Unprecedented strength in pure iron via high-pressure induced nanotwinned martensite

ABSTRACT Martensitic transformation can easily induce a maximum hardness value of 800–900 HV (Vickers hardness) for steels with carbon contents of 0.6 wt.% and above. However, the occurrence of martensitic transformation in pure iron requires exceptionally high cooling rates (105–106°C/s), and the maximum achievable hardness is only about 150 HV. Here we report an extreme hardness of 830 HV in pure iron obtained through high pressure induced martensitic transformation at a rather slow cooling rate of just 10°C/s. This unprecedented strength originates from the formation of twin-related martensitic laths with an average thickness of 3.8 nm. GRAPHICAL ABSTRACT IMPACT STATEMENT Slow cooling of pure iron under high pressure induces martensitic transformation, producing nanotwinned martensite with unprecedented strength.


Introduction
Iron has played an important role in the progress of human civilization [1]. It is well-known that pure iron is very soft ( < 100 HV) and can be hardened by making it into steels through adding carbon and other alloying elements. The maximum hardening in iron and steels is achieved by martensitic transformation up to 800-900 HV [2], i.e. a diffusionless and shear displacive transformation that changes the high temperature austenite form to fine scale martensite containing a high density of lattice defects. The hardening is mainly originated from three mechanisms: (i) solid solution transformed martensite has insignificant lattice supersaturation, a relatively low dislocation density (10 14 m −2 ) and a coarse boundary spacing ( ∼ 500 nm), and hence a hardness of only 150 HV [9].
As a thermodynamic variable independent of chemical composition, the pressure (P) can enhance the driving force for martensitic transformation and decrease the M s point by providing extra work to increase the change in Gibbs free energy [10], and consequently, tune the substructure that controls the martensite strength. A previous investigation [11] demonstrated that application of a pressure of 3-5 GPa causes the martensitic substructure in low carbon steels to be dominated by nanoscale internal twins of the type that are commonly found in martensite of high carbon steels transformed under atmospheric pressure. A recent study also showed that quenching under a pressure of 5 GPa induces lath martensite in an interstitial-free (IF) steel that otherwise does not form martensite during quenching under atmospheric pressure [12]. Herein, pure iron samples were subjected to a heating and cooling cycle under high pressure, leading to the formation of martensite. The martensite formed comprises nanoscale laths, with each lath a crystallographically equivalent variant, and adjacent variants related by a twin misorientation (60 o / < 111 > ). This nanoscale martensite exhibits an ultrahigh hardness of 830 HV. This discovery highlights an incredible hardening by martensitic transformation in pure iron.

Experimental methods
A commercial high purity iron (0.0021 wt.%C, 0.012 wt.% Mn, 0.0064 wt.% P, 0.0028 wt.% S and Fe the balance) was subjected to heating and cooling under either atmospheric pressure (10 −4 GPa; the corresponding sample is referred to as 10 −4 GPa-iron) or high pressures of 1, 2, and 3 GPa (1, 2, and 3 GPa-iron). As schematically illustratedin Figure 1(a), a hexahedron pressure equipment was used to generate high pressure by equally compressing the six sides of a cubic sample (5 × 5 × 5 mm 3 ) using a CS-I B and SPD-6-600 hexahedral multi-anvil unit. The sample assembly consisted of heater, insulator, and pressure transmitting medium. The temperature was measured with K-type thermocouples, and the pressure was estimated from previously obtained calibration curves for the multi-anvil high pressure apparatus [13,14]. The sample was gradually compressed to the desired pressure at room temperature and then was heated at 1 o C/s to the desired temperature referring the Fe-C phase diagram under hydrostatic and atmospheric pressure [15], namely, 300, 600, 720, 780, 880, 950 and 1050°C, respectively. As shown by Figure 1(b), the duration holding at each setting temperature was 1800 s, followed by cooling to ambient temperature in 90 s by switching off the power and then slowly decompressed. The cooling rate from 950°C to ambient temperature was roughly 10 o C/s. Heating and cooling under atmospheric pressure were performed in a vacuum furnace. The sample was heated to 950°C in 15 s and held for 1800 s, followed by furnace cooling to ambient temperature in 1200 s, corresponding to a cooling rate of ∼ 0.8 o C/s. The microstructure of the final products after heating and cooling cycle under pressure was characterized by optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). TEM samples were made by focused ion beam (FIB) milling with an FEI SHNNNP Scios dual-beam facility, from the area close to the corresponding hardness indentations. TEM and HRTEM observations were performed in the FEI Talos 200X TEM and JEM 2100 TEM operated at an accelerating voltage of 200 kV. The thickness of parallel laths was measured on the TEM micrographs along lines that are perpendicular to the laths. Vickers hardness was measured on a Quness 10A+ microhardness tester with a load of 0.98 N and a duration of 10 s. Each hardness value was averaged over at least ten measurements.

