Effect of Age-Hardening to Metal Structure and Tribology Characteristics of Lead-Free Bismuth Bronze Casting Containing Sulfur*1

Katsuyuki Funaki1, Kaname Fujii1, Shigeki Takago1, Toshimitu Okane2, Takeshi Kobayashi3,*2 and Takafumi Akashi4

1Department of Machinery and Metal, Industrial Research Institute of Ishikawa, Kanazawa 920–8203, Japan 2Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305–8564, Japan 3Kansai University, Osaka 564–8680, Japan 4Akashi Gohdoh Inc., Hakusan 924–0011, Japan

The age-hardening behavior and low friction characteristics of Cu-Sn-Bi alloy with 1.5 mass% nickel and 0.3 mass% sulfur (PBX alloy) have been investigated. The PBX alloy showed age-hardening when aged at the temperature range of 623 K to 743 K for over 0.6 ks after solution treatment at α+γ stable temperature. Peak hardness was obtained by aging at 673 K. After aging at the temperature range of 573 K to 743 K, the supersaturated solid solution part changed into eutectoid structure in which ne δ precipitated. The age-hardening of the PBX alloy showed a two-step hardening. The rst step began within 0.6 ks and then hardness increased slowly with aging time. Aged at 673 K, the eutectoid matrix changed into a pine needle-shaped structure. The structure is a mixed structure of a bainitic structure and quasi-martensite having mid-rib structures in several places. Aged at 743 K, the macro crystal grain size did not change in 3.6 ks, but aging over 14.4 ks caused macro crystal grain re nement. The dynamic friction coef cient of the PBX alloy in an oil bath of 333 K was smaller than that of JIS CAC603 alloy due to the eutectoid structure of the α phase and ne δ phase. [doi:10.2320/matertrans.F-M2017831]

(Received December 22, 2016; Accepted August 4, 2017; Published October 16, 2017)

Keywords: lead-free bronze, copper alloy casting, aging, phase transformation, eutectoid structure, low friction coef cient

1. Introduction We have developed a casting copper alloy which exhibits pearlitic nodules4), that is a eutectoid structure in which a Lead bronze castings that exhibit stable sliding character- larges amount of intermetallic compound (an ordered phase) istics are frequently used for sliding members of medium are layered in as cast state by adding 1.0 mass% or less of and high speed and high load applications such as metal sulfur to Cu-Sn-Ni-Bi type bronze alloy having nickel con- bearings for machine tools and diesel engines and high pres- tent of 2.0 mass% or less (hereinafter designated as the PBX sure hydraulic pump parts. In recent years, there are moves alloy)5). In this alloy, the dynamic friction coef cient is prohibiting or reducing the inclusion of speci c harmful lower and the seizure resistance is higher than these of the substances in products such as lead and cadmium according lead bronze casting JIS CAC 603 by the texture effect (here- to environmental regulations such as RoHS and ELV direc- inafter referred to as metallic texture) of the eutectoid struc- tives. Therefore, there is the growing needs in the industry ture where the hard phase and the soft phase alternate at the for lead-free copper alloy castings for sliding members, and contact surface6). In addition, the PBX alloy shows age hard- development of a lead-free bronze alloy having high seizure ening properties when it is annealed in the α+δ phase tem- resistance comparable to lead bronze and low frictional re- perature range after solution treatment in the γ phase region. sistance (dynamic friction coef cient) is demanded. This phenomenon is not observed in sulfur-free bismuth The surface texture which processes regular irregularities bronze such as JIS CAC 902 and 904, it is a phenomenon on the sliding surface has been studied as a means for im- peculiar to PBX alloy7). proving the tribological characteristics from the progress of On the other hand, Cu-Ni-Sn type nickel bronze contain- ultra- ne processing technology1–3) in recent years and has ing 5 to 20 mass% Ni shows age hardening, because of been reported to have the effect of low friction and low wear forming the modulated structure in which Sn-rich and Sn- especially in the boundary lubrication state. Also, with fric- lean zones are cyclically arranged by spinodal decomposition of metal materials without contamination on the sur- tion, and regulates in the Sn-rich zone and precipitation of face, the frictional force between dissimilar metals is lower lattice phases8). However, in Cu-Sn-Ni based alloys with than that between similar metals. These facts are important nickel content of 2.0 mass% or less, there are no reports of for considering the lubrication effect instead of the lead indi- spinodal decomposition or concentration uctuation, and vidual lubrication. For example, in the case of the eutectoid there are many unclear points about aging mechanism and structure composed of an α phase and an intermetallic com- precipitate phase of the PBX alloy. pound phase, different metals are in contact with each other Therefore, in this study, the solution treatment and aging locally. In particular, when hard and soft phases are layered treatment were applied to the PBX alloy to reprecipitate the like a pearlite structure, unevenness is formed as wear pro- ordered lattice phase, and the age hardening and the mecha- gresses, and it can be expected to generate surface texture. nism of the expression of the low friction coef cient were examined from the detailed observation of the precipitation behavior and the eutectoid structure. *1This Paper was Originally Published in Japanese in J. JFS. 87 (2015) 861–867. *2 Professor Emeritus, Kansai University 1556 K. Funaki, et al.

