In-situ micro-tensile tests in conjunction with scanning electron microscope (SEM) and electron backscatter diffraction (EBSD): (a) test setup, (b) specimen geometry, (c) true stress-strain curves obtained for X60MnAl17-1 and X30MnAl23-1; tests were carried out with a crosshead displacement rate of 0.2 mm/min, which corresponds to a quasi-static of about 0.001 s and interrupted at different strains as pointed to observe the microstructure and micro-crack development; EBSD measurement area and the region of SEM investigations on the micro-tensile specimen is shown; white arrows indicate tensile loading direction.
The EBSD inverse pole figure (IPF) maps of the undeformed samples showing grain size and orientations: (a) X60MnAl17-1, (b) X30MnAl23-1. Grain size fitted using normal distribution function shown in: (c) X60MnAl17-1, (d) X30MnAl23-1; microstructures of both alloys is completely austenite (); the average grain size of X60MnAl17-1 is 16.0 µm and X30MnAl23-1 is 12.0 µm.
SEM micrographs depicting non-metallic inclusions present in the investigated alloys: X60MnAl17-1 (a,b); X30MnAl23-1 (c,d); manganese sulphide (MnS) (black arrows) and aluminium nitride (AlN) (white arrows).
Macroscopic material response: (a) True stress-strain curves (b) Strain-hardening behavior.
EBSD image quality (IQ) maps depicting the evolution of twinning with increasing macroscopic strain: X60MnAl17-1 (a) (b) (c) (d) (left column), X30MnAl23-1 (e) (f) (g) (h) (right column); blue color in IQ maps in (a,e) indicates annealing twin boundaries and detected -3 deformation twin boundaries in (b–d,f–h); The arrows indicate annealing twin boundaries (black), nucleation of deformation twins (white), intercepting twins at grain boundary (light blue) and triple junctions (yellow); pixels in black are unindexed points.
) Stress-strain behavior extracted from the macroscopic tensile tests carried out in conjunction with digital image correlation (DIC). (b
) Local strain distribution: X60MnAl17-1—Failure at the intersection of deformation bands with large strain localization (see also Figure 7
k), X30MnAl23-1—Failure due to diffuse necking and large strain localization; rectangles indicate regions chosen to investigate deformation bands; left: macroscopic strain; right: legend for local von Mises effective plastic strain (
Deformation bands initiation, propagation and their location at the point of failure obtained from the tensile tests in conjunction with DIC: (I) X60MnAl17-1: enlarged view of – curve depicting the initiation of deformation bands (i,l), propagation of deformation bands (j,m) and failure at deformation bands (k,n), (II) plot of the deformation bands velocity versus strain, (III) X30MnAl23-1: enlarged view of – curve depicting no deformation bands initiation and propagation (u,v,x,y) and failure due to diffuse necking and strain localization (w,z), (IV) plot of the maximum local plastic strain versus strain; (i–k,u–w) shows local strain rate () distribution; (l–n,x–z) shows local strain () distribution; the legend for and distribution (right); white arrows indicate the direction of propagation of the deformation bands; the macroscopic strain at the start and end of deformation band is given; images were shown in steps of ∼0.4–0.6% macroscopic strain.
Temperature increase due to adiabatic heating in the macroscopic tensile tests carried out in conjunction with infrared thermography camera and video extensometer: (a,c) Rise in surface temperature during deformation, (b,d) Distribution of local temperature at different elongations and at failure.
Evolution of microstructure and micro-crack formation with deformation. X60MnAl17-1: (a) (b) (c) (left column), X30MnAl23-1: (d) (e) (f) (right column); red arrows indicate micro-cracks; white arrows indicate cracks in MnS inclusions at grain boundary (GB); macroscopic true strain is indicated at the top right corner.
Mechanism of micro-cracks development at a strain of . X60MnAl17-1 (a) Micro-crack initiation at triple junction and crack propagation into the neighboring grains assisted by the intercepting deformation twins, (b) Intergranular cracks nucleating at GB, X30MnAl23-1: (c) Micro-cracks initiation at GB and their propagation, (d) Intergranular cracks nucleation at grain boundaries and triple junctions; red arrows indicate micro-cracks.
Micro-cracks at inclusions. X60MnAl17-1: (a) Micro-cracks in AlN inclusions at GB and in MnS inclusions close to GB and also inside the grains, X30MnAl23-1: (b) Micro-cracks in MnS inclusion at GB, (c) Micro-cracks in MnS inclusion inside a grain; red arrows indicates cracks in inclusions at GB; white arrows indicate cracks in inclusions inside the grains.
Fracture behavior of tensile specimens. Diffuse necking and fracture surface appearance (left) and SEM images of the fracture surfaces at different magnifications (right). Average dimple size of both X60MnAl17-1 and X30MnAl23-1 is approximately ≤2.5 µm. White arrows indicate large elliptical voids at MnS and AlN inclusions.
Strain hardening in relation to the evolution of twinning; strain hardening behavior (top) and EBSD IPF maps with increasing macroscopic true strain (bottom).
A schematic illustration of micro-cracking mechanisms in TWIP steels based on the experimental observations from interrupted micro-tensile tests carried out in the SEM. Stress concentration caused by the intercepting deformation twins at GB. Void formation and their growth leading to inter-granular micro-crack initiation at GBs.
Chemical compositions (in wt.%) and the stacking fault energy (SFE) values (in mJ/m) of the investigated alloys.
Cleanliness analysis: area fraction, average and largest size of the inclusions. Size of the smallest inclusions found in both alloys is ∼0.5 µm. The scatter was estimated by analyzing four to five images taken at different locations.
|Alloy||Area Fraction (%)||Average Size (µm)||Largest Size (µm)|
|X60MnAl17-1||0.10 ± 0.01||1.23 ± 0.05||8.0 ± 1.5|
|X30MnAl23-1||0.11 ± 0.02||1.50 ± 0.10||12.0 ± 2.0|
Mechanical properties of Al-added TWIP steels: Yield/ultimate tensile strength (YS/UTS), uniform/total elongation (UE/TE), Lankford coefficient (r-value), strain hardening exponent (n-value), density (), Young’s modulus (E), shear modulus (G) and Poisson’s ratio (). All data are average values determined from at least three parallel experiments.
|X60MnAl17-1||294 ± 10||844 ± 15||65 ± 5||70 ± 5||0.90 ± 0.01||0.35 ± 0.01||7700 ± 10||188 ± 2||75 ± 1||0.267 ± 0.01|
|X30MnAl23-1||246 ± 10||693 ± 15||62 ± 2||63 ± 2||0.83 ± 0.01||0.37 ± 0.01||7715 ± 5||161 ± 1||63 ± 1||0.274 ± 0.01|