Materials Science Forum Vol. 879

Abstract: In this paper, effects of initial micro-structures on deformation behaviors of commercial pure titanium were elaborated by investigating the evolution of dislocation boundary and its adiabatic shear sensitivity. At the low to medium stain rates, the main plastic deformation mechanism of as-annealed commercial pure titanium is dislocation slipping. Meanwhile, geometrically necessary boundaries (GNBs) with different directions are generated and crossed with each other. However, new dislocation boundaries are formed in as-cold rolled plates, which are parallel to the initial ones induced by cold rolling. When the strain rate is up to 1000 s^-1, the initial dislocation boundary playes an adverse role in the adiabatic shear sensitivity of commercial pure titanium. The adiabatic shear band is the high-speed deformation characteristic micro-structure in commercial pure titanium. In addition, dynamic recrystallized grains are generated in the center of an adiabatic shear band, which is consistent with the sub-grain rotation mechanism.

Abstract: The microstructural evolution in annealed Mg-3Al-1Zn (AZ31) magnesium alloy during high density electric current pulses (ECP) treatment is investigated by using a same current density with different processing numbers. It is found when the processing number of the ECP treatment is not greater than four times, the grains are refined and more homogenized, and the texture intensity obtained from the (0002) pole figure appears an obvious enhancement from 11.22 to 22.88. However, increasing the repeated ECP processing number will cause the coarsening of grain size and the decreasing of the texture intensity. The mechanism of the microstructural evolution during ECP treatment is discussed from the point of view of grain boundary motion.

Abstract: In this paper, the martensitic transformation temperature, the microstructure and the crystal structure of the complicated martensitic phases of Ni_56-xFe₁₉Ga₂₅Co_x (x =0, 1.5, 3, 4.5, 6) alloys were investigated by DSC, XRD, SEM and TEM techniques. DSC results show that the martensitic transformation temperature T_m, which is above the room temperature, decreases with the increasing Co content. The microstructure of the Ni_56-xFe₁₉Ga₂₅Co_x (x =0, 1.5, 3, 4.5, 6) alloys is composed by the martensitic lath and randomly distributed γ phase. The 6M+14M mixed modulated martensite and the γ second phase were detected in the Ni₅₃Fe₁₉Ga₂₅Co₃ alloy by XRD and TEM tests.

Abstract: We describe here the relationship between electron microscopy and mechanical property studies in industrially processed titanium bearing microalloyed steel plates that involved processing using the recently developed ultrafast cooling (UFC) approach. Given that the segregation of manganese is generally responsible for microstructural banding in low-alloy steels, which can deteriorate the tensile property in the direction of thickness, the manganese-content was reduced by ~0.6-0.8% with the objective to obtain uniform microstructure across the thickness of the steel plate. Besides, non-uniform distribution of accelerated cooling along the thickness direction also leads to inhomogeneous microstructure across the plate thickness. In order to obtain near-uniform microstructure and similar mechanical properties from the surface to the center of plate, fast and effective cooling process is necessary. In this regard, refined and uniform microstructure that was free of microstructural banding was obtained via UFC process across the plate thickness, with strict control and faster cooling rate on the run-out table. Furthermore, grain refinement and random precipitation in the ferrite matrix contributed ~100 MPa toward yield strength. The study underscores the potential of processing medium and heavy plates of titanium bearing microalloyed steels plates with uniform and refined microstructure across the thickness via thermo-mechanical controlled processing (TMCP) involving UFC.

Abstract: Linear friction welding (LFW), an emerging automated technology, has potential for solid-state joining of dissimilar materials (bi-metals) to enable tailoring of the mechanical performance, whilst limiting the assembly weight for increased fuel efficiency. However, bi-metallic welds are quite difficult to manufacture, especially when the material combinations can lead to the formation of intermetallic (brittle) phases at the interface, such as the case with assembly of Ti base alloys with Ni base superalloys. The intermetallic phase, once formed, lowers the performance of the as-manufactured properties and its growth during elevated temperature service can lead to unreliable performance. In this project, it was demonstrated that linear friction welding can be applied to join Ti-6%Al-4%V (workhorse Ti alloy) to INCONEL^® 718 (workhorse Ni-base superalloy) with minimized interaction at the interface. Of particular merit is that no intermediate layer (between the Ti alloy and Ni-base superalloy) was needed for bonding. Characterization of the bi-metallic weld included macro-and microstructural examination of the flash and interface regions and evaluation of the hardness.