2. Experimental Procedure mens aged for 10.8 ks at 573–743 K after solution treatment. The hardness of the specimen with solution heat treatment The PBX alloy was cast into the shell sand mold for JIS A was 5 HRB softer than as cast specimen, and change in tensile test piece and machined into a cylindrical specimen hardness was small even when subjected to aging treatment of 20 mm diameter and of 12 mm height from the grip por- at 573 K or less. Hardness increased signi cantly at 623 K tion of test piece. Table 1 shows the chemical composition or more, peaked at 673 K, then decreased at 743 K. of the PBX alloy analyzed by an ICP AEM system and a Figure 3 shows the change in hardness with respect to ag- combustion infrared absorption analysis system (for sulfur ing time at 673 K showing the peak hardness and at 743 K only). In the materials of the composition used in this exper- higher than the peak hardness. Either temperature showed an iment, the inuence of copper and tin on the transformation increase in hardness with an aging of 0.6 ks and peaked at temperature is large. Therefore, the eutectoid transformation 7.2 ks in the case of 743 K. In the case of 673 K, the in- temperature considered in choosing the aging temperature crease in hardness due to aging was larger than 743 K and shall be in accordance with the binary phase diagram of did not reach its peak even after 21.6 ks aging. Cu-Sn system9) shown in Fig. 1, and the inuence of other elements was neglected. Each specimen was quenched to 3.2 Effects of heat treatment on metallographic room temperature after solution treatment at 803 K for structure 10.8 ks, so as to prevent disappearance of microsegregation As shown in Fig. 4, the aging at 743 K caused a notable between dendrites formed at the time of casting, and was change in macroscopic organization. Macroscopic crystal heated again and aged for a predetermined time between 573 K and 743 K. At this time, as the sample was broken by water quenching, the quenching was forced air cooling. In the evaluation of specimens, hardness, microstructure, change of X-ray diffraction pattern, etc. were investigated. A position sensitive detector (PSD) and a copper target were used for the X-ray diffraction, and analysis was carried out after removal of the Kα2 diffraction peaks. Hardness was measured using Rockwell hardness tester on B scale. In the observation of microstructures, a ferric chloride hydrochlo- ric acid alcohol solution was used for etching.

3. Experimental Results Fig. 2 Age hardening behavior of PBX alloy. 3.1 Age hardening behavior Figure 2 shows the change of the HRB hardness in speci-

Table 1 Chemical composition of specimen, in mass%.

Cu Sn Ni Zn Pb Bi S PBX Bal. 10.6 1.52 0.01 0.02 2.93 0.30

Fig. 3 Isothermal age hardening curve of PBX alloy.

