An effective approach to grow boron nitride nanowires directly on stainless-steel substrates

When growing one-dimensional (1D) nanomaterials via the vapour–liquid–solid (VLS) model, the substrates usually need to be coated with a layer of catalyst film. In this study, however, an effective approach for the synthesis of boron nitride (BN) nanowires directly onto commercial stainless-steel foils has been demonstrated. Growth occurs by heating boron and zinc oxide (ZnO) powders at 1100 °C under a mixture of nitrogen and hydrogen gas flow (200 ml min−1). The stainless-steel foils played an additional role of catalyst besides substrate during the VLS growth of these BN nanowires. The as-synthesized nanowires emit strong photoluminescence (PL) bands at 515, 535 and 728 nm. In addition, we found that the gas flow rate and the hydrogen content in the gas mixture strongly affected the diameter and yield of the nanowires by changing the relative concentration of the nanowire growth species in the chamber.

Selective oxidation synthesis of MnCr2O4 spinel nanowires from Commercial stainless steel foil

For the first time, MnCr₂O₄ spinel single-crystalline nanowires were simply synthesized by heating commercial stainless steel foil (Cr_0.19Fe_0.70Ni_0.11) under a reducing atmosphere. The nanowires have an average diameter of 50 nm and a length of about 10 μm. Some nanowires are sheathed with a thin layer of amorphous silicon oxide. Photoluminescence measurements revealed that the nanowires exhibit an emission band at 435 nm, which resulted from the oxygen-related defects in the silicon oxide sheath. It was found that the reducing atmosphere plays a key role for the nanowire growth. In the reducing atmosphere, the Mn and Cr elements in the stainless steel could be selectively oxidized because of their higher affinity for oxygen than the Fe and Ni elements. The Fe and Ni elements in the stainless steel, however, acted as the catalyst for the vapor–liquid–solid (VLS) growth of the MnCr₂O₄ nanowires.

Recrystallization in AISI 304 austenitic stainless steel during and after hot deformation

In order to improve the understanding of the dynamic and post-dynamic recrystallization behaviours of AISI 304 austenitic stainless steel, a series of hot torsion test have been performed under a range of deformation conditions. The mechanical and microstructural features of dynamic recrystallization (DRX) were characterized to compare and contrast them with those of the post-dynamic recrystallization. A necklace type of dynamically recrystallized microstructure was observed during hot deformation at 900 °C and at a strain rate of 0.01 s⁻¹. Following deformation, the dependency of time for 50% recrystallization, t₅₀, changed from “strain dependent” to “strain independent” at a transition strain (ε^*), which is significantly beyond the peak. This transition strain was clearly linked to the strain for 50% dynamic recrystallization during deformation. The interrelations between the fraction of dynamically recrystallized microstructure, the evolution of post-dynamically recrystallized microstructure and the final grain size have been established. The results also showed an important role of grain growth on softening of deformed austenite.

Hot deformation and recrystallization of austenitic stainless steel: Part 11. post-deformation recrystallization

The postdeformation recrystallization behavior of a hot-deformed austenitic stainless steel was investigated based on the first part of this study, in which the microstructure development during hot deformation and, in particular, the evolution of dynamic recrystallization (DRX), was studied. The effect of different parameters such as strain, strain rate, and temperature were examined. The dependency of the time for 50 pct softening, t 50, changed from “strain dependent” to “strain independent” at a transition strain (ε*) that was in the steady-state area of the hot deformation flow curve. The fully recrystallized microstructure showed a similar transition in strain sensitivity. However, this occurred at stains greater than ε*. A mathematical model was developed to predict the transition strain under different deformation conditions. Microstructural measurements show that the transition strain corresponds to approximately 50 pct DRX in the deformed structure at the point of unloading.

Hot deformation and recrystallization of austenitic stainless steel: part 1. dynamic recrystallization

The hot deformation behavior of a 304 austenitic stainless steel was investigated to characterize the evolution of the dynamically recrystallized structure as a starting point for studies of the postdeformation  recrystallization behavior. The effect of different deformation parameters such as strain, strain rate, and temperature were investigated. The flow curves showed typical signs of dynamic recrystallization (DRX) over a wide range of temperatures and strain rates (i.e., different Zener–Hollomon (Z) values). However, under very high or very low Z values, the flow curves’ shapes changed toward those of the dynamic recovery and multiple peaks, respectively. The results showed that while DRX starts at a strain as low as 60 pct of the peak strain, a fully DRX microstructure needs a high strain of almost 4.5 times the initiation strain. The DRX average grain size showed power-law functions with both the Zener–Hollomon parameter and the peak stress, although power-law breakdown was observed at high Z values.

