高强度铍铜合金QBe2 棒材室温旋转弯曲疲劳行为

Beryllium copper alloys are widely used in aerospace, electronics and oil drilling industries because of their excellent properties such as high strength, high elasticity, high fatigue limit, stress relaxation resistance, good electrical and thermal conductivity, non-magnetism, and no spark on impact. As beryllium copper alloy is a typical precipitation strengthened copper alloy, proper heat treatment is needed to further improve the mechanical properties of QBe2 alloy. Solution annealing (A), solution annealing+aging heat treatment (AT, solution annealing+cold deformation+aging heat treatment (hard and standard age heat treatment (HT)) are common heat treatment methods for QBe2 alloy. Solution annealing (A) can obtain supersaturated solid solution α phase (Be solid solution in Cu, face centered cubic lattice, with good plasticity)through solution treatment, which can be used as both softening annealing and aging treatment preparation stage. Solution annealing+aging heat treatment (AT) can achieve the effect of precipitation strengthening by precipitation of uniformly dispersed γ-strengthening phase (solid solution based on electronic compound CuBe, with ordered body centered cubic lattice, hard and brittle). Solution annealing+cold deformation+aging heat treatment (HT)can evenly precipitate γ-strengthening phase on the basis of deformation strengthening, further improve the strength, elastic limit and fatigue strength of the alloy, and obtain better strengthening effect. More importantly, beryllium copper alloy is an important elastic element material in aviation instrumentation, it needs to experience hundreds of times of repeated operation in the use process, so the reliability of components depends on the fatigue performance of beryllium copper alloy material to a large extent. The rotational bending fatigue can well simulate the complex stress state of the material under the actual service condition, and the good fatigue performance depends on the high tensile strength and the specified plastic elongation strength. Therefore, it is necessary to study the rotational bending fatigue behavior of high strength beryllium copper alloy QBe2 at room temperature. The rotational bending fatigue (RBF)behavior of beryllium copper alloy QBe2 with different stress amplitudes where the stress ratio of specimen R=-1 and specimens were rotated at 5000 r∙ min^-1 (80 Hz)was studied. The stress-life (S-N) curve was drawn by fitting the relationship between loading stress and fatigue cycle times with the experimental data. And the fatigue fracture surface morphology of the specimens were observed by scanning electron microscope ( SEM)and energy dispersive spectrometer (EDS). The results showed that the hardening effect of beryllium copper alloy QBe2 rod of HT temper (hard and standard age heat treatment)was very significant. The tensile strength was increased by about 73%, the R_m was up to 1342~1368 MPa, the specified plastic elongation strength was increased by about 125%, and the R_p0.2 was up to 1303~1331 MPa. But the fatigue limit stress of the alloy under 1×10⁷ fatigue life was only 444.3 MPa, far less than 0.27 (R_p0.2+R_m), of just about 0.3Rm. And the trend of the stress-life S-N curve was that the fatigue life increases gradually with the decrease of the stress level. When the fatigue life approached 1×10⁷ cycles, the stress decreased with the increase of the number of cycles, and the curve gradually changed gently and tended to level. Meanwhile, the functional expression of S-N curve with 50% failure probability was obtained as lnN=10.6068-0.0085S_max (R²=0.8619) by using linear fitting. In addition, combined with the fracture analysis, it was found that the rotating-bending fatigue cracks of the alloy rod at room temperature under different stresses of 440~600 MPa were all generated at the surface defects of the samples, and the macroscopic fracture surface was not smooth, and the radial plastic fatigue groove line extending from the fatigue crack initiation zone (i.e., crack source)to the crack propagation direction could be seen, which could be divided into three parts：crack initiation zone, crack propagation zone (steady-state crack propagation zone and crack instability propagation zone)and instantaneous fracture zone. The area ratio of fatigue initiation zone and crack propagation zone was more than 80%. The surface of the instantaneous fracture zone was slightly rough and irregular. With the increase of stress amplitude, the area of the instantaneous fracture zone increased and moved toward the center of the section. There were different degrees of second phase and inclusion particles in different areas of the fatigue fracture surface, and the crack originated from the boundary between the particles near the surface and the matrix. Therefore, it could be concluded that the mechanism of fatigue fracture of QBe2 alloy rod was that the second phase and inclusion particles near the surface of the sample led to the initiation of cracks in this weak spot. It could also be said that if the metallurgical quality of the alloy could be further improved (such as the number and size of β second phase and inclusion particles could be reduced), the fatigue resistance of the alloy would be improved positively.