Results
It is surprising that remarkable hardening was obtained in pure iron after the heating and cooling cycle under high pressures, as shown in Figure 2. The 3 GPa-iron sample exhibited an unprecedented ultrahigh hardness of 830 HV for holding temperatures above 950°C. This hardness value is one order of magnitude higher than that of the starting material (80 HV). This value is 5-6 times higher than that of pure iron cooled under atmospheric pressure at 10 5 -10 6°C /s (150 HV) [9] and is comparable to that (800 ∼ 900 HV) of martensite in carbon steels containing > 0.6 wt.% C [3]. The hardening varies with depth and depends upon the holding temperature and applied pressure. From Figure 2(a), the hardness of 3 GPa-iron decreases continuously to 400 HV at a depth of 1 mm, and then reaches 200 HV in the sample center. With decreasing holding temperature, the hardening becomes increasingly less significant and eventually no hardening is observed for holding temperatures below 720°C, as shown in Figure 2(b). At a holding temperature of 950°C, the hardness decreases to 700 and 400 HV as the pressure is reduced to 2 and 1 GPa, respectively, as shown in Figure 2(c).
All processed pure iron samples have the bodycentered cubic (bcc) structure, which was verified by Xray diffraction. Optical microscopy observations showed that the bcc phases in the high pressure samples with holding temperatures below 720°C are characterized by coarse and irregular grains with clean interiors, like  [16], and the boundaries enclosing the three packets P 1 , P 3 , and P 4 are marked by dashed, dotted, and full lines, respectively. that in the 10 −4 GPa-iron shown in Figure 3(a). As the holding temperature exceeds 720°C, substructures are observed inside the grain interiors of the high pressure samples (Figure 3(b)), resembling the hierarchical packet-block-lath structure of the lath martensite observed in IF steel or low carbon steels [16]. Notably, a gradient structure was present inside the cubic sample, showing lath martensite with different structural scales.
The lath martensite at the center of 3 GPa-iron with a holding temperature of 950°C has a relatively coarse substructure, as demonstrated by the EBSD characterization (Figure 4(a)). Three packets of several to tens of micrometers in width, labeled P 1 , P 3 and P 4 in the figure, are seen within the observed area of a grain. Each packet contains regions of similar colors, representing blocks with roughly the same crystal orientation. The orientation relationship (O.R.) between the martensite (m) and the parent austenite (A) follows the Kurdjumov-Sachs (K-S) OR [17], {111} A //{011} m and < 110 > A // < 111 > m , which is revealed by the good agreement between the experimentally measured martensite orientations (blue cloud) and those of the 24 K-S variants (black solid circle) in the {001} pole figure shown in Figure 4(b). Each block was observed to contain only one martensitic variant, and adjacent variants in the same packet show a twin misorientation with an axis/angle pair of < 110 > /70.5°or < 111 > /60°, which corresponds to the misorientation for twin-related variants [16]. These misorientations lead to the peak around 60°in the histogram of misorientation distribution in Figure 4(d). TEM reveals that each block in Figure 4(c) contained laths with an average thickness of 166 nm and predominantly low misorientation angles (Figure 4(e)). The habit plane of the martensite laths was close to (101) m and (111) A , which is in agreement with that of typical lath martensite in IF steels and low carbon steels [18].
Martensite induced close to the sample surface has fine-scale substructure, with a K-S O.R. between austenite and martensite, and (111) A -type habit plane (deviation of 5.5°), as shown in Figure 5(a). The packets are reduced in size to tens of nanometers in width (for instance 40-50 nm for packets d and e), and the block thickness ranges from 2 to 10 nm with an average of 3.8 nm ( Figure 5(b,f)). This length scale is 1-2 orders of magnitude smaller than that of the coarse lath martensite ( ∼ 166 nm) in the sample center and is even smaller than that (5-10 nm) of high carbon martensite [3]. Each block corresponds to a single variant that is twin-related to adjacent blocks in the same packet (variant pair: V 1 -V 2 and V 15 -V 16 ), as evident from the mirror symmetric bcc [110] diffraction patterns ( Figure 5(c)). Based on the high resolution TEM images ( Figure 5(d,e)), the (1-10) crystallographic planes (solid lines) from twin-related variants are mirror symmetric. Such a martensitic structure is referred to as nanotwinned martensite hereafter in this paper.

