior-deformation, strength, electrical conductivity, precipitation 1. Introduction Age-hardenable copper based alloys are widely used in electrical applications such as for connectors and leadframes. The Cu-Be alloys exhibit by far an excellent balance of mechanical strength (more than 1000 MPa) and electrical conductivity (30% IASC 1) ). 2,3) However, the substitution of Cu-Be alloys is required, due to the relatively high costs and potential health hazards of beryllium. Cu-Ti alloys containing approximately 1 to 6 mol% Ti are an attractive substitute because their mechanical strength is comparable to that of Cu-Be alloys, and they exhibit good stress-relaxation behavior and higher thermostability. 4 7) While, Cu-Ti alloys are inferior with respect to electrical conductivity, due to the much larger contribution of the solute Ti to the resistivity than that of beryllium in the Cu-Be alloys. 8) To extend the industrial applicability of Cu-Ti alloys, it is strongly desirable to provide Cu-Ti alloys that exhibit both high strength and high conductivity. It has been reported that the electrical conductivity of Cu-Ti dilute alloys is significantly improved by aging in a hydrogen atmosphere rather than conventional aging in vacuum. 9 11) Improvement of the conductivity is due to a significant reduction in the concentration of the solute Ti in the matrix, which is caused by the formation of not only Cu 4 Ti, but also the titanium hydride (TiH 2 ) phase. In subsequent research, it was demonstrated that the adjustment of aging conditions such as temperature and hydrogen pressure was effective to improve the balance of conductivity and strength for these alloys; 12,13) aging at a low temperature provided a reasonable balance of these properties, although a long aging-time was required. Aging under high hydrogen pressure allowed for a rapid improvement in the conductivity, *This Paper was Originally Published in Japanese in J. JRI Cu. 50 (2011) although the use of high pressure hydrogen gas should be avoided from a safety perspective. Thereby, in order to apply these preliminary findings for the practical fabrication of Cu-Ti alloys with high-strength and high-conductivity, it is necessary to understand the influence of the alloy composition and pre-treatment, together with the aging conditions, on the mechanical and electrical properties. Recently, we proposed that cold-rolling deformation could improve the balance of hardness and conductivity in Cu-Ti alloys during aging in a hydrogen atmosphere within a short aging time, compared to non-deformed specimens aged under the same atmospheric conditions. 14) However, the published data are confined to a mild deformation up to a reduction in thickness of 15%. In this work, we first examined systematically the strength and conductivity of Cu-Ti dilute alloys that were processed using a solutiontreatment and cold-working to reduce thickness by 0 to 60%, followed by aging in a hydrogen atmosphere, to clarify the effect of prior-deformation on the resulting properties. A promising process-condition of the extent of thickness reduction during prior-deformation and the aging temperature were then adopted and demonstrated for the fabrication of alloys with high-strength and high conductivity comparable to those of Cu-Be alloys. The microstructural evolution of the alloys prepared was also confirmed using transmission electron microscopy (TEM). 2. Experimental Procedure The nominal composition of the alloy used in this work was Cu-4.2 mol% Ti, which is the same as a commercial high-strength alloy. Sheets of the alloy were prepared by melting pure copper (99.99%) and titanium (99.99%) as raw materials and then hot-rolling to 0.22 mm thickness. The sheets were solution-treated at 1223 K for 15 min in air and immediately quenched in water, which resulted in a recrys-

2 2 S. Semboshi, S. Orimo, H. Suda, W. Gao and A. Sugawara tallized microstructure with a grain size of approximately 10 mm. Some of the sheets were unidirectionally cold-rolled down to 0.18, 0.15 and 0.08 mm thickness, which correspond to a reduction in thickness of 15, 30 and 60%, respectively. The sheets were cut into plates of 50 mm long and 6 mm wide, and tensile specimens of 20 mm long and 4 mm wide. The longitudinal direction of each tensile specimen was parallel to the rolling direction. The specimens were mechanically polished with 2000 grade SiC paper to remove the surface oxide layer and were then aged at temperatures ranging from 653 and 623 K under a hydrogen pressure of 0.8 MPa. The electrical conductivity of the aged specimens was measured at room temperature using the standard DC fourprobe technique. The Vickers hardness was examined with an applied load of 500 g and a holding time of 10 s. The hardness number was determined by averaging the results of more than ten tests, excluding the maximum and minimum values. Tensile tests were conducted