Abstract: The paper deals with the investigation of microstructure and mechanical properties of electron beam welded joints of high strength steel grades S690QL and S960QL in quenched and tempered condition. The microstructure of base metal was composed of bainite and martensite mixture at hardness about 270HV10 and 340HV10 for the S690QL and S960QL steels, respectively. The weldment was composed of several characteristic subzones revealed on the transverse sections. The central region of the weldment consisted mainly of coarse columnar dendritic grains which perpendicular to the fusion zone boundary. The microstructure of the heataffected zone near the fusion line consisted mainly of martensite, however, in both steels the microstructure varied with the distance from the fusion line. The tensile strengths of welded joints were R_m=850 MPa (S690QL) and 1074MPa (S960QL) and corresponded the tensile strengths of the base materials.

Abstract: The saturation of primary tensile twins in heavily textured Mg-alloy AZ31 is investigated, and their strain accommodation limit is evaluated. EBSD results suggest that the mean number of twins per grain saturate rapidly, followed by the stop of twin growth. Twinning saturation is included in a physical model of twin evolution.

Abstract: Flash butt welding (FBW) of railway rails was investigated in this work. For this purpose samples of R260 rail steel and 60E1 profile were instrumented and subsequently welded on a Schlatter GAA 100 welding machine under industrial conditions. The intention is to gain in depth process knowledge by more accurately depicting thermal cycles for an entire welding sequence in the immediate proximity of the weld as well as in the heat affected zone (HAZ). A detailed characterization of the single stages of the heat up phase of the process is important. Additionally, the secondary welding voltage was measured simultaneously during the experiments to characterize the transient heat input. Moreover, these data were used in the analysis of the temperature signals to better cope with electrical interferences. Additionally, a finite element (FE) model of this FBW process was developed in the present work. Its implementation and solution is realized with the help of ESI’s FE-software SYSWELD. A strong coupled thermo-electrokinetical and metallurgical calculation routine was used. The model comprises the transition resistance at the welding surfaces as the main heat source to the process. Temperature dependent material properties and a corresponding metallurgical model based on an experimental CCT diagram of the rail steel R350HT are implemented in the simulation as well.

Abstract: Three AISI 1045 steels: a base steel, one modified with vanadium (V), and one modified with V and niobium (Nb) were studied to evaluate microstructural conditioning prior to induction hardening. Simulated bar rolling histories were evaluated using fixed-end hot torsion tests with a Gleeble® 3500. The effects of chemical composition and thermomechanical treatment on final microstructures were examined through analysis of laboratory simulations of steel bar rolling and induction hardening processes in order to provide additional insights into the morphological evolution of austenite of microalloyed steels. Analysis of prior austenite grain size (PAGS) is complemented with analysis of austenite recrystallization and pancaking during rolling. The potential for utilizing TMP, in conjunction with microalloy additions, to enhance bar steel microstructures and subsequent performance is assessed by evaluating the induction hardening response of each steel systematically processed with different preconditioning treatments.

Abstract: Welding of thick walled components with an electron beam has great potential due to the minimal heat input, high reproducibility and cost-efficiency. In the present work electron beam welding was used to weld 50mm thick plates of cast Ni-base alloy A625 to ferritic/martensitic 9% Cr steel plates. The welds were creep exposed at 625°C with stress levels ranging between 156 - 100MPa. Microstructure analysis of the weld-seam and the heat affected zone was carried out using metallography and scanning electron microscopy employing the EBSD technique to determine the location of the creep rupture. Creep fracture is located in the heat affected zone of the 9% Cr steel. Electron beam welded samples were compared to shield metal arc welded samples regarding welding and creep resistance. The performance and related microstructure properties of the electron beam welded specimen are more than competitive to conventional metal-arc-welding procedures.