Chapter 3
for twenty times on each sample at different locations for each cold-rolled work hardening and thermal ageing condition to develop the data reliability.
Fig. 3.1 Micro-hardness examination using PC interfaced hardness tester.
Basically, micro-hardness values were measured for all categories of samples whether untreated or thermally treated under each temperature of thermal ageing conditions at each cold-rolled work hardening level. Therefore, samples were divided into two groups of which the first group were of as-cast and cold rolled but free from heat treatment to observe the individual effect of work hardening. Few samples from the second group were heat treated isochronally at various temperatures, such as, 25oC, 100oC, 150oC, 200oC, 250oC, 300oC, 350oC, 400oC, 450oC and 500oC for a period of one hour and their micro-hardness tests were carried out. Following the experimental results of micro-hardness obtained for isochronally aged samples, the remaining samples of the second group were heat treated isothermally at 150oC, 200oC and 250oC corresponding to the higher micro-hardness values with the variation of ageing period from 15 minutes to 240 minutes and series of tests were carried out to investigate the effect of ageing time on micro-hardness.
Microstructures were examined before and after applying the cold-rolled work hardening and ageing at different thermal conditions using optical electronic microscope (OEM) of model BW-S500. In addition, the surface morphologies of work hardened (75% deformation level) sample materials were investigated using a field emission
scanning electron microscope (FE-SEM) of model JEOL JSM-7600F with the magnification of x30000. EDX spectra of samples were recorded by an electron dispersive spectrometer of model JEOL EX-37001 associated with the FE-SEM set up as shown in fig.3.2.
Fig. 3.2 Microstructure examination using scanning electron microscope.
3.3 Analysis of Micro-hardness Results 3.3.1 Effect of Alloying
Measured micro-hardness values of four sample materials, i.e., material-I (pure Cu), material-II (Cu-Sn alloy), material-III (Cu-Pb alloy) and material-IV (SnPb-solder affected Cu) are analyzed before and after the work hardening. As cast condition, the average micro-hardness values of four materials attained from twenty measurement readings at room temperature are found to be 55.05 HV, 68.42 HV, 50.97 HV and 65.24 HV, respectively, as shown in fig. 3.3. It indicates that the addition of about 1.134% Sn with copper in material-II has increased the hardness by 24.3%, whereas the addition of about 1.197% Pb for material-III has decreased the hardness by 7.41%, and the addition of about 1.257% Sn and 1.195% Pb together for material-IV has increased the hardness by 18.52% from that of material-I. It may be mentioned here that the micro-hardness values of Sn and Pb have been found as 8.5 HV and 6.4 HV respectively. So, the Cu-Sn and Cu-Pb alloys should have less micro-hardness than that of pure Cu, and the obtained results for Cu-Pb alloy with 7.41% less is quite matching. But problem has occurred with
Cu-Sn alloys with the increase of micro-hardness by 24.3% and 18.52% for material-II and material-IV respectively. The reason behind such increase in micro-hardness with the addition of Sn is mainly due to the formation of CuxSny intermetallic compounds [32- 36]. On the contrary, the decrease of micro-hardness with the addition of Pb is considered to be the effect of no propensity of forming any intermetallic compounds while alloyed with copper [37-41.
Fig. 3.3 Micro-hardness variation against cold-rolled work hardening conditions at room temperature for copper and the solder affected copper alloys.
3.3.2 Effect of Cold Rolling Deformation
When the sample materials have undergone cold-rolled deformation, they have shown different characteristics from their as-cast condition. The micro-hardness values of all four sample materials have been found to be increased with the increase of deformation level as it generally occurs to a certain limiting level of cold-rolling [42].
However, the incremental rates of micro-harness values are found to be different for different sample materials as shown in Fig. 3.3. The micro-hardness values are also observed to be varied nonlinearly or can be said curvilinear against the cold-rolled deformation level of sample materials. The first increment from as-cast to 5% cold-rolled condition, i.e., the initial slopes of the curves in Figure 1 are 0.37, 0.33, 0.49 and 0.41, respectively, while all four curves become flat with almost zero slope at the final stage of work hardening. At 75% cold-rolled deformation condition, the micro-hardness values
of the four sample materials have reached to 64.18 HV, 77.51 HV, 67.56 HV and 79.67 HV, respectively, which indicate that the hardness values have increased by 16.59%, 13.29%, 32.56% and 22.13% in comparison to their respective values of as-cast condition. In other words, material-II, -III and -IV have become 1.21, 1.05 and 1.24 times harder than material-I at the cold-rolled deformation level of 75%, respectively.
