Brazing of Be with CuCrZr-bronze using copper-based filler metal STEMET

At the D.V. Efremov Scientific Research Institute of Electrophysical Apparatus (SRIEA), elements of the ITER first wall (beryllium tiles and heat removal CuCrZr-bronze) are currently joined by ``fast'' brazing (heating with a rate of 150K/min) using the method of passing a current through brazed samples. For this purpose, rapidly quenched brazing filler metal STEMET 1101 is used. The thermal cycle of brazing consists of a fast heating of the assembly to a temperature of 700–720°C and a subsequent cooling (rate of 60K/min). However at these temperatures, processes resulting to a loss of the tensile and shear strength of CuCrZr can occur. Therefore, reducing the brazing temperature is a very important task. Optimization of the composition of the Cu–Ni–Sn–P system filler metals and comparative tests of filler metals of various compositions have been carried out in this paper to reduce the brazing temperature of beryllium with CuCrZr. Alloys of the following compositions Cu–6.4Ni–9.2Sn–6.3P (STEMET 1105) and Cu–9.1Ni–3.6Sn–8.0P (STEMET 1101) were made in the form of rapidly quenched ribbons with a thickness of 50µm and a width of 50mm. They were used to perform furnace brazing by Joule heating (with a rate of 15K/min) of beryllium with CuCrZr (Be/CuCrZr) at temperatures of 650, 700 and 750°C for 15min. Metallographic investigations of the zone of brazing and mechanical shear tests of joints before and after the heat treatment at 350°C for 30h have been conducted. It was found that the joints of Be/CuCrZr brazed at 650°C using STEMET 1105 (τs=230MPa) and at 750°C using STEMET 1101 (τs=260MPa) had the best shear strength properties. However, there is a significant decrease of the microhardness of CuCrZr from 1570 to 1140MPa at 750°C, which indicates a significant loss of its strength. The results obtained suggest that the brazing of beryllium with CuCrZr using STEMET 1105 at 650–700°C will not adversely affect the CuCrZr

Optimization of mechanical alloying and spark-plasma sintering regimes to obtain ferrite–martensitic ODS steel

The results of structure investigation, distribution uniformity of dispersed particles of Y2O3, porosity and density of the ferritic/martensitic reactor steel EP-450 (0.12C–13Cr–2Mo–Nb–V–B, wt%) produced by spark-plasma sintering (SPS) are presented. More than 140 samples were produced using different combinations of mechanical alloying (time, speed of attritor rotation) and SPS parameters (temperature, speed of reaching preset temperature, pressure and time of exposure under pressure, concentration of strengthening particles). It is determined that the absence of strengthening Y2O3 nano-particles in local volumes of sintered specimens is connected with the imperfection of mechanical alloying, namely, the formation of agglomerates of matrix steel powder containing no oxide nano-particles. It has been determined that the time of mechanical alloying should not exceed 30h to provide minimum powder agglomeration, uniform distribution of Y2O3 particles in the powder mixture and minimum porosity of sintered samples. Spark-plasma sintering should be performed at the lowest possible temperature. As a result it was found that samples with 99% theoretical density can be obtained using the following optimized SPS-parameters: sintering temperature is 1098÷1163K; speed for reaching the preset temperature is >573K/min; load is 70÷80MPa; time of exposure under pressure – either without isothermal exposure, or exposure during ≥3min; optimum quantity of Y2O3 is 0.2÷0.5wt%