Fig. 1 Binary phase diagram of the Cu-Sn system9). Fig. 4 Macrostructure change during aging process at 743 K. Effect of Age-Hardening to Metal Structure and Tribology Characteristics of Lead-Free Bismuth Bronze Casting Containing Sulfur 1557 grains did not change until the aging time of 3.6 ks, but mac- Cu2S were also observed in other elds of view. When aging roscopic crystal grains re ned at 14.4 ks. In the case of dis- at 673 K, growth of undissolved δ phase and many ne pre- location introduced by external force, recrystallization oc- cipitates appeared as in Fig. 6 (b). In addition, a part of the α curs in a relatively short time. It is speculated that phase transforms into a pine-needle-shaped structure, and recrystallization after 3.6 ks or more has been recrystallized microcrystalline grains are re ned to about 10 μm from the due to the introduction of large amount of dislocation due to orientation indicated by the structure. There were many ne large shear stress such as lattice transformation during ag- precipitates in the pine-needle-shaped structure, whereas ing. In the specimen with the time of 21.6 ks, further re ne- precipitates grew aky and decreased in number in untrans- ment advanced, macroscopic structure became blurred and unclear. In addition, that this macroscopic change was not observed in the specimen aged at 673 K. Figure 5 shows microstructures obtained by solution heat treatment of PBX alloy only and solution aging at 673 K and 743 K for 10.8 ks. The microstructure of as cast PBX alloy was similar to 673 K aged specimen. Copper sul de (Cu2S) of tens of micrometers, intermetallic compound of copper and tin (δ phase: Cu4Sn) and ne spherical particles were observed between dendrites only by solution treatment. At aging temperature 673 K, ne intermetallic compounds precipitated to surround Cu2S and Cu4Sn, and changed to α + δ eutectoid structure where the dendrite structure clearly appeared. At the aging temperature of 743 K, the eutectoid structure between the dendrites was hardly etched, and the precipitated intermetallic compound was coarser than 673 K. The change of this eutectoid structure suggests that the base structure differs depending on the aging temperature, and it seems to be related to the difference in the hardness of the aging. Also, in the case of bismuth bronze such as JIS CAC904, no change in the microstructure due to ne precipitates or age hardening was observed.7) It is considered that the age hardening in this alloy was attributable to the eutectoid structure generated between dendrites.

3.3 SEM observation of precipitated phases Figure 6 shows SEM observation of dendrite intervals. Figure 6 (a) shows only the solution heat treatment, Fig. 6 (b) shows the microstructure aged 10.3 ks at 673 K after the solution. Analysis with EDS revealed that the ne spherical particles observed between dendrites in Fig. 5 (a) are bismuth and their size is about several micrometers. In addition, undissolved intermetallic compounds and sul des was also con rmed, and its stoichiometric composition was Cu4Sn and Cu2S. In the eld of view of Fig. 6 (a), only the α phase and the Fig. 6 SEM observation of microstructure of PBX alloy solid solution at bismuth phase were observed, but undissolved δ phase and 803 K for 10.8 ks. (a) as solution (b) 673 K-10.8 ks aging.

Fig. 5 Microstructure change during aging process. 1558 K. Funaki, et al. formed α phase. Since precipitation occurs at nuclear places structure. where stacking faults and dislocation densities are high, it is As discussed in Section 3.2, microstructures were differ- considered that pine-needle-shaped structure contains a large ent at aging temperature 673 K and 743 K. Figure 8 shows amount of dislocations and lattice defects. In addition, the α (200) diffraction line obtained by aging at 10.8 ks at EBSD analysis veri ed that the area of the pine-needle- these temperatures after solution treatment. The main dif- shaped structure increased with time and that the pine-nee- fraction line of the specimen only solution treatment was dle-shaped structure consisted of not an fcc (α phase) but a normally distributed, and a small peak adjacent to the low bcc structure. This structural feature suggests that it was angle side (around 48.8 degrees) was observed. When aging generated by diffusion-controlled phase transformation. at 673 K, the intensity of the main diffraction line decreases but it moves to the high angle side while maintaining the 3.4 X-ray diffraction of aged specimen normal distribution, and adjacent small peaks are recognized Figure 7 shows the aging time and the change in the independently. In this case, it is presumed that the solid solu- X-ray diffraction patterns for the sample aged at 673 K after tion element decreases due to precipitation, the crystal lat- the solution treatment. Only the solution treatment, α (111) tice shrinks, and the atomic scattering factors such as the of the main diffraction line and the diffraction peak of δ matching distortion between the intermetallic compound (120) due to the intermetallic compound of copper and tin precipitated by aging and the stacking fault are increased. which was undissolved were observed. On the other hand, when 10.8 ks aged at 743 K, the normal In the aged specimens, small diffraction peaks were distribution of the main diffraction line collapsed and moved shown on both sides of the main diffraction line, and the dif- to the low angle side. In this case, it seems that the crystal fraction intensity increased with progress of aging. The peak lattice has unevenly expanded during the precipitation pro- around 45 degrees showed good agreement with the diffrac- cess, and it is inferred that not the concentration change of tion peak of δ (121). On the other hand, the peak M appear- the solid solution element but the tensile strain accompany- ing around 40 degrees agreed with the diffraction angle of ing the phase transformation occurred. In addition, even at neither α-Cu nor δ-Cu. Therefore, it is considered to be a 743 K, small peaks adjacent to each other were observed in- metastable phase caused by the pine-needle-shaped dependently, suggesting that metastable phases having the same crystal system were formed and grown in the aging process, but details are unknown.

