Papers by Keyword: Deformation Mechanism

Abstract: In order to control the ratio of nano/ultrafine structure grains of warm/cold rolled 316L stainless steel after annealing, the influence of deformation amount and temperature on martensite content and microstructure was investigated, and a model of the content of stain-induced martensite and deformation amount and temperature was established. Results showed that the content of stain-induced martensite was nonlinear with deformation amount, but with an incubation period. And it’s generally exponential. Martensitic transition occurred in large deformation stage. 58.23% strain-induced martensite was formed when deformation amount was 80% at 200°C. The content of martensite is the most significant factor that affects austenite grain size after annealing. With more strain-induced martensite, the average austenite grain size decreased, and the uniformity of grain size was improved, which was generally monotonous.

Abstract: The beta-titanium alloys are used mainly in bioapplications for artificial joints and other implants. They posses interesting properties such as, high corrosion resistance, low Young’s modulus, good plasticity or superelasticity etc. In this work the effect of solution treatment temperature on deformation and fracture properties has been studied. The alloy Ti-35Nb-2Zr was processed via powder metallurgy process (cold isostatic pressing, sintering and subsequent swaging). Swaged alloy was annealed at 800, 850, 900, 950 and 1000 °C. Tensile tests have been performed on such heat treated specimens and the fracture surface has been studied in correlation with microstructure. With increasing annealing temperature both tensile strength (from 925 MPa to 990 MPa) and elongation (from 13 to 25 %) increased where the maximum values were obtained for 900 °C annealed specimens and subsequently slight decrease has been observed. The simultaneous increase of strength and elongation was attached to change of deformation mechanisms which was described by studying fracture surfaces and microstructure of deformed (tensile tested) specimens.

Abstract: The hot deformation behavior of FGH720Li superalloy was investigated by hot compressive tests on Gleeble-1500D thermal simulation test machine in different temperatures and strain rates. The true stress-strain curves were obtained, and based on the deformation data, the constitutive equation of FGH720Li superalloy was built. The Deformation Active Energy of FGH720Li was determined to be Q=787.6KJ/mol. The main deformation modes were dislocation glide and twinning. At the beginning of the deformation, a large number of dislocations generated, glided and scrambled in the alloy, then entwisted to form dislocation cells, which were the recrystallization nucleus. At the later period of the deformation, the dislocation would rotate to easy glide direction through twinning, inducing that the deformation of the alloy ensued. At the same time, the deformation had significant effect on grain refinement and the crushed of the primary particle boundary.

Abstract: By means of creep curves measurement and diffraction contrast analysis of dislocation configurations, the creep behavior and effect factors of a Ru-free nickel-based single crystal superalloy within the temperature range of 740~780°C are investigated. Results indicated that, the cubical γ′ phase transformed into the bunch-like structure. During creep at 760°C/750Mpa, no rafted features of γ′ phase is detected, but the degree of distortion of the cubic γ′ phase near fracture regions is increased. During creep, more super-dislocations shearing into γ′ phase of Ru-free alloy are decomposed on {111} plane to form the partials plus stacking fault, which is attributed to the interaction of the elements to decrease the stacking fault energy of the alloy. Moreover, the screw dislocations shearing into γ' phase of Ru-free alloy during creep at 760 °C may cross-slip from {111} to {100} planes for decomposing to form the configuration of the K-W locks, which is attributed to the higher alloying extent for decreasing the diffusion rate of elements. And the dislocation configuration may restrain the slipping of them to improve the creep resistance of alloy due to the non-plane core structure of them.

