Study On The Internal Friction And Magnetic Non-destructive Testing Method Of DBTT For Metal Materials

With the development of nuclear energy technology,the issue of nuclear safety is becoming more and more important.The extreme environment of nuclear reactors will cause the hardening,embrittlement,etc.of nuclear materials,especially plasma-facing first wall materials and structural materials.Real-time monitoring of its comprehensive performance is the key to ensuring nuclear safety.The ductile-to-brittle transition temperature(DBTT)is one of the important indicators to measure the embrittlement of materials.Therefore,it is imperative to explore a non-destructive testing method,especially for the safety and life assessment of advanced nuclear energy devices.In view of the fact that there is no mature and reliable nondestructive technology for testing of DBTT,in this paper we select W-0.5wt.%ZrC alloy and martensite steel,which are candidate materials for plasma first wall material and nuclear structural material,to explore and in-depth study of DBTT nondestructive testing methods.In this section,a newly developed testing technique based on the amplitude-dependent internal friction(ADIF)was developed to determine the DBTT of W-0.5wt%ZrC alloy.First of all,1 mm thick W-0.5wt.%ZrC alloy plates were prepared by mechanical milling,hot pressing sintering and multistep hot and cold rolling.Then the tensile test and internal friction test were carried out on W-0.5 wt.%ZrC alloy at different temperatures by tensile test method and the large-strain internal friction test method,respectively.On the one hand,the analysis of tensile result showed that DBTT of the W-0.5 wt.%ZrC alloy was between 50 and 80?,and the specimen had higher tensile strength at both 80? and 400?,which are about 1413 MPa and 900 MPa,respectively.On the other hand,as the temperature increasing,the change trend of the critical amplitude transition point(?c)obtained by ADIF is consistence with the yield point obtained by the tensile test.And both of them began to appear at the temperature of 80 ?,indicating that the DBTT obtained by ADIF technique is in the range of 50 to 80 ?.In addition,the ADIF fracture interface of the test specimen characterized by SEM begins to show some plastic deformation at 80?.This phenomenon further confirmed the feasibility and accuracy of the ADIF technique in determining the DBTT of the materials.By subsequent microstructural characterization,combined with the preparation process of the W-0.5wt.%ZrC alloy,the effect of microstructure such as dislocation density and distribution,grain size as well as dispersed particles size and distribution were analyzed and explored on the relative low DBTT,the high strength and the ADIF properties of the W-0.5wt%ZrC alloy.In the second section,a non-destructive testing method based on the magnetic testing technique was developed and used to determine the ductile-to-brittle transition temperature(DBTT)of martensitic steel.On the one hand,martensitic steels such as F82H,EUROFER97,SCRAM,T91 and rolling T91 were tested with the Charpy impact test,and DBTT of each sample was obtained.On the other hand,to study the magnetic property of these steels,the coercivity(HC)and magnetization(M)in a small magnetic field were measured at different temperatures.After the analysis of results,it was found that the change trends of ln(HC)-T and d(lnM)/d(1/T)-T changed after a critical temperature point(TC),and linear approximately increase with the temperature above and below the TC.The TC of each martensitic steel corresponds to their own DBTT respectively.The TCs of F82H,EUROFER97,SCRAM,T91 and rolling T91 martensite steels were 189,190,221,215 and 271 K,respectively,which corresponding to their respective DBTTs(187,186,218,217 and 275 K,respectively).These phenomena indicate that the magnetic non-destructive method was repeatable with high sensitivity,and also suitable for specimens with microstructure change and deformation.It is mainly because in brittle region the thermal activity energy of dislocations is lower and interaction energy between dislocations and magnetic domain walls is higher than that of ductile region,hence,it is difficult for domain walls to move.The results indicate that using the relationship between magnetic properties and mechanical properties to detect DBTT is a promising candidate method for DBTT detection.The establishment of nondestructive testing technology for DBTT will provide a fast and effective detection methods for the hardening,embrittlement and DBTT changes of reactor structural materials,and will also provide a technical support for the rapid measurement of DBTT of other materials.