Therefore, the influence of work hardening is more prominent for Cu alloys than that of pure Cu. It is also interesting to note that material-III (Cu-Pb alloy) is softer than material-I (pure Cu) before cold-rolling condition. But the micro-hardness value of material-III has crossed over that of material-I at the cold-rolled level of about 35% and thereafter material-III has remained harder than material-I at higher deformation levels.
Again, initially the micro-hardness of material-IV is also less than that of material-II before cold-rolling, but the micro-hardness curve of material-IV overshoots that of material-II at the same deformation level of about 35% and remains at higher position onwards. So, a deduction may be drawn that the effect of cold-rolling on micro-hardness of Cu under the presence of Pb is quite different from that of Sn. This phenomenon may be considered as the elastic misfit interaction, i.e., size effect due to the resultant elastic distortion that interacts with onward distortion around the dislocations with the presence of Pb in Cu before and after the work hardening [14,39]. On the contrary, Cu-Sn system is a complex structure due to allotropic variation, and its intermetallic compounds are not overwhelmingly affected by the elastic misfit interaction after cold rolling which results in a smaller amount of increase in hardness compared to that of Pb in Cu [34-36].
3.3.3 Effect of Thermal Ageing
Once the work hardened samples are aged isochronally at various temperatures ranging from 25oC to 500oC, the hardness values of all four sample materials are found to be almost steady and sustainable up to ageing temperature of 150oC as depicted in Fig.
3.4. However, the ageing temperature range for the sustainable hardness is found to be higher for material-II, -III and -IV than that of material-I. With further rise of ageing temperature, the hardness values of all four sample materials are found to be reduced significantly as shown in Figures 3.4(a), 3.4(b), 3.4(c) and 3.4(d) for cold-rolled work hardening levels of 0%, 25%, 50% and 75%, respectively. It is observed that thermal ageing together with cold-rolling has remarkable effect on hardness of Cu up to ageing temperature of 200oC, which agrees with the previous findings of the authors [23,43] and
the effect on micro-hardness on pure Cu becomes especially severe for higher percentage of cold-rolling deformation. However, the changes in hardness values against ageing temperature for three alloy materials are not that radical compared to pure Cu; rather they are somehow gradual in nature, especially for lesser work hardening levels. At the ageing temperature of 400oC and above, micro-hardness values of pure Cu have been found almost the constant (about 30 HV) for all the work hardening levels, i.e., the work hardening of pure Cu has hardly any effect on hardness after thermal ageing at higher temperatures, such as, 400oC or so on. But it is not the case for three alloy samples; they have shown different micro-hardness values for different work hardening levels while aged at higher temperatures. It indicates that little addition of SnPb-solder in Cu has increased the sustainable temperature range for micro-hardness values of the alloys to be used at elevated temperature. However, the micro-hardness values of three alloy materials have become the lowest (around 30-40 HV) for heavy cold-rolling conditions, such as, 50% and 75% together with the thermal ageing at temperatures like 450oC and 500oC.
Fig. 3.4 Micro-hardness of copper and solder-affected copper alloys against isochronal ageing temperature (aged for 1 hour): (a) 0%, (b) 25%, (c) 50%,
and (d) 75% cold-rolled work hardening.
Fig. 3.5 Micro-hardness of copper and solder-affected copper alloys against ageing
period for isothermal ageing at 150oC: (a) 0%, (b) 25%, (c) 50%, and (d) 75% cold-rolled work hardening.
Since the isochronal ageing curves of Fig. 3.4 elucidate that the micro-hardness values are stable for all four sample materials at the ageing temperature up to 250oC for as-cast, 200oC for 25% cold-rolled and 150oC for 50%/75% cold-rolled conditions and the down falling of micro-hardness values have become clear for thermal ageing at elevated temperatures, isothermal ageing is done at the temperatures of 150oC, 200oC and 250oC for cold-rolled work-hardening samples. The variations of micro-hardness values for the samples with respect to ageing periods are presented in Figures 3.5, 3.6 and 3.7 for the aforesaid three isothermal ageing temperatures, respectively. All these three figures show that micro-hardness values of all four sample materials fluctuate like a transient variation at the initial period of isothermal ageing, i.e., 15 minutes and 30 minutes. Thereafter, while ageing period is increased, the results are of steady state nature
showing almost horizontal graphs for all four materials. However, Figures 3.6 and 3.7 depict relatively smaller fluctuations in micro-hardness values than that of Fig. 3.5.
Another point to be noted that the micro-hardness values of Cu is decreased remarkably with the increase in ageing period, especially, at the ageing temperature of 250oC for four work hardening conditions with the maximum drop of micro-hardness values for 75%
cold-rolling as observed in Fig. 3.7. It indicates that the hardness values of alloy materials are more sustainable than that of pure copper during ageing at elevated temperature.