using a universal tensile testing machine (Shimadzu Autograph AG-Xplus) at room temperature with a strain rate of 1: s 1. The microstructure of the specimens aged at 623 K under the hydrogen pressure of 0.8 MPa was examined using TEM (Jeol JEM-3010). Thin-foil samples for TEM observations were first ground to less than 80 mm thick and then electropolished in a solution of 10 vol% nitric acid in methanol at 243 K with a DC voltage of less than 5 V, followed by low-angle ion milling using an argon ion beam accelerated at 3 kv. Vickers hardness, Hv /- Conductivity, σ (%IACS) (a) (b) 60% rolled 30% rolled 15% rolled Quenched Aging time, t/ h Fig. 1 (a) Variations of electrical conductivity and (b) Vickers hardness for Cu-4.2 mol% Ti alloy cold-rolled to reduction in thickness of 0 to 60% and then aged at 653 K in a hydrogen atmosphere of 0.8 MPa. The conductivity is expressed in terms of % IACS, a percentage for the conductivity of annealed pure copper at 298 K. 3. Results and Discussion 3.1 Electrical conductivity and strength Figure 1 shows the variations of the electrical conductivity (a) and Vickers hardness (b) for specimens aged at 653 K under a hydrogen pressure of 0.8 MPa after deformation by 0 (as solid-solution), 15, 30 and 60% reduction of thickness. The conductivity for all of the deformed specimens before aging was approximately 3.5% IACS. This indicates no obvious influence of the deformation on the conductivity within the accuracy of this study. The conductivities of all specimens increased steadily with time during aging in a hydrogen atmosphere (Fig. 1(a)), which exceeded the increase in conductivity of approximately 12% to 14% IACS for the specimen aged in vacuum. 8,15) The conductivity for the specimen deformed to a greater extent increased more rapidly during aging in the hydrogen atmosphere. Vickers hardness values for the specimens deformed by a reduction of 0, 15, 30, and 60% were HV 127, 187, 209, and 215, respectively, which was due to working-hardening effects. For the specimen deformed by a reduction of 0%, the hardness kept increasing during aging in the hydrogen atmosphere, even after h (Fig. 1(b)). The hardness reached a maximum at 96 h for the specimens deformed by a reduction in thickness of 15%, at 48 h for 30% reduction and at 6 h for 60% after aging in the hydrogen atmosphere. Prior-deformation was therefore effective to accelerate the time to reach maximum hardness during aging in the same atmosphere. The maximum-hardness was obtained for the specimen deformed to a greater extent and then aged. Tensile strength, σ UTS /MPa % rolled 30% rolled 15% rolled Conductivity, σ (% IACS) Fig. 2 Relationship between the electrical conductivity and tensile strength of Cu-4.2 mol% Ti alloys cold-rolled to a reduction of 15, 30, and 60% and then aged at 653 K in a hydrogen atmosphere of 0.8 MPa. The number beside each plot indicates the aging time in hours. Figure 2 shows the relationship between the conductivity and tensile strength of the specimens prior-deformed at 15, 30, and 60% reduction in thickness and then aged at 653 K under a hydrogen pressure of 0.8 MPa. Both the strength and electrical conductivity increased before reaching the maximum strength, followed by a trade-off relationship between the conductivity and strength. In the specimen deformed to a 72

3 Aging of Copper-Titanium Dilute Alloys in Hydrogen Atmosphere: Influence of Prior-Deformation Tensile strength, σ UTS /MPa Aged at 623 K Aged at 653 K nm Fig. 4 BF TEM image of Cu-4.2 mol% Ti alloy solution-treated, and then aged at 653 K for 48 h in a hydrogen atmosphere of 0.8 MPa Conductivity, σ (% IACS) Fig. 3 Relationship between the electrical conductivity and tensile strength of Cu-4.2 mol% Ti alloys cold-rolled to a reduction of 60% and then aged at 623 K in a hydrogen atmosphere of 0.8 MPa. The number beside each plot indicates the aging time in hours. For comparison, the relationships of alloys aged at 653 K (shown in Fig. 2), representative Cu-Ti alloys conventionally aged in vacuum, and commercial Cu-Be alloys are also given (dotted lines). 16) greater extent, the maximum value of the tensile strength was enhanced during aging in the hydrogen atmosphere, which was consistent with the variation of the hardness (Fig. 1(b)). Furthermore, Fig. 2 shows that the plots in the over-aging stage were shifted to the upper-right area for the specimen deformed to a greater extent. Therefore, a reasonable improvement of the balance between the conductivity and strength of the specimen deformed to a greater extent can be achieved by aging in a hydrogen atmosphere. Aging of the specimen deformed more severely after solution-treatment in a hydrogen atmosphere rendered a more favorable balance of strength and conductivity in a shorter aging period. In addition, it has been reported that aging at low temperature under a high hydrogen pressure can also improve the balance of properties for solution-treated specimens. 