Microstructural evolution in hot compressed TiHy 600 titanium alloy

TiHy 600 alloy is a near alpha titanium alloy, widely used for gas turbine engine applications such as disc and blades for high pressure compressors. One drawback of this alloy is that it is susceptible to cold dwell fatigue, which is due to the presence of micro-textured zones. Thus, appropriate processing parameters (i.e. temperature, strain and strain rate) are required to reduce the size of the micro-textured region. In order to find out the optimized processing parameters, hot compression tests were performed up to 50% engineering strain at temperatures range of 900oC-1050oC and strain rate range of 10-3 to 101 s-1 using thermo-mechanical simulator (Gleeble 3800®). Flow behavior characteristics were studied from the data obtained during hot compression and processing map was developed at true strain of 0.6 using Dynamic Materials Modeling (DMM) approach. Microstructural examination of deformed TiHy 600 titanium alloy were carried out at a particular strain rate of 10-3 s-1 and temperatures of 900oC, 950oC, 975oC, 1000oC and 1050oC. Microstructural examination consists of orientation image mapping along compression direction using electron backscatter diffraction. Hot compression mostly resulted into new dynamic recrystallized (DRX) alpha grains at 900oC, mixture of deformed large alpha grains containing subgrain boundaries and transformed beta phase consisting of secondary alpha laths at 950oC and 975oC and alpha laths transformed from deformed beta grains at 1000oC and 1050oC

Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries.

BACKGROUND: As global initiatives increase patient access to surgical treatments, there remains a need to understand the adverse effects of surgery and define appropriate levels of perioperative care. METHODS: We designed a prospective international 7-day cohort study of outcomes following elective adult inpatient surgery in 27 countries. The primary outcome was in-hospital complications. Secondary outcomes were death following a complication (failure to rescue) and death in hospital. Process measures were admission to critical care immediately after surgery or to treat a complication and duration of hospital stay. A single definition of critical care was used for all countries. RESULTS: A total of 474 hospitals in 19 high-, 7 middle- and 1 low-income country were included in the primary analysis. Data included 44 814 patients with a median hospital stay of 4 (range 2-7) days. A total of 7508 patients (16.8%) developed one or more postoperative complication and 207 died (0.5%). The overall mortality among patients who developed complications was 2.8%. Mortality following complications ranged from 2.4% for pulmonary embolism to 43.9% for cardiac arrest. A total of 4360 (9.7%) patients were admitted to a critical care unit as routine immediately after surgery, of whom 2198 (50.4%) developed a complication, with 105 (2.4%) deaths. A total of 1233 patients (16.4%) were admitted to a critical care unit to treat complications, with 119 (9.7%) deaths. Despite lower baseline risk, outcomes were similar in low- and middle-income compared with high-income countries. CONCLUSIONS: Poor patient outcomes are common after inpatient surgery. Global initiatives to increase access to surgical treatments should also address the need for safe perioperative care. STUDY REGISTRATION: ISRCTN5181700

Thermomechanical Processing, Microstructure and Texture Studies of TiHy 600 Alloy (a Near Alpha Titanium Alloy)