4. Discussion

4.1 Phase transformation of PBX alloy Bainite transformation10) is one mode of phase transformation accompanied by rich precipitation of solute atoms by individual motion of each atom. In Cu-Sn alloys, the γ phase of DO3 ordered structure performs eutectoid transformation at 793 K. Slow cooling or isothermal annealing at 793– 723 K induces pearlitic transformation to α+δ, whereas isothermal annealing at 723 K or lower encourages bainite 11) transformation to γB + δ . It is noteworthy that γB has a hcp structure and that it does not appear in the phase diagram of Fig. 1. The PBX alloy aged at 673 K demonstrated ne precipi- Fig. 7 Change in X-ray diffraction pattern aged at 673 K. tates as well as the pine-needle-shaped structure, suggesting that the bainite transformation of γ B + δ took place. This case is interpreted as diffusion-controlled hardening behavior as in Fig. 3, but initial hardening in which hardness increased greatly by aging of only 0.6 ks cannot be explained. Figure 9 shows a magni ed view of the pine-needle- shaped structure in Fig. 6(b). The matrix was similar to bainite (γB) in steel, and a martensite-like structure was intermingled having a mid-rib structure with partial surface relief (dashed line in Fig. 9(b)). This result suggests that a hardness rise in the early stages of aging is based on strengthening because of martensite-like formation. Figure 10 is a backscattered electron image from the eutectoid structure of the PBX alloy aged for 21.6 ks at 743 K. The microstructure grains are as large as about 10 µm in mean particle diameter because the growth was inhibited by δ precipitates, as indicated by the arrow. Bondt et al.11) re- Fig. 8 α (200) diffraction line aged at 673 K and 743 K for 10.8 ks. ports pearlitic transformation of Cu-Sn alloy to α+δ at this Effect of Age-Hardening to Metal Structure and Tribology Characteristics of Lead-Free Bismuth Bronze Casting Containing Sulfur 1559

temperature. Each grain presents a thin lamellar structure, but the lamellar spacing is extremely narrower as compared with the pine needle-shaped structure of Fig. 9(a). This structure is not presumed to be the multilayer of α and δ like perlite of ferrous materials, but steps resulting from stacking faults in α phase12), or merely the lamellar appearance of α (fcc) and δ (hcp) attributable to (111) fcc// (0001) hcp relation13). Re nement of macro grains took place in specimens aged for 14.4 ks at 743 K, as shown in Fig. 4. Precipitated δ in Fig. 10 was broadly distributed in a diameter of 0.1–1 µm. Also, δ on recrystallized grain boundaries were coarsened because of Ostwald ripening. In general, the size distribution of precipitated particles is narrow in a diffusion-controlled system and broad in an interfacial reaction-controlled sys- tem14). Accordingly, precipitation in this specimen is assumed as interfacial-reaction-controlled. In an aging process, δ closely precipitates between highly supersaturated dendrites at rst, then Ostwald ripening prevails according to an interfacial reaction control mechanism afterward. It is assumed that the volume discrepancy between δ and the matrix extends because adjacent grains interfere as the grain growth of δ advances, and because a stress eld involving plastic deformation occurs around each particle to cancel this discrepancy. If individual atomic motion is superposed onto dislocations taking place in a stress eld, then it is presumed that sub-boundaries are generated to absorb lattice defects in grain. When aged at 743 K, slow diffusion of Sn in Cu allows particles on sub-boundaries with many defects to coarsen preferentially, so that the stress eld surrounding particles might be enhanced. The continuous process of the above sequence is presumed to encourage dynamic recrystallization toward grain re nement. No macroscopic struc- Fig. 9 SEM observation of eutectoid structure of 673 K aged specimen. ture re nement was observed in specimens aged at 623 K. (a) SE image (b) schematic of metal structures. Because of rise of recrystallizing temperature by Sn and Ni as solute elements of the PBX alloy and the low growth rate of precipitated particles are considered.

