Effect of heat treatment on the microstructure and mechanical properties of yttrium metal

Rare earth oxides prepared by magnetron sputtering of rare earth metal targets are ideal high k gate dielectric materials. The production of targets has drawn more and more attention. In this work, the yttrium (Y) metal target is subjected to cold rolling, resulting in an accumulated deformation of 40%. Subsequently, the deformed Y was heat-treated at various temperatures. Microstructural and mechanical characteristics of Y subjected to deformation and heat treatment were systematically examined utilizing an electron backscatter diffractometer and a hardness tester. The main coarse grains of rolled Y metal were replaced by fine grains with heat treatment temperatures of 550, 650, and 750 °C. Heat treated at 750 °C, grain size distribution was quite homogeneous and the grain growth was not obvious. As for recovery and recrystallization processes, the recovery process occurred after 450-°C heat treatment. And the recrystallized process is fully completed at 750 °C. Besides, the High-Angle Grain Boundaries (HAGB) fraction increased with a rise in heat treatment temperatures from 550 to 750 °C. The microhardness decreased with the heat treatment temperatures increased until 650 °C, then remained stable. This work can provide an important reference for the optimal heat treatment temperature of Y target materials.


Introduction
In the past few years, rare earth oxides (REOs) have been largely applied in special functional thin film materials, due to their excellent optical, high dielectric, fluorescence, and photoluminescence properties [1,2].REOs preparation by magnetron sputtering of rare earth metals are ideal high k gate dielectric materials.Yttrium oxide (Y 2 O 3 ) has caught more and more interest as a semiconductor thin film material, due to its high dielectric constant and thermal stability.It exhibits high energy barriers to holes and electrons in silicon [3][4][5][6].
The rare earth metal shows coarse microstructure, which can't meet the demand of industrial sputtering targets due to the special solidification characteristics.However, preparing a homogeneous, finely-grained microstructure is quite challenging.The manufacture of yttrium (Y) metal target has not been well researched, due to its hardness being sensitive to oxygen content and temperature properties.There is little research on the deformed process and heat treatment of Y metal.The different temperatures of heat treatment seriously affect the deformation performance and the final microstructure of the target material [7].
In this paper, our strategy is to achieve grain refinement through forging and rolling deformation processing of Y metal.Most importantly, the influence of heat treatment temperature from 450 °C to 750 °C in argon atmosphere on the microstructure and microhardness was thoroughly discussed, aiming to disclose the relationship between the heating temperature and the recrystallization microstructure.The purpose is to offer theoretical foundations for the manufacturing of Y targets.

Experimental details
The Y metal, with a minimum purity of 99.9%, underwent cold rolling to achieve an accumulated deformation of 40%.The rolled samples, measuring 150 mm × 60 mm × 9 mm in dimensions, were prepared through electric spark cutting machining to obtain heat treatment samples.The cold rolled Y metals were subsequently heat-treated at 450, 550, 650, and 750°C, with a dwell time of 1 hour in an argon atmosphere.The heat-treated samples were sectioned vertically to obtain cross sections.The specimen was prepared using standard metallographic techniques and subsequently polished to investigate microstructure evolution and conduct microhardness testing.The electron backscatter diffraction (EBSD) technique can provide important crystallographic information on the microstructure transformation process.The crystallographic information of the Y metal was investigated through EBSD analysis using a JSM-F100 scanning electron microscope.The polished surface of Y metal was tested for microhardness using Vickers measurements (HVS-1000A) with a 3-Newton load and a 10-second hold time.To ensure the veracity of the testing results, a minimum of five Vickers microhardness measurements were conducted on the heat-treated samples, and their average value was calculated.Thereby, the credibility of experimental data was enhanced.