Discussion
Here, the martensitic transformation could proceed under slow cooling as high pressure was applied to raise the driving force by increasing the change in Gibbs free energy [10], to suppress atomic diffusion [21], and to shift the continuous cooling transformation curve rightward [22]. Previous investigations [11,23] reported that an application of 3-5 GPa pressure causes the martensitic transformation to proceed at a cooling rate of 100°C/s in Fe-C and Fe-Cr alloys; a recent study [12] even demonstrated that a martensitic structure could be formed in an IF steel cooled under a pressure of 5 GPa at 8°C/s. The present study shows that lath martensite in pure iron can be induced to form under a pressure of 1-3 GPa at a cooling rate of 10°C/s which is 4-5 orders of magnitude slower than that required for martensitic transformation under atmospheric pressure. The lath martensite has the typical packet-block-lath structure holding the K-S O.R. with the parent austenite and (111) A -type habit planes, as observed in low carbon steels [18]. However, it is surprising to observe that the adjacent blocks in the same packet comprise twin-related variants, and the block sizes are substantially reduced to the nanoscale with increasing pressure. According to the phenomenological theory of martensite crystallography, approximate twin-related variants are more efficient in accommodating transformation strain than other pairs [24]. Moreover twin-related variant selection is enhanced as more carbon is added to decrease the M s point [25,26], which is accompanied by substantially reduced block and packet sizes due to the harder austenite against deformation [27]. Theoretically, the application of every 1 GPa pressure decreases the M s point by 82 K [11], whereas a 100 K temperature drop strengthens austenite at high temperatures by 18 MPa [28]. In addition, the flow stress (σ y ) of a material subjected to high pressure is linearly related to the pressure amplitude (P) (σ y = (1 + 3kP) σ 0) , where σ 0 is the flow stress at atmospheric pressure, and k is the pressure coefficient; for steels, k = 0.02 [29]. Consequently, the present pressure of 3 GPa may induce M s point drop of 246 K, equivalent to the addition of 0.77 wt% C, following M s = 320 K/wt %C [28], and an austenite strengthening by 53 MPa. These changes facilitate the formation of nanoscale twin-related laths. However, such martensite was neither observed in pure iron cooled under 5-7 GPa pressure that was introduced by piston-cylinder type high pressure apparatus [11] nor in IF steel cooled under 5 GPa pressure that was produced by the constrained volume expansion of NaCl in melting [12]. This difference may point to an effect of the procedure by which the high pressure is generated on the martensitic transformation. The current procedure of applying high pressure by equally pressing on the six sides of a cubic sample with hexahedron pressure equipment may restrict the volume change more effectively than other methods, rendering the occurrence of martensitic transformation at relatively low pressures.
The present hardening of pure iron by one order's of magnitude highlights the significance of martensitic hardening via structural refinement. When the double lath thickness value (2t av ) [21,30] is viewed as the slip length (d) for the H-P relationship ( σ = K HP d −1/2 , where σ is the strengthening ( HV/3), K HP is the H-P coefficient), a K HP of 0.2 GPaμm 1/2 was obtained as shown in Figure 5(g), comparable to that for a polycrystalline pure iron [19]. However, the present data points and the previously reported ones [9,12,20] lie above the H-P line for the polycrystalline pure iron (dashed line in Figure 5 [19]. Literature data on the martensite of pure iron [9], IF steel [12] and Fe-C [20] were inserted for comparison. The white circle in (b) marks the area where the SAED pattern in (c) was obtained.

Conclusion
Martensitic transformation was induced in pure iron under high pressure with a slow cooling of 10 o C/s. The martensite was characterized by twin-related laths with an average thickness of 3.8 nm. The martensitic transformation resulted in an unprecedented strengthening by a factor over 10. These new findings may expand the processing space for tuning the structure and strength of iron and, showing great technical importance for a wide range of industrial applications.

Disclosure statement
No potential conflict of interest was reported by the authors.