Effect of the ITER FW Manufacturing Process on the Microstructure and Properties of a CuCrZr Alloy

Transcription

1 Effect of the ITER FW Manufacturing Process on the Microstructure and Properties of a CuCrZr Alloy LIU Danhua ( ), WANG Pinghuai ( ), SONG Yi ( ), LI Qian ( ), CHEN Jiming ( ) Southwestern Institute of Physics, Chengdu , China Abstract The first wall (FW) is one of the core components in ITER. As the heat sink material, the CuCrZr alloy shall be properly jointed with beryllium and stainless steel. At present, the grains of CuCrZr are prone to coarsen seriously in the thermal cycle process of FW manufacturing, which has become a critical issue for ITER parties. To investigate the mirostructure and mechanical properties of the optimized CuCrZr alloy in the first wall fabricating thermal cycle, simulative experiments have been done in this study. The alloy ingot was forged and hot rolled into plates, and then solid solution annealed, cold rolled and aged for strengthening. Several heat treatments were done to the CuCrZr samples, and the changes of microstructure, micro-hardness and tensile strength were investigated. The results indicated that the original elongated grains had changed into equiaxed ones, and the vickers hardness had declined to about 60 after experiencing the process of CuCrZr/316L(N) bi-metallic plate manufacturing, either by hot isostatic pressing at a higher temperature or by explosion welding followed by solution annealing. Joining Be/CuCrZr by hot isostatic pressing acts as an aging process for CuCrZr, so after the simulated heat treatment, the hardness of the alloy increased to about 110 HV and the tensile yield strength at 250 Crose to about 170 MPa. Meanwhile, the average grain size was controlled below 200 μm. Keywords: ITER, first wall, CuCrZr alloy, hot isostatic pressing, explosive welding PACS: Fa DOI: / /17/10/13 (Some figures may appear in colour only in the online journal) 1 Introduction The first wall (FW) of the shield blanket in ITER is composed of beryllium, a CuCrZr alloy, and stainless steel (SS) [1], and it mainly plays the role of neutron shielding and heat transmission. The FW would suffer high heat flux and neutron wall loading from the plasma, as it would run in a high temperature and pressure condition, since the machining and manufacturing requirements of FW are quite rigorous. The production of the Be/CuCrZr/SS composite plate includes first joining the CuCrZr and SS plates by hot isostatic pressing (HIP) at a higher temperature, then solid solution treatment for the CuCrZr/SS composite plate, and finally joining the Be tile on the CuCrZr/SS composite plate at a lower temperature by HIP [2]. However, the grains of the CuCrZr alloy are prone to coarsen in this process, and the average grain size could not meet the ITER requirement [3]. In order to prevent the grains of the CuCrZr alloy from coarsening in the process of FW fabrication, the preparation of as-received CuCrZr has been improved. Additionally, the manufacturing technology of FW has been changed, and alternative explosive welding has been adopted for CuCrZr and SS bonding instead of HIPing at a higher temperature [4]. To investigate the behavior of optimized CuCrZr in FW fabrication, the two abovementioned fabricating technologies have been simulated and compared. Several CuCrZr samples were taken to experience two series of thermal cycles, and the microstructure and mechanical property change of CuCrZr were inspected and analyzed in this experiment. Some reference data could be gained from the results to optimize the CuCrZr alloy preparation and FW fabrication. 2 Experimental procedures An ITER-Grade optimized CuCrZr alloy was adopted in this experiment. The chemical composition of CuCrZr is: 0.71%Cr, 0.088%Zr and %O. The preparation of as-received CuCrZr includes forging (upsetting), hot rolling, solution annealing at 980 Cfor 0.5 h with water quench and then cold rolling into a 1500 mm (length) 300 mm (width) 12 mm (thick- supported by the International Nuclear Thermonuclear Experimental Reactor (ITER) Specific Program of China (No. 2014GB126000) 887