The titanium IMI 834 alloy is near α (hcp) titanium alloy, widely utilized for gas turbine engine parts such as compressor disc. It shows good creep and fatigue properties at high temperature (operating temperature: 600 °C). Low dwell fatigue life is identified to cause early failure in titanium based aero engine parts. The main reason for the decreasing dwell fatigue life is associated with the presence of large micro texture regions (macro zones) in near alpha titanium alloys. Hence, the degree of random micro texture with fine grain microstructure should be increased in order to improve the dwell fatigue life of near alpha titanium alloys. Thermomechanical processing is a useful technique to generate different kind of texture and microstructure of titanium alloys. Thermomechanical processing conditions are greatly influence the microstructural evolution and mechanical behavior. The main objective of the current Ph.D. work is to perform the hot deformation at various strain rates (10-3-10s-1) in different phase regions 900 °C (α rich region), 950 °C (α+β region), 975 °C (α+β rich region), 1000° C (rich β region) and 1050 °C (β region) of near alpha titanium alloy (TiHy 600 alloy) up to 50% deformation, mainly to generate its processing maps and to compare it with the generated microstructures and textures. Following targets are identify during the processing of TiHy 600 alloy. • Flow behaviour characteristic studies of hot compressed TiHy 600 alloy using peak stress values and further utilizing the Avrami model to verify the DRX fraction from the experiments. • Developing the processing maps through DMM (Dynamic Material Modeling) approach at various true strain values from 0.3 to 0.6. Correlating the processing map (0.6 true strains) domains with microstructures of related processing parameters (temperature and strain rate). • Evolution of microstructure and texture studies of TiHy 600 alloy at 900 °C (α rich region) with various strain rates (10-3-10 s-1). • Texture analysis of reconstructed parent β (bcc) phase at 1050 °C with various stain rates ((10-3-10 s-1) and its deformation studies through texture. TiHy 600 alloy which is similar to titanium IMI 834 alloy is received in the form of pan cake (12 mm height and 250 mm diameter) from TAG (Titanium Alloy Group) at DMRL Hyderabad (Defense Metallurgical Research Laboratory). It showed bimodal microstructure with primary alpha (αp) (50%) and transformed beta (β) having secondary alpha lamellar (αs) (50%). Hot compression is the best processing method to get the workability conditions of the near alpha titanium alloys with introducing large strains in one step. The thermo-mechanical simulator Gleeble 3800 is utilized for the hot compression of as received TiHy 600 alloy. Flow behaviour characteristics of TiHy 600 alloy is studied by developing the true stress-true strain curves up to 50% deformation at constant strain rates with various phase regions (α region, α+β region, α+β rich region, rich β region and β region). Dynamic recrystallization (DRX) is noticed to be the dominant mechanism at lower strain rate 10-3 s-1 at 900 °C and 975 °C and for 10-1 s-1 strain rate at 1000 °C. Dynamic recovery (DRV) is observed at higher temperature 1050 °C (β region) at strain rates of 10-3-10 s-1. The peak stress increases with decreasing deformation temperatures and increasing strain rates. TiHy 600 alloy material constants (A, α and n of the Arrhenius equation), obtained by using hyperbolic-sine law, are determined from the peak stress values of the flow curves. Activation energy (Q) values of 50% deformed TiHy 600 alloy are calculated at various regions such as α rich region (900 °C), α+β region (950-1000 °C) and β region (1050°C) which are 384 kJ/mol, 307 kJ/mol to 255 kJ/mol and 251 kJ/mol, respectively. The high activation energy of 384 kJ/mol for α region indicates hard hcp phase character which results in undergoing DRX phenomenon. Similarly, low activation energy of 251 kJ/mol for β region indicates soft bcc character which leads to undergoing DRV phenomenon. Predicted peak stress values are consistent with the experimentally obtained peak stress values. According to the Zener-Holloman parameter, the peak flow stress follows the unified constitutive equation observed from the plot of lnZ vs. ln [sinh (ασp)]. The developed Cingara equation is consistent with the experimental stress curves, as verified from the correlation coefficient factor. Dynamic recrystallization (DRX) fraction is calculated from the Avrami equation for 900 °C, 950 °C and 975 °C at 10-3 s-1 strain rate conditions and is compared with the DRX fraction obtained from the EBSD studied microstructures with one example taken is 900 °C, with 10-3 s-1 strain rate condition. Processing maps are developed at 0.3, 0.4, 0.5 and 0.6 true strain with clear deformation of stable and unstable domains at different temperatures and strain rates. Microstructure studies at selective domains in 0.6 true strain processing map are performed on image quality maps with grain boundaries. Hot compression at 900°C (α rich region) in unstable domain ([A] 900 °C-1 s-1) mostly resulted into new fine dynamic recrystallization equiaxed α grains along grain boundaries of large deformed α grains and large amount of low angle grain boundaries (LAGB) misorientation. Several fine DRX grains (more than higher strain rate deformed microstructure) among large deformed equiaxed αp grains and less amount of LAGB misorientation in the large equiaxed αp grains are observed in stable domain ([B] 900 °C-10-3 s-1) of α region deformed microstructure. Stable domains ([C] 950 °C-10-2 s-1 to [D] 975 °C-10-3 s-1) deformation in the α+β region had resulted in deformation of both α phase and β phase. The 950 °C deformed at 10-2 s-1 strain rate microstructure contained large LAGBs inside large equiaxed αp grains and is drastically reduced in 975 °C and 10-3 s-1 strain rate deformed microstructure. Deformation in stable domain ([E] 1000 °C-10-1 s-1) resulted in very small fraction of equiaxed αp grains with no LAGB and mostly remaining αs laths formed from the deformed β. Another stable domain ([F] 1050 °C-10-1 s-1) clearly showed only fine αs laths formed from β. Texture studies of hot compressed TiHy 600 alloy at 900 °C (α rich region hcp phase) with various strain rates (10-3-10 s-1) are performed. Deformation grains and recrystallization grains are segregated based on grain size diameter chart to study the texture analysis of both the grains separately. Both the grains found having similar oriented texture components but only differed in their texture intensities. At lower strain rate (10-3 s-1) the major texture component found is fiber texture . Higher strain rate (10 s-1) showed similar texture component like lower strain rate (10-3 s-1) with an additional texture component which is present due to generation of twins having misorientation angle of 65° around axes. Crystallographic texture studies of the reconstructed parent (bcc β) orientation maps of (1050 °C) TiHy 600 alloy after hot compression at various strain rates (10-3-10 s-1) is studied. Reconstructed β (bcc) phase microstructure contained coarser β grains. Deformed texture studies of parent (bcc β) phase are studied with various strain rates (10-3-10 s-1). It showed one major texture component at lower strain rate (10-3 s-1), ║compression direction (CD). Further increasing of the strain rate (10 s-1) to higher value showed {100} ׀׀ CD along with ׀׀ CD. At lower strain rate (10-3 s-1) orientated grains are activated due to having the possibility of more than five crystallographic slip systems. Increasing the strain rate activated less preferable orientation of grains having the possibility of only three crystallographic slip systems along with the oriented grains