12,13) Based on these results, we attempted to fabricate Cu-4.2 mol% Ti alloys with an excellent combination of conductivity and strength using a synergic effect of priordeformation and aging conditions. Prior-deformed specimens, reduced by 60% and aged at a low temperature of 623 K under a high hydrogen pressure of 0.8 MPa, were adopted. Here, the hydrogen pressure employed was possible that was still safe. Figure 3 shows the relationship between the electrical conductivity and tensile strength of the specimens, in addition to that for Cu-Ti alloys conventionally aged in vacuum and commercial Cu-Be alloys. 16) The specimen that were severely prior-deformed and then aged at a low temperature of 623 K exhibited an improved balance of conductivity and strength, compared to that for the conventional alloys and the specimens aged at 653 K, even though it required a long aging period; the specimen exhibited a maximum strength of 1052 MPa with a conductivity of 14% IACS by aging for 24 h. After aging for 120 h, the specimen still had a strength of more than 1000 MPa and high conductivity of 30% IACS. The combination of strength and conductivity were comparable with that for some commercial Cu-Be alloys. The reason why the specimen aged in a lower temperature improved in the balance of the strength and conductivity is primary because finer dispersion of Cu 4 Ti precipitates and a larger number of TiH 2 proceeded in parallel with each other during the aging. The detail has been discussed in the previous work. 12) 3.2 Microstructural evolution The microstructural evolution of Cu-Ti dilute alloys solution-treated and then aged at 673 K and 773 K in a hydrogen atmosphere has already been investigated; 10,12,17) in the early stage of aging, spinodal decomposition progresses in the solid solution phase, followed by the precipitation and dispersion of Cu 4 Ti (MoNi 4 structure: space group I4=m, a ¼ 0:583 nm, c ¼ 0:362 nm). 4,5) In the subsequent stage, particles of TiH 2 (fcc: Fm3m, a ¼ 0:444 nm 18) ), were also formed by the reaction of dissolved hydrogen atoms with Ti atoms in the matrix or in Cu 4 Ti precipitates. The coprecipitation of Cu 4 Ti and TiH 2 promotes the decrease in the concentration of solute Ti in the solid solution phase. On furthermore aging, TiH 2 particles continue to grow at the expense of Cu 4 Ti particles, and the microstructure eventually consists of two phase, of a much diluted solid solution and TiH 2 particles. The microstructural evolution must be essentially the same as that for the specimen aged at a temperature lower than 673 K in a hydrogen atmosphere. Figure 4 shows a bright field (BF) TEM image of the specimen solution-treated and then aged at 653 K for 48 h in the hydrogen atmosphere. This image shows the modulated structure attributed to spinodal decomposition, 4,5) but no formation of Cu 4 Ti and TiH 2 precipitates is evident. Therefore, in the case of the solution-treated specimen, aging for 48 h is not sufficient, and aging for a longer period is required for the Cu 4 Ti and TiH 2 precipitates to appear.

4 4 S. Semboshi, S. Orimo, H. Suda, W. Gao and A. Sugawara (a) (b) 111 Cu 220 Cu nm 50 nm Fig. 5 BF TEM images of Cu-4.2 mol% Ti alloys cold-rolled to a reduction of 60%. The rolled (RD) and normal (ND) directions are indicated by the arrows in (a). The higher magnification image in (b) shows some twin boundaries in the matrix phase, which were confirmed by the corresponding SAD pattern. Figure 5 shows BF TEM images and a selected area diffraction (SAD) pattern of a specimen deformed by a reduction of 60%. The image from the transverse direction (Fig. 5(a)) shows the grains extend to the rolling direction. In the higher magnified image (Fig. 5(b)), a high density of dislocations and deformation twins with an average width of 15 nm are evident. According to the SAD pattern taken from the twin boundary, the twin planes were (111) Cu. The specimens deformed by a reduction of 15 and 30% also contained deformation twins with widths of approximately 50 and 20 nm, respectively. Thus, prior-deformation to a more severe extent introduces a higher-density of lattice defects in the specimens. Figure 6 shows BF TEM images and a SAD pattern of a specimen deformed by a reduction of 60% and then aged at 653 K for 3, 6, 48, and h under a hydrogen pressure of 0.8 MPa. The deformation twins generated by cold-rolling, were not recovered by aging for 3 to h. In the BF TEM image of the specimen aged for 3 h (Fig. 6(a)), moiré contrasts of several nano-meters in size were observed especially on the twin boundaries, although the only diffracted spots detected in the SAD pattern were from the matrix phase of copper solid solution (fcc, a ¼ 0:361 nm). Fine dispersion of the precipitation of Cu 4 Ti is evident in the early stage of aging in a hydrogen atmosphere, 10,12,17) therefore, it