3.1 Mechanical Properties
Three groups of alloys were solidified at 850°C for 1 h and then aged at 550°C. The mechanical and electrical properties of the solid solution alloys during the subsequent aging treatment are shown in Fig. 1. As can be seen in Fig. 1(a), the hardness curves of the two groups of alloys increased rapidly to the peak value and then decreased after the aging time was prolonged. Both groups of alloys have an obvious aging-strengthening phenomenon. As can be seen from Fig. 1(b), the electrical conductivity curves of the two groups of alloys are all stabilized after the aging time is prolonged by a rapid increase to the peak value.
As can be seen from Fig. 1, after aging for 4 h, the FT12 alloy reaches the peak hardness, at which time the alloy hardness is 112.3 HV, and the conductivity is 27% IACS; after aging for 2 h, the FT11 alloy reaches the peak hardness, at which time the alloy hardness is 107.2 HV, and the conductivity is 34.6% IACS; after aging for 2 h, the FT21 alloy reaches the peak hardness, at which time the alloy hardness is 102.1 HV, and the conductivity is 34.6% IACS. After aging for 2 h, the FT21 alloy reaches the peak hardness of 107.2 HV and an electrical conductivity of 39.7% IACS.Comparing the peak performance of the three groups of alloys, it can be found that the peak hardness of the alloys is the highest when the ratio of iron to titanium is 1:2, and the peak hardness of the alloys is higher than the value of FT21 alloy by 10.2 HV, but at the same time the electrical conductivity of the FT12 alloy is seriously decreased, and it reduces 11.3% IACS compared with the FT21 alloy. It can be seen that when the Fe-Ti ratio is 2ะ1, the overall performance of the alloy is better than other ratios.
The alloy was cold rolled with a deformation of 70% and then aged at 550°C. The mechanical and electrical properties of the alloy in the cold-rolled state during the subsequent aging treatment are shown in Fig. 2. As can be seen in Fig. 2(a), the hardness curves of the alloys all increased rapidly to the peak and then decreased after the aging time was prolonged. The alloys have an obvious aging strengthening phenomenon. As seen in Fig. 2(b), the electrical conductivity curves of the alloys are all stabilized after a rapid increase to the peak value at the extension of the aging time.
As can be seen from Fig. 2, the peak aging time of FT12 alloy is 1 h, the peak hardness of the alloy is 189.5 HV, and the conductivity is 44.2% IACS; the peak aging time of FT11 alloy is 1 h, the peak hardness of the alloy is 173.9 HV, and the conductivity is 51.3% IACS; the peak aging time of FT21 alloy is 1 h, the peak hardness of the alloy is 166.5 HV, and the conductivity is 64.3% IACS. The peak hardness of the FT21 alloy was 166.5 HV and the electrical conductivity was 64.1% IACS.
3.2 Microstructures of the alloy
Figure 3 shows the TEM images and corresponding selected area diffractograms (SADP) of peak-aged samples of three alloys, FT12, FT11, and FT21, at 550°C, where Fig. 3 (a1,a2) is for the FT12 alloy, Fig. 3 (b1,b2) is for the FT11 alloy, and Fig. 3 (c1,c2) is for the FT12 alloy, and the yellow portion in the SADP is labeled as the precipitation phase and the red part is the Cu phase. From Fig. 3(a1,a2), it can be seen that the precipitated phase is in the form of particles, and the diffraction spots of the corresponding precipitated particles appear clearly in the corresponding SADP. After the analysis of the electron diffraction pattern, it can be found that the precipitated particles can be defined as the Fe2Ti phase, and the lattice constants of the hexagonal crystalline system of the Fe2Ti phase are a = 0.478 nm, c = 0.781 nm, and there exists a [\(\text{120}\)]Fe2Ti//[\(\stackrel{\text{-}}{\text{1}}\text{10}\)]Cu site-directional relationship. From Fig. 3(b1,b2), it can be seen that the precipitated phase of FT11 alloy also shows a granular shape, which can be defined as the Fe2Ti phase, and there is a site-directional relationship of [\(\text{100}\)]Fe2Ti//[\(\stackrel{\text{-}}{\text{1}}\text{10}\)]Cu between the Fe2Ti phase and the matrix. Figure 3(c1,c2) shows the peak aging precipitated phase state of FT12 alloy; the precipitated particles can also be defined as Fe2Ti phase, and there is a site-directional relationship of [\(\stackrel{\text{-}}{\text{1}}\stackrel{\text{-}}{\text{1}}\text{1}\)]Fe2Ti//[\(\stackrel{\text{-}}{\text{1}}\text{21}\)]Cu between the Fe2Ti phase and the matrix. Figure 4 shows the HRTEM image of the precipitated phase of the FT12 alloy, which was transformed by FFT to reveal that the precipitated crystal structure is similar to the Fe2Ti phase, with a crystal plane spacing measured at 0.3645 nm.