Friction Stir Welding of Austenitic Stainless Steel by PCBN Tool and its Joint Analyses

AISI-316 plates of thickness 4 mm are joined by friction stir welding (FSW) with varying welding parameters like tool rotation speed (rpm) and welding speed (mm/min). All welded samples are observed by optical microscopy followed by their tensile tests. Mechanical strength of 104% of the base material with comparable elongation is achieved in FSW sample of 1100 rpm rotation speed and 8 mm/min welding spee

Mechanical properties and microstructural characterization of friction stir welded aIsI 316 austenitic stainless steel

The aim of this work is to check feasibility of joining austenitic stainless steel using friction stir welding (FSW). Defect free weld is observed achieved in 4 mm thick austenitic stainless steel friction stir welded with tool rotation speed of 1100 RPM and welding speed of 8 mm/min. The welds are tested mechanically by tensile tests and their Vickers hardness values are measured while their images are obtained by optical microscopy. Nearly equal ultimate tensile strength of the weld with increase of hardness in nugget zone with respect to the base material is achieved

Peak stress studies of hot compressed TiHy 600 alloy

TiHy 600 is a near alpha titanium alloy similar to IMI 834 Ti alloy, widely used for gas turbine engine applications such as disc and blades for high pressure compressors. Flow behavior of TiHy 600 alloy is investigated by conducting hot compression tests at temperatures ranging from 900°C to 1050°C with in the strain rate range of (0.001/s) respectively up to 50% deformation. Constitute modeling of work hardening is established, using Cingara equation to verify the stress up to peak stress. It was found that experimental true stress-true strain curves are in good agreement with Cingara equation curves for all temperatures at strain rate of (0.001/s)up to their peak stress values. The average activation energy (Q) is calculated from the peak stress of the flow curves using the hyperbolic-sine law equation (Arrhenius equation) i.e. 851.506KJ/mol. The microstructures of 50% deformed samples are correlated with the post-peak stress flow curve, performed mainly to compare the various softening processes (DRV or DRX)

Flow behaviour of TiHy 600 alloy under hot deformation using gleeble 3800

To understand deformation behaviour of TiHy 600 alloy at higher temperatures, hot compression tests are performed in α region (1173 K), α + β regions (1223, 1248, and 1273 K) and β region (1323 K) at strain rates (0.001, 0.01, 0.1, 1 and 10/s) for up to 50% deformation in Gleeble 3800® thermo-mechanical simulator. Flow curve plots are drawn at each strain rates and temperatures and it is observed that dominant deformation mechanism at higher temperature 1323 K (β region) and strain rates (1 and 10/s) is dynamic recovery (DRV) whereas dynamic recrystallization (DRX) is mostly observed at lower strain rates (0.001, 0.01/s) in medium temperature range of 1223 K (α region) to 1248 K (α + β region). Hyperbolic sine law equation is used to calculate the activation energy (Q) and other material sensitive parameters (A, α and n1). The activation energies for DRX in α region and DRV in β region are obtained as 384 and 251 kJ/mol. Experimental peak stress values are compared with predicted peak stress values (R2 = 96.2%) and Zener-Hollomon parameter (R2 = 94.3%). The flow stress behavior up to the peak stress is verified with Cingara equation. Finally, calculated prediction results of DRX volume fraction obtained from Avrami equation is compared with experimental observed microstructure

Comparative study on Ti-Nb binary alloys fabricated through spark plasma sintering and conventional P/M routes for biomedical application

The main purpose of this work is to obtain homogenous, single β phase in binary Ti-xNb (x = 18.75, 25, and 31.25 at.%) alloys by simple mixing of pure elemental powders using different sintering techniques such as spark plasma sintering (pressure-assisted sintering) and conventional powder metallurgy (pressure-less sintering). Synthesis parameters such as sintering temperature and holding time etc. are optimized in both techniques in order to get homogenous microstructure. In spark plasma sintering (SPS), complete homogeneous β phase is achieved in Ti25at.%Nb using 1300 °C sintering temperature with 60 min holding time under 50 MPa pressure. On the other hand, complete β phase is obtained in Ti25at.%Nb through conventional powder metallurgy (P/M) route using sintering temperature of 1400 °C for 120 min holding time which are adopted from the dilatometry studies. Nano-indentation is carried out for mechanical properties such as Young's modulus and nano-hardness. Elastic properties of binary Ti-xNb compositions are fallen within the range of 80–90 GPa. Cytotoxicity as well as cell adhesion studies carried out using MG63, osteoblast-like cells showed excellent biocompatibility of thus developed Ti25at.%Nb surface irrespective of fabrication route