2 ness) plate, and lastly, aging the CuCrZr plate at 475 C for 3 h according to ITER specification. The plate samplesizeis20mm 20 mm 12 mm (plate thickness). The samples were divided into two groups to experience the simulated thermal cycles corresponding to the two fabrication technologies of FW. The 1# thermal cycle includes annealing at 980 C for 2 h with Ar gas cooling to simulate the CuCrZr/SS HIP process, then solid solution annealing at 980 C for 30 min with Ar gas cooling, and at last aging at 580 C for 2 h with Ar gas cooling to simulate the Be/CuCrZr HIP process. The 2# thermal cycle includes only the last two heat treatment steps of the 1# thermal cycle. After grinding, polishing and etching, the surfaces of the samples were observed with an optical microscope. After that, the observed surface was grinded and polished again for the hardness test. In accordance with the ITER requirement, hardness and tensile tests were performed, respectively. The Vickers hardness (HV) test was performed with a load of 1.96 N and a duration of 30 s. The tensile test was carried out in a high vacuum environment. Three samples were tested at 250 C, the force and test temperature fluctuated in ±0.5% and ± 5 C, respectively. The tensile rate was 0.5 mm/min when the strain was below 2% and it was 2 mm/min when the strain was above 2%. annealing at 980 C for 30 min with gas cooling. 3 Results and discussion 3.1 1# thermal cycle: Simulating the fabrication of FW with the CuCrZr and SS joined by HIP at a higher temperature It can be seen in Fig. 1 that the grains of the asreceived CuCrZr sample are elongated along the rolling direction, squashed, as fibers [5]. Although they recrystallized partly after aging annealing, the microstructure still kept significant deformation from cold rolling as a result of the lower aging temperature [6]. There are a large number of structural defects such as dislocations and vacancies. The second phase precipitated in the as-received CuCrZr alloy [7]. As a result, the hardness of the as-received sample reached about 166. It is shown that the sample s fibrous structures from cold rolling disappeared and the grains changed into equiaxed ones after annealing at 980 Cfor2hwith gas cooling. The average grain size was about 100 μm, and the hardness reduced to about 55 when the work hardening was weakened or even eliminated by recrystallizing. Additionally, the aging precipitates and is reconstituted into the solid solution [8]. The grain size almost remains stable at a certain temperature, and a grain coarsening temperature exists the grains would dramatically coarsen when it is exceeded [9]. Consequently, there is little change in the average grain size of the sample annealed at 980 Cfor 2 h with gas cooling and after subsequent solid solution Fig.1 Microstructures of the samples in 1# thermal cycle, (a) As-received CuCrZr, (b) CuCrZr annealed at 980 Cfor 2 h with gas cooling, (c) CuCrZr annealed at 980 Cfor2 h with gas cooling followed by solution annealing at 980 C for 30 min with gas cooling, (d) CuCrZr annealed at 980 C for 2 h with gas cooling followed by solution annealing at 980 C for 30 min with gas cooling, then aged at 580 Cfor 2 h with gas cooling Thesamplesinthe1#thermalcyclehavealmostthe same average grain size, but on account of age strength- 888

3 LIU Danhua et al.: Effect of the ITER FW Manufacturing Process on the Microstructure and Properties ening, the hardness of the sample that has experienced the whole 1# thermal cycle rebounded to about 110, whereas the hardness of the samples without aging treatment was just 55 or so, as shown in Table 1 and Fig. 2. In the process of aging at 580 C for 2 h with gas cooling, there were a large number of dispersed second phase particles separated out from the supersaturated solid solution, uniformly dispersed. The fine particles could effectively prevent the movement of grain boundaries and dislocations [10], and exert an obvious dispersion strengthening effect; in this way the hardness of the CuCrZr alloy could be significantly improved. A study has shown that [11] there are some Cr phases and Cu, Zr compound phases generated from a CuCrZr alloy base in the aging process. The precipitates of Cr and Zr become much finer, and meanwhile their shape changes from sheet into granular as a result of the interaction between Cr and Zr atoms, and consequently the material strength could be significantly improved. The tensile yield strength at 250 C of the samples that experienced the whole 1# thermal cycle is about 170 MPa and the elongation is 30%, as shown in Fig. 3. Fig.2 Vickers hardness trend of CuCrZr alloy in 1# thermal cycle Fig.3 The stress-strain curve at 250 C of the CuCrZr alloy experiencing the whole 1# thermal cycle It can be seen in Fig. 1 that the grains of the CuCrZr alloy are uniform, and there are no abnormally large ones after annealing at 980 C. Additionally, higher hardness, tensile yield strength and elongation were obtained after the simulated thermal cycle. This has confirmed that a deformation-aging heat treatment process could improve not only the strength and plasticity of the alloy, but also the thermal stability of its microstructure [12]. The SEM and EDS results of the samples microstructures in 1# thermal cycle are shown in Fig. 4. Some precipitates dispersed in each sample, and there seems to be no relationship between the number or shape of the precipitates and the hardness of the samples. It shows that the hardness variation of the Cu- CrZr alloy in the thermal cycle does not mainly depend on the precipitates. This phenomenon just verifies the previous research by Batra I S et al [13]. They believe that there are two kinds of dispersed phases in the Cu- 0.8Cr-0.008Zr alloy, ones are coarse Cr phases, which would not redissolve in solid solution annealing, while the others are nanoscale precipitates generated from a supersaturated solid solution. The latter s precipitation sequence is: Supersaturated solid solution Rich solid solution atoms region Metastable and ordered fcc phases Ordered bcc precipitates. The EDS results of the CuCrZr samples in the 1# thermal cycle suggested that the precipitates observed in SEM are almost coarse Cr phases. It seems that they have no contribution to the hardness change of the CuCrZr alloy in the 1# thermal cycle. So the main factor that causes CuCrZr alloy age strengthening in this experiment should be the nanoscale precipitates mentioned by Batra I S et al # thermal cycle: Simulating the fabrication of FW with the CuCrZr and SS joined by explosive welding In the 2# thermal cycle, the CuCrZr alloy was also recrystallized in solid solution annealing at 980 Cfor 30 min with gas cooling to eliminate the interface deformation caused by explosive welding and prepared for subsequent aging. After that, the average grain size was about 100 μm. Meanwhile, the dislocations caused by cold rolling and work hardening were also reduced or even disappeared. As the aging precipitates redissolved, the age strengthening effect was also weakened. Therefore, the hardness of the CuCrZr alloy reduced from 166 to about 55 after solid solution annealing, as shown in Table 2. Table 1. Hardness of the samples in 1# thermal cycle Heat As-received 980 C/2 h 980 C/2 h (Gas cooling) 980 C/2 h (Gas cooling) treatment (Gas cooling) +980 C/0.5 h C/0.5 h (Gas cooling) (Gas cooling) C/2 h (Gas cooling) HV