is suggested that the moiré contrasts in the BF TEM image should correspond to the precipitation of Cu 4 Ti. During the peak-hardened period of aging for the 6 h aged specimen (Fig. 6(b)), such moiré contrasts were formed in the overall. In the specimen aged for 48 h (Fig. 6(c)), not only moiré contrasts, but also bright granular contrasts of approximately 5 nm were visible between the twin boundaries. In the corresponding SAD pattern, weak spots due to TiH 2 (marked by solid circles) and those of its double diffractions (by dotted circles), together with fundamental spots from the matrix phase, were detected. Therefore, the granular contrasts correspond to TiH 2. TiH 2 particles were grown between the twin boundaries by aging for h (Fig. 6(d)). The size of the TiH 2 particles was approximately 5 nm, which is similar to that for 48 h aging. The TiH 2 particles were not significantly coarsened during the aging process, because the twin boundaries suppressed the growth of TiH 2 particles. 3.3 Influence of prior-deformation Strain and lattice defects such as dislocations and deformation twins are accumulated by cold-rolling the solution-treated specimens. In particular, the width of the deformation twins is less than 50 nm by a reduction in thickness of more than 15%, which is much smaller than that for pure copper. 19) This suggests that the supersaturated solid solution of Ti in Cu exhibits a lower stacking fault energy than pure copper. Figures 6(a) and 6(c) shows that during aging in a hydrogen atmosphere, precipitates of Cu 4 Ti and TiH 2 are preferentially nucleated and grown in dislocations and deformation twins, because of their high fault energies; the lattice defects in the matrix phase behave as nucleation sites for the precipitation. In addition, a comparison of Figs. 4 and 6(c) shows that the precipitates of Cu 4 Ti and TiH 2 are developed more rapidly in the prior-deformed specimen than in the non-rolled specimen after the same period of aging in a hydrogen atmosphere. This is because the strain and lattice defects assist in nucleation of the precipitates, similar to that for aging in vacuum. 20,21) Thus, prior-deformation to a more severe extent causes more strain and a large number of lattice defects, which resulting in an increase of the number of nucleation sites and the nucleation rate for precipitates of Cu 4 Ti and TiH 2. Strengthening of the Cu-Ti dilute alloys by aging in a hydrogen atmosphere is primarily due to the precipitation of Cu 4 Ti particles, 10) and the improvement in conductivity is controlled by the concentration of Ti atoms dissolved in the

5 Aging of Copper-Titanium Dilute Alloys in Hydrogen Atmosphere: Influence of Prior-Deformation 5 (a) (b) 50 nm 50 nm (c) (d) TiH 2 50 nm 50 nm Fig. 6 BF TEM images of Cu-4.2 mol% Ti alloys cold-rolled to a reduction of 60%, and then aged at 653 K for (a) 3, (b) 6 (peak-aging condition), (c) 48, and (d) h in a hydrogen atmosphere of 0.8 MPa. The image in (a) shows moiré patterns indicated by circles. The image in (c) shows TiH 2 particles between twin boundaries, which were confirmed by the corresponding SAD pattern (inset). The weak spots marked by solid circles in the SAD pattern are from TiH 2, and the other weak spots marked by dotted circles are due to double diffraction. matrix, which is efficiently reduced by the formation of TiH 2. 9) In the severely deformed specimens, the number density of Cu 4 Ti precipitates is effectively increased during aging in a hydrogen atmosphere, which results in efficient dispersion-strengthening. Furthermore, the nucleation of Cu 4 Ti and TiH 2 precipitates is accelerated in the deformed specimen, so that the period for maximum-strengthening is shortened and the conductivity is increased more rapidly during aging in a hydrogen atmosphere. 4. Conclusion The effect of the processing conditions on the strength and electrical conductivity of Cu-4.2 mol% Ti alloys aged isothermally at 623 to 653 K under the hydrogen pressure of 0.8 MPa was investigated, together with the microstructural evolution of the alloys. The salient results obtained are summarized as follows. (1) The conductivity increased more rapidly in those specimens prior-deformed by a greater reduction in thickness. In addition, the maximum values of hardness and tensile strength were enhanced within a shorter aging time. Controlling the conditions of not only the extent of prior-deformation, but also the aging temperature and hydrogen pressure, imparts an excellent balance of strength and conductivity, of more than 1000 MPa and 30% IACS, respectively. (2) Dislocations and deformation twins with average sizes of several tens of nano-meters in width were generated in specimens prior-deformed to a reduction in thickness of more than 15%. These lattice defects were effective to increase the nucleation sites in the specimen and

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