3.3 High temperature softening resistance
According to the aging performance change curve in Fig. 2, the alloy was selected after cold rolling 70% and aging at 550 ℃ for high-temperature resistance to softening test at different temperature conditions of insulation for 1 hour, measure the hardness of the alloy and draw its high-temperature resistance to softening curve. Figure 5 shows the high-temperature softening curves of three groups of alloys.
Figure 5 shows that in the annealing temperature range of 460 ℃-500 ℃, the hardness of the three groups of alloys decreases slowly, and the hardness of the alloys in this annealing temperature range can reach more than 95% of the initial value, and the trend of hardness decrease is not significant. In the annealing temperature range of 520 ℃-660 ℃, the hardness of the three groups of alloys decreased at an increasing rate and the hardness values decreased significantly. The difference in the decreasing trend of alloy hardness between the two annealing temperature ranges indicates the existence of different softening mechanisms in the alloys at different annealing stages. The reason for the decrease in alloy hardness with the increase in holding temperature is that at lower holding temperatures, the diffusion power of atoms inside the alloy increases, the precipitated phase grows and coarsens, the ability to hinder dislocation movement decreases, and the alloy hardness decreases, but due to the lower holding temperature temperature, the over-ageing reaction is not significant, and the decrease in alloy hardness is not obvious. When the insulation temperature is too high, the alloy occurs more serious over-ageing, at this time the alloy hardness decline increased, so in the 520 ℃ -660 ℃ annealing temperature range, the alloy hardness significantly reduced. It can be seen from the figure that the hardness of FT12 alloy has been higher than the remaining two groups of alloys, and FT11 alloy in the insulation temperature is lower when the hardness value is higher than that of FT21 alloy, but when the insulation temperature is more than 540 ℃, the alloy hardness decreases rapidly, and the overall hardness curve decreases, FT21 alloy is more FT11 alloy gently. The hardness value of the alloy decreased to 80% of the initial value after 1h of annealing treatment, at which time the corresponding annealing temperature was the softening temperature of the alloy. It can be seen that the FT11 alloy has a softening temperature of 560°C, the FT12 alloy has a softening temperature of 580°C, and the FT21 alloy has a softening temperature of 600°C. The FT21 alloy has a softening temperature of 580°C. Compared with FT11 alloy, the softening temperature of FT21 alloy is increased by 40 ℃, and that of FT12 alloy is increased by 20 ℃, indicating that the alloy has the best ability to resist softening at high temperatures when the ratio of iron to titanium is 2:1.
Figure 6 shows the EBSD plots of three groups of Cu-Fe-Mg-Ti alloys after peak aging at 500 ℃ and 620 ℃ for 1h after cold rolling with 70% deformation. Figure 6 shows the EBSD plots of three sets of Cu-Fe-Mg-Ti alloys after peak aging at 500 ℃ and 620 ℃ for 70% of deformation. Table 2 shows the EBSD plots of Fig. 6 with the percentage of recrystallization of the alloys. Table 2 shows the percentage of recrystallization of the alloy in the EBSD plots of Fig. 6. As shown in Fig. 6(a,b), the recrystallization percentage of FT21 alloy is 2.0% after holding at 500 ℃×1 h. The recrystallization percentage of FT21 alloy is 2.0% after holding at 620 ℃×1 h. As shown in Fig. 6(a,b), the recrystallization percentage of FT21 alloy is 2.8% after 500 ℃×1 h holding time, and the recrystallization percentage of FT21 alloy held at 620 ℃×1 h is obviously higher, reaching 28.6%.