4 (a) (b) (c) (d) Fig.4 Microstructure (SEM) and energy spectrum of the samples in the 1# thermal cycle, (a) As-received CuCrZr, (b) CuCrZr annealed at 980 C for 2 h with gas cooling, (c) CuCrZr annealed at 980 C for 2 h with gas cooling and solution annealed at 980 C for 30 min with gas cooling, (d) CuCrZr annealed at 980 C for 2 h with gas cooling and solution annealed at 980 C for 30 min with gas cooling, then aged at 580 C for 2 h with gas cooling Table 2. Hardness of the samples in 2# thermal cycle Heat As 980 C/0.5 h 980 C/0.5 h treatment -received (Gas cooling) (Gas cooling) C/2 h (Gas cooling) HV It is shown in Fig. 5 that the average grain size of the CuCrZr alloy almost remains the same after aging at 580 C for 2 h with gas cooling. However, there were a large number of second phase particles generated from the supersaturated solid solution, and they dispersed in the matrix after this heat treatment. As could be found in the result shown in Table 2, Figs. 6 and 7, the hardness of the CuCrZr alloy also rebounded to about 110 and the tensile yield strength at 250 C reached 170 MPa or so. The SEM and EDS results of the samples in 2# thermal cycle are shown in Fig. 8. They are similar to the results of the samples in 1# thermal cycle. There are also some precipitates in each sample in the 2# thermal cycle. The particles are still the Cr-rich phases and there is no obvious difference as compared with the ones in the samples in 1# thermal cycle, either in quantity or shape. It proves once again that the age strengthening of the CuCrZr alloy does not depend on these 890

5 LIU Danhua et al.: Effect of the ITER FW Manufacturing Process on the Microstructure and Properties Cr-rich phases, and the nanoscale precipitates should be the contributors. (a) Fig.5 Microstructure of the samples in the 2# thermal cycle, (a) CuCrZr annealed at 980 C for 30 min with gas cooling, (b) CuCrZr annealed at 980 C for 30 min with gas cooling, then aged at 580 C for 2 h with gas cooling (b) Fig.6 Vickers hardness trend of the samples in the 2# thermal cycle Fig.8 Microstructure (SEM) and energy spectrum of the samples in 2# thermal cycle (a) CuCrZr annealed at 980 C for 30 min with gas cooling, (b) CuCrZr annealed at 980 C for 30 min with gas cooling, then aged at 580 Cfor2h with gas cooling 4 Conclusion Fig.7 The stress-strain curve at 250 C of the CuCrZr alloy experiencing the whole 2# thermal cycle The as-received CuCrZr alloy was prepared by forging, hot-rolling, solution heat treatment, cold rolling and then aging. The average grain size of this material could be controlled to 100 μm bothin1#and 2# thermal cycles which simulate two different fabrication technologies of ITER FW. Meanwhile, the final harness could be up to about 110, and the ultimate tensile yield strengths at 250 C in the two thermal cycles both reached 170 MPa or so. All of them could meet the ITER requirement. 891

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