Results and discussion
EBSD analysis conclusions of Y metal with a rolling reduction of 40% and samples subjected to heat treatment with various temperatures are presented in Figure 1.The microstructure exhibits a nonuniform distribution, as depicted in Figure 1 (a).There were extremely coarse grains and extremely small broken grains.According to previous reports [7], the coarse grains were original deformation microstructure.After heat treatment at 450°C, the non-uniform microstructure is shown in Figure 2(b).And most of the microstructures were still the original deformed coarse grains.This indicated that the heat treatment temperature of 450 °C cannot promote the occurrence of the recrystallization process.After annealing at 550 °C, very small-sized nucleated recrystallization grains can be observed clearly in the partial microstructure in Figure 2(c) due to the partial recrystallization process.However some coarse grains still existed, which made the microstructure significantly inhomogeneous.The primary factor was the low thermal activation energy of the recrystallized processes and sluggish nucleation proportion of recrystal grains at 550 °C heat treatment process for Y metal [7].After 650 °C heat treatment, the fine and uniform equiaxed grains are shown in Figure 2(d).With the increasing heattreated temperature, the thermal activation energy of the recrystal process was augmented, leading to an intensified rate of nucleation for recrystal grains, which resulted in a uniform and fine-grain microstructure within the same heat treatment time.After 750 °C heat treatment, grown equiaxed grains showed the average and uniform distribution in microstructure, which is displayed in Figure 1(e).At this heat treatment temperature, the grain size was larger than that under 650 °C heat treatment, since the recrystallization process was more complete and the grains grew larger.The heat treatment with various temperatures indicates that heat treatment temperature had an important influence on the growth of recrystallization grains.The microstructure distribution was uniform when the heat treatment temperature was 650 °C and 750 °C.The grain size was the crucial parameter for measuring the qualities of metal targets [8].The grain size distributions of rolled and heat treatment of Y metal can be observed in Figure 2. In Figure 2(a), a substantial amount of broken grain sizes of rolled Y were less than 10 μm, but the coarse grain size was more than 100 μm.After measurements, the average grain size of the non-uniform microstructure was 41 μm.As revealed in Figure 2(b), for 450 °C heat treatment samples, the grain size still exhibited an extremely non-uniform distribution, the original deformed coarse grains size was more than 170 μm.The average grain size was recorded as 73 μm.The result indicates there was no occurrence of the recrystallization process under 450 °C heat treatment.The significant difference in average grain size of the two samples revealed the microstructure showed non-uniform distribution before the rolling process.The grain size distributions were relatively uniform in Figure 2(c), (d), (e), and the grain size is less than 65 μm.After 550, 650, and 750 °C heat treatment, the average grain size was 19 μm, 13 μm and 16 μm, respectively.At 750 °C, the grain size distribution exhibited a high degree of uniformity, while the occurrence of grain growth was not prominently observed.The grain distribution characteristics maps of EBSD results can provide recrystallization behavior and microstructure evolution process.The grain distribution characteristics of the Y metal after rolling and heat treatment at various temperatures are illustrated in Figure 3.For the rolled and lowtemperature heat-treated samples as shown in Figures 3(a) and 3(b), blue represented original undeformed microstructures, yellow represented the deformed substructure, and red represented the broken grains.After measurements, the ratio of yellow and red area accounted for 80% of the 450 °C heat treatment, which indicated the microstructure was predominantly observed in the form of a deformed substructure.The sample was heat-treated at a temperature of 550 °C, and blue represented the recrystallization grains and a small amount of undeformed microstructure.The recrystallization grains constituted 84.6% of the sample.The present process, the static recovery, and partial recrystallization occurred.The recrystallization grain ratio increased significantly from 60.8% to 90.3% with the rise in heat treatment temperature, specifically from 650 °C to 750 °C.The recrystallization process was fully accomplished at the temperature of 750 °C.The misorientation angle reveals the distribution of grain boundary character.The corresponding misorientation angle distributions of Y metal rolled and heat treatment is shown in Figure 4.The misorientation angles that are less than 15 o represent Low-Angle Grain Boundaries (LAGBs), and angles that are more than 15 o represent High-Angle Grain Boundaries (HAGBs).After 450 °C heat treatment progress, the main misorientation angles were still dominated by LAGBs.The HAGB fraction exhibits an upward trend with the rise in heat treatment enhanced from low-temperature to 750 °C, as illustrated in Figures 4(c), (d), and (e), which was similar to the previous studies [9,10].The result indicated a release of the stored energy of rolling deformation and the finish of the recrystallization process.The evolution of microhardness values can evaluate the recovery processes effectively [7].The microhardness of Y metal is shown in Figure 5.The microhardness of rolled Y metal was 108 HV, which was obviously higher than heat-treated samples.The presence of numerous stored dislocations in the deformed microstructures and deformed substructures contributed to the observed phenomenon.After 450 °C heat treatment, the microhardness significantly decreased from 108 HV to 80 HV, which can be attributed to the occurrence of recovery processes.The recovery processes released the stored energy of dislocations, but it cannot change the microstructure morphology.This result was not contradicted by the microstructure evolution within the preceding context.The microhardness was 74 HV after being heat-treated at 550 °C.At this stage, recovery and partial recrystallisation processes occurred at the same time.The microhardness reduced to 65 HV and 66 HV, respectively, after being heat treated at temperatures of 650 and 750 °C.The microhardness remained stable, due to the completion of the recovery process, and the main process was recrystallization.

Conclusion
In this article, the influence of different temperatures on both microstructure and mechanical properties is systematically analysed.The main coarse grains of rolled Y metal were replaced by fine equiaxed grains with heat treatment at 550, 650, and 750 °C.Grain size distribution exhibited a high degree of uniformity, while the extent of grain growth remained inconspicuous at 750 °C.The average grain size was measured to be 16 μm.The recovery processes occurred after 450 °C heat treatment.The recrystallization process reached its full completion at the temperature of 750 °C.The fraction of LAGBs progressively decreased, leading to a subsequent rise in HAGBs as a result of an elevated ratio of recrystallization.The microhardness gradually reduced with the heat treatment temperatures increasing.Nevertheless, microhardness remains stable when the temperature is greater than 650 °C.This work can offer an important reference for the optimal heat treatment temperature of Y target materials.

Figure 2 .
Figure 2. The grain size maps for Y metal rolled and heat treatment with various temperatures.

Figure 4 .
Figure 4.The corresponding misorientation angle distributions of Y rolled and heat treatment with various temperatures.

Figure 5 .
Figure 5. Microhardness of Y metal rolled and heat treatment with various temperatures.