Revealing the mechanical and microstructural performance of multiphase steels during tensile, forming and flanging operations

The mechanical performance of Dual Phase (DP) and Complex Phase (CP) steels was investigated by SEM analysis, tensile testing, Forming Limit Curve investigation and flange formability testing. The alloys of interest were Dual Phase (DP) untempered, Dual Phase (DP) tempered and Complex Phase (CP) steels. Phase content analysis showed that the distribution of the ferrite and martensite phases was the same for the two DP alloys, but the grain size and condition (tempered/untempered) for the martensite islands was much different in the two alloys. In the tempered DP steel, the smaller grain size for the martensite and the tempering process resulted in increased elongation, more formability and ability to form a flange (flangeability). In CP steels the soft ferrite phase is replaced by harder bainite, yielding a bainitic-martensitic microstructure. Bainite reduced the total elongation of the alloy during tensile testing, reduced the formability (especially under plane strain conditions) of the alloy but improved the flangeability of the alloy. Under flanging conditions, CP steels deformed to higher strains, at tighter radii with minimum springback. Microstructural inspections at the outer radius of the flanged specimens revealed that in CP steels bainite deforms similarly to martensite, therefore the strain partitioning is smaller in CP steels in comparison to DP steels. Plastic deformation in CP steels upon flanging occurs with the formation of strong slip bands in both martensite and bainite. In contrast, the martensite and ferrite grains in DP steels deform quite differently leading to strong strain localisations. Void nucleation and cracking occurred at the martensite islands or within the soft ferrite phase next to the martensite islands. In CP steels no voids or damage was observed within the matrix. A special case study was done with a thicker and stronger alloy, a Martensitic 1400 steel to reveal the flangeability limits for advanced high strength steels. Neither cracks nor damage were observed visually on the flanged specimens. However SEM observations at the outer radius of the flanged samples revealed significant void growth at inclusion sites and cracks nucleating within the matrix adjacent to the inclusions.Publisher Statement: This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/)</p

Multiscale characterisation of the mechanical properties of austenitic stainless steel joints

A multiscale investigation was pursued in order to obtain the strain distribution and evolution during tensile testing both at the macro- and micro-scale for a diffusion bonded 316L stainless steel. The samples were designed for the purpose to demonstrate that the bond line properties were equal or better than the parent material in a sample geometry that was extracted from a larger component. The macroscopic stress-strain curves were coupled to the strain distributions using a camera-based 2D – Digital Image Correlation system. Results showed significant amount of plastic deformation predominantly concentrated in shear bands which were extended over a large region, crossing through the joint area. Yet it was not possible to be certain whether the joint has shown significant plastic deformation. In order to obtain the joints’ mechanical response in more detail, in situ micromechanical testing was conducted in the SEM chamber that allowed areas of 1x1 mm2 and 50x50 mm2 to be investigated. The size of the welded region was rather small to be accurately captured from the camera based DIC system. Therefore a microscale investigation was pursued where the samples were tested within an SEM chamber. Low magnification SEM imaging was utilised in order to cover a viewing area of 1 mm×1 mm while high magnification SEM imaging was employed to provide evidence of the occurrence of plastic deformation within the joint, at an area of just 50 μm×50 μm. The strain evolution over the microstructural level, within the joint and at the base material was obtained. The local strains were highly non-homogeneous through the whole test. Final failure occurred approximately 0.2 mm away from the joint. Large local strains were measured within the joint region, while SEM imaging showed that plastic deformation occurs via the formation of strong slip bands, followed by the activation of additional slip systems upon further plastic deformation which end up in additional slip bands to form on the surface. Plastic deformation occurred by slip and twinning mechanisms. Upon necking, significant out of plane deformations and slip deformation mechanisms were observed which suggested that plastic deformation was also happening at the last stages of damage evolution for the specific alloy. This was also evident from the large difference between the 600 MPa UTS stress value and the low stress values before final failure (which in many cases was below 30 MPa)

Deformation-induced microstructural banding in TRIP steels

Microstructure inhomogeneities can strongly influence the mechanical properties of advanced high-strength steels in a detrimental manner. This study of a transformation-induced plasticity (TRIP) steel investigates the effect of pre-existing contiguous grain boundary networks (CGBNs) of hard second-phases and shows how these develop into bands during tensile testing using in situ observations in conjunction with digital image correlation (DIC). The bands form by the lateral contraction of the soft ferrite matrix, which rotates and displaces the CGBNs of second-phases and the individual features within them to become aligned with the loading direction. The more extensive pre-existing CGBNs that were before the deformation already aligned with the loading direction are the most critical microstructural feature for damage initiation and propagation. They induce micro-void formation between the hard second-phases along them, which coalesce and develop into long macroscopic fissures. The hard phases, retained austenite and martensite, were not differentiated as it was found that the individual phases do not play a role in the formation of these bands. It is suggested that minimizing the presence of CGBNs of hard second-phases in the initial microstructure will increase the formability

A FIM/AP INVESTIGATION OF A RAPIDLY SOLIDIFIED ALUMINIUM-CHROMIUM ALLOY

A rapidly solidified Al - 6 to 9wt%Cr alloy formed by vapour deposition has been studied by field-ion microscopy and atom probe microanalysis (FIM/AP). The as-deposited material was found to contain brightly imaging Cr-rich particles, both spherical and rod-shaped, of about 0.5-1nm in diameter and up to 5nm long. Random area AP analysis showed the presence of Cr-enrichment on a very fine scale, with estimated particle compositions of 30-50at%Cr. No change in particle size or composition was observed for aging at 350°C for up to 20 hours. By comparison, an Al-4wt%Cr wire formed by in-rotating-water quenching showed no indication of precipitation in its as-received state by either FIM or AP

Calciothermic reduction of Ti-6AI-4V

Previous studies have shown that calciothermic reduction can be applied to reduce the oxygen content of commercially pure titanium and titanium dioxide. In this investigation, the calciothermic reduction route was examined to assess whether the process could be applied to the commercial alloy Ti-6A1-4V. Ti-6A1-4V rods were sealed in a metal container and reacted with calcium in a molten flux. The resultant material was characterised using X-Ray Diffraction (XRD),Focused Ion Beam-Secondary Ion Mass Spectrometry (FIB-SI MS) and Inert Gas Fusion to determine the hulk oxygen concentration as well as the oxygen concentration profiles. The removal of oxygen from the samples was achieved in both an inert atmosphere and also when air was present within the vessel