Fig. 3.6 Micro-hardness of copper and solder-affected copper alloys against ageing
period for isothermal ageing at 200oC: (a) 0%, (b) 25%, (c) 50%, and (d) 75% cold-rolled work hardening.
Fig. 3.7 Micro-hardness of copper and solder-affected copper alloys against ageing
period for isothermal ageing at 250oC: (a) 0%, (b) 25%, (c) 50%, and (d) 75% cold-rolled work hardening.
3.3.4 Microstructure Observation
The micrographs (100x) of sample materials as their cast condition are displayed in Fig. 3.8. Fig. 3.8(a) indicates clear look of face centered cubic (FCC) lattice of non- polymorphous Cu. However, the micrograph of Fig. 3.8(b) is quite different from that of Fig. 3.8(a) due to addition of a very little amount of Sn in Cu to form material-II as a Cu- Sn system with the indications of forming CuxSny intermetallic compounds [32-36,44].
The microstructure of the as-cast Cu-Sn alloy has consisted of a dendritic primary-α phase and an α+δ-CuxSny eutectoid phase, which has resulted in high hardness values as observed in Fig. 3.3. On the other hand, Fig. 3.8(c) and (d) are showing distributed
presence of Pb as a solute in the boundaries in Cu solvent of material-III and material- IV [38-41] as their cast condition resulting in less hardness values compared to material- I and material-II, respectively. The grain sizes are also observed to be varied clearly for all the cast alloys.
Fig. 3.8 Optical micrographs of copper and solder-affected copper alloys as-cast condition: (a) material-I (pure Cu), (b) material-II (Cu-Sn alloy), (c) material-III (Cu-
Pb alloy) and (d) material-IV (solder affected Cu).
Fig. 3.9 presents the micrographs of sample materials after cold-rolled work hardening level of 75% deformation. The microstructure indicates the work hardening effect as a geometrical deformation at crystalline level along with grain size refinement.
They also show varying dislocation slip lines along with elementary structure [44]. The elongated appearance of deformed crystals has contributed increase the hardness of all four materials. Basically, when the copper based materials are heavily work hardened, plastic deformation results in the defects to changing the crystalline makeup, and thus the micrographs of Fig. 3.9 can be characterized with the diffused boundaries and the
shear bands. The present results do not fully agree with the general understanding that the microstructure of heavily rolled copper consists of long, thin, elongated cells with sharp boundaries, however, it agrees with Malin & Hatherly [45].
Fig. 3.10 depicts the microstructural images of the four sample materials when subjected to cold-rolled work hardening level of 75% deformation together with thermal aging performed at 400oC. After thermal ageing at this higher temperature, the cold- rolled samples have availed a state of relaxation, and thus the micrographs have some grain reshaping indications. As a result, elongated crystalline structure is missing and dislocations are not being also detected. This has attributed to recrystallization effect which has lowered the hardness to a significant level as consequence of thermal ageing.
Fig. 3.9 Optical micrographs of four materials after work hardening through cold rolling of 75% deformation: (a) material-I (pure Cu), (b) material-II (Cu-Sn alloy),
(c) material-III (Cu-Pb alloy) and (d) material-IV (solder affected Cu).
Fig. 3.10 Optical micrographs of copper and solder-affected copper alloys after cold work level of 75% and thermal ageing at 400℃: (a) material-I (pure Cu), (b) material- II (Cu-Sn alloy), (c) material-III (Cu-Pb alloy) and (d) material-IV (solder affected Cu).
The FE-SEM images of material-I (pure Cu), material-II (Cu-Sn alloy), material-III (Cu-Pb alloy) and material-IV (Cu-Sn-Pb alloy) have been observed after cold attaining 75% cold-rolled work hardening deformation with magnification factors of 500, 1000,
2000 and 30000; amongst them images with magnification factor of 30000 along with their EDX spectra are presented in Fig. 3.11. SEM micrograph of Fig. 3.11(a) depicts that the material-I contains fine regular grains of Cu with relatively straight boundaries. Here, the presence of any trace of intermetallic compounds is not observed.
The corresponding results of EDX spectra, shown in Fig. 3.11(b), affirm the purity level of sample material-I, i.e., pure Cu. In Fig. 3.11(c), the SEM image of material-II indicates the microstructural trace of CuxSny system and the corresponding EDX result provides also the similar trace of CuxSny system (see Fig. 3.11(d)). The presence of Sn in Cu results in different microstructure of material-II from the SEM image of material-I. It is because of the solidification of the molten Sn in Cu, which develops the intermetallic compounds, such as, Cu6Sn5, Cu3Sn, Cu5Sn, etc. These intermetallic compounds have reasonably
changed crystalline structures, which eventually causes the electro-mechanical properties of material-II to change, as observed for hardness. In the diffusion process, the solubility of Sn in Cu gets changed up to saturation level and the excess Cu remains in elemental form around the intermetallic compounds [32-36]. As a result, inclusion of only about 1% Sn having micro hardness value of only 8.5 HV has increased the hardness of material-II (Cu-Pb alloy) significantly.