4.2 Friction characteristics and metallographic texture Frictional force F occurs between two metal pieces that are mutually sliding in response to load W, as depicted in Fig. 11 and expressed by eq. (1), which combines Force F′ that shears adhesion generated by plastic deformation of unevenness on a friction surface and excavate force F′′ that occurs when a hard object moves the contacting parts im-

Fig. 10 Backscattered electron image aged at 743 K for 21.6 ks. Fig. 11 Fundamental structural components of frictional force. 1560 K. Funaki, et al. mersed in a soft object. remaining hard δ phase is exposed and projects, excavate force F′′ to scratch the contacting material occurs. However, F = F + F = Aτ + A H (1) the PBX alloy requires a smaller force τ to shear adhesion In that equation, A stands for the real contact area, A′ rep- because of high matrix hardness and point contact on the ad- resents the breakout area, τ denotes the shear strength, and hesion surface of the eutectoid structure being divided by in- H signi es hardness. termetallic compound, i.e., τ/H in eq. (2) is smaller. Dynamic friction coef cient μd is given as frictional force Furthermore, it is presumed that stretched and aky interme- divided by load and is expressed by eq. (2)15): tallic compound precipitates are so nely distributed in the PBX alloy that area A′ of projecting and biting the mating µ = F/W = F/AH = τ/H + A /A. (2) d material is small even when friction advances, so that exca- ′ ′′ Equation (2) shows that μd gets smaller when τ and A are vate force F is small and negligible. This mechanism is as- smaller or H and A are greater. A′ is far smaller than A in sumed to have made a difference in dynamic friction general, so that what is necessary to reduce μd is merely to coef cients. decrease τ/H. A soft material with small τ has great A at a contact region, whereas a hard material with small A shows 5. Conclusion great τ. Consequently, it is considered that both advantages are obtained simultaneously in a eutectoid structure consist- Bismuth bronze containing 1.5 mass% of Ni and ing of a soft phase and a hard phase. Microstructures of dif- 0.3 mass% of S (PBX alloy) was subjected to solution treat- ferent hardness, primary α (HV 100) and α + ne δ eutec- ment and aging. An intermetallic compound phase was toid structure (HV 150), are intermingled in the PBX alloy. re-precipitated. The mechanism of age hardening and low Accordingly, the PBX alloy and lead bronze JIS CAC603 friction characteristics was discussed based on detailed ob- (LBC3) with no ne eutectoid structure were subjected to servation of the precipitation behavior and eutectoid struc- ring-on disk frictional wear test6), and dynamic friction coef- ture. The following major results were obtained. cients were compared. The test was conducted by 2-hour (1) Cu-Sn-Ni-Bi-S PBX alloy exhibited age hardening by operation in SAE 10 W Class engine oil at 333 K on the aging at 623–743 K after solution treatment. Peak hardness friction conditions of a circumferential velocity of 5 m/s and was attained by aging at 673 K. Two-step hardening behav- a pushing pressure of 5 MPa (PV = 1,500 MPa-m/min). ior was observed, in which the hardness rose drastically by Table 2 presents the average friction coef cient obtained us- aging for only 0.6 ks at any temperature. Then hardness in- ing the frictional wear test and the matrix hardness of the creased gradually with aging time thereafter. specimens. The PBX alloy with the metallographic texture (2) The matrix of the PBX alloy aged for 10.8 ks at 673 K of eutectoid structure is harder than JIS CAC603 by 15% in performed phase transformation to a pine-needle-shaped HRB. The average friction coef cient is as extremely small structure containing ne precipitates, where microscopic as 0.04. The sliding surfaces after the test were smooth. No grains were re ned to about 10 µm. In addition, a marten- unevenness was observed. site-like structure was intermingled, having a mid-rib struc- Figure 12 shows schematically drawing friction behavior ture with partial surface relief. This intermingling suggests attributable to different matrix microstructure between JIS that the hardness rise in the early stages of aging is based on CAC603 with α + granular δ and the PBX alloy with α + strengthening because of martensite-like structure formation. ne δ eutectoid structure.6) Because adhesion is apt to occur The subsequent hardness rise occurs because of advanced in α phase, JIS CAC603 having a great areal fraction of α phase transformation. phase in the matrix requires a great force τ to shear the ad- (3) By aging of the PBX alloy at 743 K, no change was hesion. In addition, when the soft phase is worn out and the observed in macroscopic grains until aging time of 3.6 ks, but their re nement was observed at 14.4 ks. This result is hypothesized as occuring because of dynamic recrystallization related to a stress eld involving plastic deformation Table 2 Friction coef cient and hardness of specimens. caused by interference between precipitated particles when δ Average friction coef cient, μ Hardness, HRB phase precipitating adjacently conducts Ostwald ripening. PBX alloy 0.04 69 (4) The dynamic friction coef cient of the PBX alloy in CAC603 0.06 60 oil at 333 K was smaller than that of JIS CAC603. It is considered that because the adhesion was divided by intermetallic compound precipitates, so that a smaller force was re- quired for shearing it, and the excavate force for aky intermetallic compound precipitates was negligibly small.

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