Table 2
recrystallization degree of FT12 alloy %
temperature
|
FT12
|
500 ℃
|
2.8
|
620 ℃
|
28.6
|
Figure 7, Fig. 8 shows the TEM images of three groups of Cu-Fe-Mg-Ti alloys after peak aging samples were annealed at different temperatures for 1 h after 70% cold rolling. From Fig. 7, it can be seen that after the three groups of alloys were insulated at 500 ℃ × 1 h, there were high-density dislocations and dislocation entanglement inside the alloys. When the three groups of alloys after 580 ℃ × 1 h heat preservation treatment, the density of dislocations in the alloy is smaller than that of the alloy after 500 ℃ × 1 h heat preservation treatment, this is due to the high temperature annealing, the alloy occurs to revert to the recrystallization, dislocations movement, the same slip surface of the dislocations of the same number of dislocations are attracted to each other to cancel, dislocations arrangement gradually regularization leads to. When the three groups of alloys are annealed at 620 ℃ for 1 h, the dislocation density decreases dramatically, the phenomenon of mutual entanglement between dislocations and the dislocation cell basically disappears, and the dislocations in the cell wall are gradually transformed into the dislocation network in the low-energy state, and there is a thin and long dislocation line. As can be seen in Fig. 8, the recrystallized grains of the three groups of alloys after annealing at 500 ℃ for 1 h can be clearly observed, and when the alloy is annealed at 580 ℃ for 1 h, the degree of recrystallization is intensified, and the recrystallized grains have a certain degree of growth. When the alloy is annealed at 620 ℃ for 1 h, there is a merging of sub-grains. The merging of subgrains makes the orientation of two or more subgrains become consistent, which can be used as the nucleus of recrystallization and promote the occurrence of recrystallization phenomenon. And the recrystallized grains of both groups of alloys have obvious increase.
From Fig. 5, it can be seen that the softening phenomenon is not obvious when the three groups of Cu-Fe-Mg-Ti alloys are annealed in the temperature interval from 460 ℃ to 500 ℃. From the TEM diagram in Fig. 8, it can be seen that after 400 ℃×1h heat preservation treatment, the alloy has a recrystallization phenomenon, which indicates that when the alloy is annealed in this temperature range, the alloy's recovery and recrystallization is the reason for the reduction of its hardness, but due to the low degree of recrystallization, the hardness value of the alloy does not change much. When the three groups of alloys are annealed in the temperature range from 500 ℃ to 580 ℃, the degree of alloy restitution and recrystallization is intensified, consuming some of the dislocations, and it can be observed from Fig. 7 that the density of dislocations decreases, the alloy softening effect is obvious, and the hardness begins to decrease. When the alloy is annealed at a temperature range of 580 ℃ to 660 ℃, the degree of alloy recovery and recrystallization is further aggravated, the dislocation density decreases sharply, the internal dislocation density of the alloy further decreases, the recrystallized grains grow up significantly, and the phenomenon of alloy recovery and recrystallization is further aggravated.
In summary, when the Cu-Fe-Mg-Ti alloys are annealed from 460 ℃ to 580 ℃, the softening is caused by restitution and recrystallization, and when the alloys are annealed from 580 ℃ to 660 ℃, the alloys soften as a result of the combined effect of the restitution and recrystallization phenomena and the coarsening of the particles in the precipitated phase. The recrystallization ratio of FT21 alloy is lower than that of FT12 alloy and FT11 alloy, which indicates that the recrystallization ratio of the alloy decreases and the softening property of the alloy improves when the ratio of iron to titanium is 2:1. Therefore, the softening temperature of FT21 alloy is higher than that of FT12 and FT11 alloys, and the softening temperature of Cu-Fe-Mg-Ti alloy can be effectively increased when the ratio of iron to titanium is 2:1.