Effects Of Micro-Alloying Elements On Microdefects And D Electrons In Ni-Cr-Co Superalloys

By means of the characteristics of positron annihilation, a systemwith 2 high purity Ge detectors and 2-dimension multichannel analyserfor measuring coincidence Doppler broadening spectra of positronannihilation radiation has been built in this work. With this system, thebackground of Doppler broadening spectrum of positron annihilationradiation are reduced remarkably. By using this system and positronannihilation lifetime spectroscopy, the distribution of electron momentumin pure metals, the microdefects of Ni-Co-Cr superalloys in the recoveryand recrystallization process and the effect of P on electron density andmicrodefect in Ni-Co-Cr superalloys have been studied.(1) The background of Doppler broadening spectrum of positronannihilation radiation can be reduced by using coincidence technique.Doppler broadening spectrum presents information about themomentum distribution of the annihilating positron-electron pair. In orderto obtain the signal of core electron with high momentum in an atom, it is necessary to reduce the background of Doppler broadening spectrum ofpositron annihilation radiation.In the conventional single-detector Doppler technique, with apeak-to-background ratio of about 200, the intensity of thehigh-momentum annihilation quanta is comparable to the background,which makes it difficult to make any conclusions on the shapes andmagnitudes of the Doppler curve.In a Ge-NaI detector system, we have used a NaI scintillationdetector in a coincidence with a HP Ge detector. The Ge detector is usedto detect the annihilationγspectrum. The NaI detector is placed incollinear geometry with the Ge detector in order to detect the two 511keVy rays from the e⁺-e^- annihilation. When pulses from both detectors arrivein coincidence, the pulse from the Ge detector is recorded in the memoryof a multichannel analyzer (MCA). In this experiment the background isremoved from the high-energy side of the 511-keV peak, but in thelow-energy side the background largely remains due to the Comptonscattering of the 511-keVγrays. With this setup, we can get apeak-to-background ratio of about 104on the high energy side of the peak.In order to reduce the background further, a system with 2 highpurity Ge detectors and 2-dimension multichannel analyser has been built.For every coincident event, the energies of both gamma rays (denoted byE₁ and E₂) are registered in two high purity Ge detectors arranged at 180° to each other. These energies form the horizontal and vertical axes, andthe count corresponding to each E₁ and E₂ combination depending ontheir absolute values. There is an intense central peak centered at E₁=E₂=511 keV, which corresponds to annihilations with the valence electrons.On either side of this central peak, the spectrum shows horizontal andvertical bands. The elliptical region extending diagonally with E₁+E₂2m₀C²=1022 keV (E₁=m₀c²-pc/2 and E₂=m₀c²+pc/2) originates fromannihilations with high momentum electrons. A cut along the diagonalcan then be analyzed to observe variations in shape due to thecontribution of core electrons. With this system, we can get apeak-to-background ratio of higher than 10⁵.(2) In analyzing the Doppler broadening spectra of positronannihilation radiation measured by using an improved two-HP Gedetectors coincidence system, the 3d electron signal for the elements ofthe fourth period can be extracted. The experimental results show that thepeaks in the ratio curves moved to the direction of high-momentum, andthe average electrons momentum of different elements increased with theincreasing of atomic number. The heights of the peaks in the ratio curvesincrease with an increasing in 3d electrons.(3) The Doppler broadening spectra of cold-rolled Ni-Cr-Co alloysafter annealing at 530℃or 570℃) for different time, indicate that thedensity of microdefects decrease with the increase of annealing time, and the probability of positron-3d electron annihilations increase with theincrease of annealing time. It also has a similar effect by increasing theannealing temperature.(4) The measurements of Doppler broadening spectra of theNi-Cr-Co alloys with different contents of phosphor have been shownthat P segregate to grain boundaries, and reduce the energy of grainboundaries. The probability of positron-3d electron annihilationsincreases with the increase of P content, for 3p electrons of P atoms and3d electrons of Ni-Cr-Co alloys are localized to form strong covalentbonds due to strong 3d- 3p hybridization. Positron lifetime results havebeen shown that the free electron densities in bulk and grain boundary ofNi-Cr-Co superalloys decrease with the increase of P content. Thesegregation of P may form strong covalent bonds in the face of grainboundaries, while decrease cohesion of metallic bonds along upright theface of grain boundaries. This may decrease the cohesion of the grainboundaries.