The FE-SEM image of material-III presented in Fig. 3.11(e) is quite different from the images of material-I and material-II, as shown in figures 3.11(a) and (c), respectively.
The grains are not very regular like the FE-SEM image of material-I, rather grain boundaries are of different look for Cu-Pb alloy [37-40], but there is no trace of any intermetallic compound in the FE-SEM image of material-III. That means the solidification after inclusion of Pb in Cu has not developed intermetallic compounds as observed in the case of Cu-Sn system. The corresponding EDX spectra also provide no indication of forming any intermetallic compounds (see Fig. 3.11(f)). As a result, inclusion of Pb having micro hardness value of only 6.4 HV has lowered the hardness of material-III (Cu-Pb alloy).
The FE-SEM image of material-IV presented in Fig. 3.11(g) seems again different from the above mentioned three FE-SEM images but closer to that of material-II shown in Fig. 3.11(c) and the reason is the role of Sn with the formation of Cu-Sn system. Here also, intermetallic compounds do exist and EDX result in Fig. 3.11(h) is affirming the microstructure interpretation of FE-SEM image having Cu-Sn system along with solid solution of Cu and Pb. As a whole, the intermetallic compounds formed due to presence of Sn have contributed significantly to increase the micro-hardness of material-II and - IV as observed in Fig. 3.3.
The FE-SEM image of material-III presented in Fig. 3.11(c) is again different from the images of material-I and material-II shown in Fig. 3.11(a) and (b) respectively. The grains of material-III are not very regular like material-I as in Fig 3.11(a), but again no trace of any intermetallic compound is observed in SEM image of material-III shown in Fig. 3.11(b). That means the inclusion of Pb in Cu has not developed intermetallic compounds as observed in the case of Cu-Sn system. The FE-SEM image of material-IV
presented in Fig. 4.11(d) looks similar to that of material-II along with the presence of intermetallic compounds of Cu-Sn system and solid solution of Cu and Pb.
Fig. 3.11 FE-SEM images along with EDX spectra after cold work level of 75% and ageing at 150oC for 1 hr: (a) SEM of material-I, (b) EDX of material-I, (c) SEM of material-II, (d) EDX of material-II, (e) SEM of material-III, (f) EDX of material-III,
(g) SEM of material-IV and (h) EDX of material-IV.
3.4 Summary
The present research shows that tiny amount of tin ( 1%) can contribute significantly to increase the micro-hardness of copper (24%), while similar amount of lead decreases the corresponding micro-hardness by an amount of around 7%. Similarly, micro- hardness of solder affected copper is 18.52% higher than that pure copper. Since the micro-hardness values of Sn and Pb are only 8.5 HV and 6.4 HV respectively, material- II (Cu-Sn alloy) and material-III (Cu-Pb alloy) were supposed to have less micro- hardness than that of pure Cu. In that respect, the obtained result for Cu-Pb alloy seems reasonable, but Cu-Sn alloys with the increase of micro-hardness needs to look for reasoning. The increase in micro-hardness with the addition of Sn is mainly due to the formation of CuxSny intermetallic compounds. The intermetallic compounds, such as, Cu6Sn5, Cu3Sn, Cu5Sn, etc. formed due to presence of tin in copper has changed crystalline structures and in turn the hardness of material-II and -IV.
Micro-hardness values of all four sample materials, i.e., material-I (pure Cu), material-II (Cu-Sn alloy), material-III (Cu-Pb alloy) and material-IV (solder affected Cu) have been severely affected by cold work and are increased with the increase of cold work. The probable reasons for rising hardness are grain shape alteration, dislocations and grain size refinement.
The increase of hardness has occurred initially in curvilinear pattern with the rise of cold-rolling level up to 50~55% and then got flatten. It also shows that the micro- hardness of solder affected copper alloys can be increased to a great extent when they are subjected to cold-rolling. Here, Pb is found to dominate more than Sn to increase hardness for cold work. As such, the hardness curves of material-III and -IV have superseded that of material-I and -II respectively. Moreover, the cold-rolled work- hardening level of ~35% may be considered as a critical deformation level where material-III (Cu-Pb alloy) supersedes material-I (pure Cu) and material-IV (SnPb-solder affected Cu) material-II (Cu-Sn alloy) on hardness. The thermal ageing has changed the crystal structures and affected the hardness which was developed by cold work and consequently the hardness values of all four sample materials have become less at the elevated temperature. However, a post hardening thermal treatment shows that the alloys can offer a higher stable range of hardness against ageing temperature than that of pure Cu.