The effect of prestrain and bake hardening on the low-cycle fatigue properties of TRIP steel

The scope of this study was to examine the effects of plane strain prestrain, induced via cold-rolling, and subsequent automotive paint bake hardening cycle on both tensile and fatigue properties of a hot rolled TRIP780 multiphase steel. Strain-life data has been generated for as-received (0% prestrain), 10% and 20% prestrained samples, in both baked and unbaked conditions. Cold rolling  increased the number of strain reversals to failure at high cyclic strain amplitudes with no effect at low strain amplitudes. Bake hardening increased the number of reversals to failure at high cyclic strain amplitudes. The prestrained material exhibited partial cyclic softening, with some residual strength increase. The residual strength increase was attributed to the austenite to martensite transformation that occurred during the prestraining process.

EBSD analysis of a Ti-IF steel subjected to hot torsion in the ferritic region

The effect of grain size on the warm deformation behaviour of a titanium stabilized interstitial free steel was investigated using hot torsion. Tests were performed at temperatures between 765 °C and 850 °C at strain rates between 0.003 s−1 and 1 s−1 for samples with grain sizes of 25 μm, 75 μm and 150 μm. The structures were observed using EBSD analysis and are consistent with those expected for materials dominated by dynamic recovery. Some evidence was found for small amounts of thermally induced migration of pre-existing boundary (bulging) and for the generation of new segments of high angle boundaries by continuous dynamic recrystallization. The early onset of a steady-state flow stress in the finer grained samples is attributed to one or a combination of thermally induced boundary migration and enhanced rates of recovery near subgrain (and grain) boundaries.

Experimental and numerical investigation of low pressure tube hydroforming on 409 stainless steel

Tube hydroforming has been widely used to produce automotive structural components due to the superior properties of the hydroformed parts in terms of their light weight and structural rigidity. Compared to the traditional manufacturing process for a closed-section member including stamping and followed by welding, tube-hydro forming leads to cost savings due to reduced tooling and material handling. However, the high pressure pumps and high tonnage press required in hydroforming, lead to increased capital investment reducing the cost benefits. This study explores low pressure tube hydro forming which reduces the internal fluid pressure and die closing force required to produce the hydroformed part. The experimental and numerical analysis was for low pressure hydro formed stainless steel tubes. Die filling conditions and thickness distributions are measured and critically analysed.

Experimental and numerical evaluation of forming and fracture behaviour of high strength steel

Car manufacturers are under pressure to reduce vehicle mass while maintaining comfort and passenger safety for current and future vehicles. To meet this demand the steel industry has developed Advanced High Strength Steels (AHSS) that promise higher strength and improved formability compared to conventional steel grades. Even though significant research has already been performed to evaluate the material properties and forming behaviour of most AHSS types, only a limited literature is available on their necking and fracture behaviour and the effect on formability. This paper examines and compares the thinning, necking and fracture behaviour of two AHSS and one conventional steel type, namely TRIP, DP and HSLA. Uniaxial, plane and biaxial strain conditions are investigated by tensile, cup drawing and stretch forming tests and by using numerical methods. The test results indicate that significant differences exist in necking and fracture behaviour between all three steel types.

Effect of grain size on deformation twinning behaviour of high MN steel

The effect of grain size on the mechanical properties and deformation twinning behaviour in high manganese steel was investigated. In order to generate different grain sizes, the samples were subjected to hot rolling, cold rolling and annealing. Room temperature tensile testing of the steel with different grain sizes (5-50 µm) indicated the occurrence of twinning induced plasticity (TWIP) in all the samples. Also, changes in work-hardening behaviour accompanied changes in the grain size. The results are discussed in terms of the enhanced sensitivity of twinning to the grain size.

A study of the strengthening mechanism in the thermomechanically processed TRIP/TWIP steel

The strengthening mechanism responsible for the unique combination of ultimate tensile strength and elongation in a multiphase Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel was studied. The microstructures with different volume fraction of polygonal fenite, bainite and retained austenite were simulated by controlled thermomechanical processing. The interupted tensile test was used to study the bainitic ferrite, retained austenite and polygonal ferrite behavior as a function of plastic strain. X-ray analysis was used to characterize the volume fraction and carbon content of retained austenite. TEM and heat-tinting were utilized to analyze the effect of bainitic fenite morphology on the strain induced transformation of retained austenite and retained austenite twinning as a function of strain in the bulk material. The study has shown that the austenite twinning mechanism is more preferable than the transformation induced plasticity mechanism during the early stages of deformation for a microstructure containing I5% polygonal ferrite, while the transformation induced plasticity effect is the main mechanism in when there is 50% of polygonal ferrite in the microstructure. The baillitic fenite morphology affects the deformation mode of retained austenite during straining. The polygonal fenite behavior during straining depends on dislocation substructure tonned due to the deformation and the additional mobile dislocations caused by the TRIP effect. TRIP and TWIP effects depend not only on the chemical and mechanical stability of retained austenite, but also on the interaction of the phases during straining.