Variant selection during the c-to-ab phase transformation in hot-rolled bainitic TRIP-aided steels

The variant selection phenomenon during the austenite to bainite phase transformation in hot-rolled TRIP-aided steels was quantitatively characterized at the level of individual austenite grains. The reconstruction of the electron backscatter diffraction maps provided evidence that bainite grows by packets of laths sharing a common {1 1 1}_y plane in the austenite. The affect of hot deformation is to reduce the number of packets that form. It is suggested that slip activity is important in understanding this effect.

Thermodynamics approach to the hydrogen diffusion and phase transformation in titanium particles

The hydrogen diffusion and phase transformation in a titanium particle were studied based on thermodynamic calculation. The mechanisms of hydrogen diffusion in different phases (alpha-Ti, beta-Ti and TiHx) were analyzed. A mobility database was developed for titanium– hydrogen system based on the experimental works on hydrogen diffusion coefficient reported in literature and the fundamental of diffusion. To implement the calculation, a commercial software package for the simulation of diffusion-controlled phase transformation was used. The hydrogen diffusion process, hydrogen distribution, phase transformation and phase growth rate during hydrogenization of a titanium particle at temperatures of 560 K, 800K and 1000K were discussed. The thermodynamics and kinetics analysis provided quantitative insight into the diffusion process and improved the understanding of diffusion mechanism and phase transformation.

Study of phase transformation and work hardening phenomenon during drilling of Ti-5553 and Ti-64

Titanium 5553 is a recently developed modification of Russian near-β titanium alloy VT-22 which has applications and potential particularly in the aerospace industry for such key components as landing gear. However, indications are that Ti-5553 has poorer machinability characteristics than other Ti alloys and a comprehensive and far-reaching analysis is a necessary research imperative. This paper presents the result of phase transformation and work hardening during drilling of Ti-5553 compared with Ti-64. The aim of this research work is to optimise the machining condition for Ti-5553, in which the β to a phase transformation, together with material work hardening could be fully understood. Analysis of machinability indicators, such as subsurface micrograph and hardness of drilled samples and drilling forces and torques, demonstrated that Ti-5553 generally has poorer machinability characteristics than Ti-64 and to some extent this variation has been quantified to allow for further and more detailed investigation.

Deformation induced phase transformation during machining of Ti-5553

Ti-5553 is a relatively new titanium alloy with applications particularly in the aerospace industry for such key structural components as landing gear. However, during machining of Ti-5553, the elevated temperature and high strain at tool-workpiece interface may alter workpiece microstructure and result in ß to a phase transformation. During phase transformation, some intermediated phase such as w phase may form which is brittle and hard to machine, and it could reduce the fatigue life of machined components. The aim of this research work is to optimize the machining condition for Ti-5553, in which its hot deformation behavior in terms of ß to a phase transformation could be fully understood. Analysis of variables such as micrographs of phase components and cutting zone temperature demonstrates that the cutting temperature governs the formation of final phase components and to some extent this variation has been quantified to allow for further and more detailed investigation.

Phase transformation evolution in NiTi shape memory alloy under cyclic nanoindentation loadings at dissimilar rates

Hysteresis energy decreased significantly as nanocrystalline NiTi shape memory alloy was under triangular cyclic nanoindentation loadings at high rate. Jagged curves evidenced discrete stress relaxations. With a large recovery state of maximum deformation in each cycle, this behavior concluded in several nucleation sites of phase transformation in stressed bulk. Additionally, the higher initial propagation velocity of interface and thermal activation volume, and higher levels of phase transition stress in subsequent cycles explained the monotonic decreasing trend of dissipated energy. In contrast, the dissipated energy showed an opposite increasing trend during triangular cyclic loadings at a low rate and 60âsec holding time after each unloading stage. Due to the isothermal loading rate and the holding time, a major part of the released latent heat was transferred during the cyclic loading resulting in an unchanged phase transition stress. This fact with the reorientation phenomenon explained the monotonic increasing trend of hysteresis energy.

Machinability and phase transformation study of nanobainite steels

There is an increasing demand for high strength materials with the development of technology and critical applications. Nano materials are newly developed materials with extremely high strength for this purpose. Nanobainite is a dual phase material containing alternate layers of bainitic ferrite in nano dimensions and retained austenite. Nanobainite is produced by isothermally holding austenitized steel at a temperature of 200°C or less, depending on the chemical composition, for 6 10 days until bainite forms and then cooling to room temperature using austempering. The experimental design consisted of face milling under 12 combinations of Depth of Cut (DOC)-1, 2 and 3mm; cutting speed-100 and 150m/min; constant feed-0.15mm/rev and coolant on/off. The machinability of the material is assessed by means of analysis, such as surface texture and microhardness. The assessment also involves microstructural comparisons before and after milling. Future work involves quantifying the microstructural phase before and after milling using XRD. The results obtained are used to assess the most favorable condition to cut this new variety of steel.

Recent advances in machining of Austempered Ductile Iron to avoid machining induced microstructural phase transformation reaction

Austempered Ductile Iron (ADI) is a type of nodular, ductile cast iron subjected to heat treatments-austenitising and austempering. Whilst machining is conducted prior to heat treatment and offers no significant difficulty, machining post heat treatment is demanding and often avoided. Phase transformation of retained austenite to martensite leading to poor machinability characteristics is a common problem experienced during machining. Study of phase transformations is an investigative study on the factors-plastic strain (εp) and thermal energy (Q) which effect phase transformations during machining. The experimental design consists of face milling grade 1200 at variable Depth of Cut (DoC) range from 1 to 4 mm, coolant on/off, at constant speed, 1992 rpm and feed rate, 0.1 mm/tooth. Plastic strain (εp) and martensite content (M) at fracture point for each grade was evaluated by tensile testing. The effect of thermal energy (Q) on phase transformations was also verified through temperature measurements at DoC 3 and 1 mm using thermocouples embedded into the workpiece. Finally, the amount of plastic strain (εp) and thermal energy (Q) responsible for a given martensite increase (M) during milling was related and calculated using a mathematical function, M=f (εp, Q). The future work of the thesis involves an in-depth study on the new link discovered through this research: mathematical model relating the role of plastic strain and thermal energy in martensite formation.

Effect of transformation volume contraction on the toughness of superelastic shape memory alloys

Shape memory alloys (SMAs) exhibit two very important properties: shape memory phenomenon and superelastic deformation due to intrinsic thermoelastic martensitic transformation. To fully exploit the potential of SMAs in developing functional structures or smart structures in mechanical and biomechanical engineering, it is important to understand and quantify the failure mechanisms of SMAs. This paper presents a theoretical study of the effect of phase-transformation-induced volume contraction on the fracture properties of superelastic SMAs. A simple model is employed to account for the forward and reverse phase transformation with pure volume change, which is then applied to numerically study the transformation field near the tip of a tensile crack. The results reveal that during steady-state crack propagation, the transformation zone extends ahead of the crack tip due to forward transformation while partial reverse transformation occurs in the wake. Furthermore, as a result of the volume contraction associated with the austenite-to-martensite transformation, the induced stress-intensity factor is positive. This is in stark contrast with the negative stress-intensity factor achieved in zirconia ceramics, which undergoes volume expansion during phase transformation. The reverse transformation has been found to have a negligible effect on the induced stress-intensity factor. An important implication of the present results is that the phase transformation with volume contraction in SMAs tends to reduce their fracture resistance and increase the brittleness.