Abstract: By means of creep properties measurement and microstructure observations, the effect of microstructure evolution on creep strength of the single crystal Ni-based alloy with different orientations is investigated. Results show that, during creep at 1040 oC/137 MPa, the γ′ phase in [001]-oriented alloy is transformed into mesh-like N-type rafted structure, the one in [011]-oriented alloy is transformed into the stripe-like rafted structure along [001] direction, the one in [111]-oriented alloy is transformed into the mesh-like rafted structure along (010) plane. The γ′ phase with different configurations results in the different creep resistance of alloys, the sequence of creep resistance of alloys with different orientations are determined to be σ_[001] > σ_[111] > σ_[011]. But after pre-compressed treated along [100] direction of [011] oriented alloy, the γ′ phase in [011] oriented alloy is transformed into the stripe-like rafted structure along [100] direction, which makes the creep life of the alloy enhancing from 11 h to 174 h at 850 oC/400 MPa, and from 74 h to 165 h at 1040 oC/137 MPa. This is attributed to the pre-treatment eliminating the gable channels to form the “labyrinth” structure of γ′ phase, which may increase the resistance of dislocation motion.

Abstract: Mechanical properties of extruded WZ72 magnesium alloy with long-period stacking ordered (LPSO) phase were investigated during compression loading at room temperature and at a constant strain rate of 10^-3 s^-1. The samples of (8 x 4 x 4) mm³ were compressed along three directions with respect to the lamellar LPSO-phase: parallel (ED), perpendicular (TD) and under 45°. Concurrently with the deformation tests, the acoustic emission (AE) response of the specimens was recorded. The AE measurements revealed that both the twinning activity and the kinking of the LPSO phase significantly depend on the orientation of LPSO phase. The highest strength was observed for the sample which was compressed parallel to the LPSO phase (extrusion direction). The highest AE activity was also measured in this sample.

Abstract: The occurrence of size effects in cobalt was examined by the analysis of mechanical properties of samples with thickness t, in a large range of grain size d giving a number of grains across the thickness t/d. On Hall-Petch plots, from the very beginning of plastic strain, two linear behaviors are notable: the polycrystalline one for higher t/d and the multicrystalline one for lower t/d in which the flow stress is strongly reduced. (t/d)_c is the threshold value between the two behaviors taking a value of around 14. This high value is directly linked to the low stacking fault energy of cobalt. The microstructure of the polycrystalline samples exhibits a strong basal texture and a small proportion of a secondary face-centered cubic phase in a hexagonal close-packed main phase was evidenced. TEM analysis enables to characterize the dislocations and the stacking faults present in the two phases. To complete the analysis, two plasticity stages can be distinguished: stage A corresponding to dislocations gliding and stage B driven by twinning. Size effects in cobalt are found to occur during gliding process and could be related to surface effects as previously shown in face-centered cubic metals.

Abstract: The powder sintering process is adopted for preparing the porous Cu-35Ni-15Al alloy.The microstructure was observed by using scanning electron microscopy. Mechanical properties were tested and compared at different temperatures and stress conditions. This thesis also studied the impact of stress on creep properties of Cu-35Ni-15Al alloy,The results show there are roughly three deformation stages of high temperature compression deformation.The microscopic deformation was mainly the curved, collapsed and folded holes.The creep resistance performance of porous Cu-35Ni-15Al alloys weakens with the increasing pressure.

Abstract: The domains of the existence of deformation mechanisms in a map associated with phase transformation and mechanical effects related to aging processes were investigated in austenitic stainless steels. It was also discussed the participation of grain boundary sliding, both as an additional deformation mechanism and a damage accumulation process. A prediction analysis for two typical high temperature engineering systems was attempted based on the map information. This prediction indicates the possibility of grain boundary sliding and creep strain jumps to interfere with the expected operational life of components in these systems operating at high temperatures.

Abstract: High efficiency and high speed are the development directions of modern manufacturing technologies. In the last two decades, high speed machining is successfully applied in cutting steel and alumina alloy, due to its unique advantages. However, it is not yet prevalent in powder metallurgy (PM) superalloy machining. This work focuses on the shear angles and influencing rules in high speed machining PM nickel based superalloy, in order to provide reliable theoretical and practical methods in high efficiency/speed machining this kind of material in production practice.