Designing ultrastrong maraging stainless steels with improved uniform plastic strain via controlled precipitation of coherent nanoparticles

doi:10.1016/j.jmst.2021.04.011

Abstract

Abstract:

The development of ultrastrong maraging stainless steels (MSSs) is always in high demand. However, traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic (BCC) lath martensitic matrix. Here, we design a novel ultrahigh strength MSS (Fe-5.30Cr-13.47Ni-3.10Al-1.22Mo-0.50W-0.23Nb-0.03C-0.005B, wt.%) using a cluster formula approach. A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-NiAl nanoprecipitates (3 ~ 5 nm) in BCC martensitic matrix was successfully obtained. This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa, but also a decent uniform elongation of 4.2 ~ 5.1%, which is almost triple of the value observed in existing MSSs. We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit high-performance MSSs.

Key words: Maraging stainless steel, Ultrahigh strength, Coherent precipitation, Strengthening mechanisms, Uniform deformation

Z.H. Wang, B. Niu, Q. Wang, C. Dong, J.C. Jie, T.M. Wang, T.G. Nieh. Designing ultrastrong maraging stainless steels with improved uniform plastic strain via controlled precipitation of coherent nanoparticles[J]. J. Mater. Sci. Technol., 2021, 93: 60-70.

Figures/Tables 10

Fig. 1. XRD pattern and microstructure of the designed Fe-5.30Cr-13.47Ni-3.10Al-1.22Mo-0.50W-0.23Nb-0.03C-0.005B alloy after solutionized treatment. (a, b): XRD pattern and OM image showing the martensitic matrix. (c): TEM bright-field (BF) image and corresponding SAED patterns showing that the nano-sized NbC particles dispersed into the BCC matrix. (d): High-resolution TEM image and its FFT pattern indicating that the NbC particle is incoherent with the BCC matrix.

Fig. 2. TEM characterization of the aged alloy at 773 K for 8 h. (a, b): TEM BF images and the corresponding SAED patterns showing that ultrafine B2 nanoprecipitates are dispersed into the martensitic matrix. (c, c-1, c-2): HRTEM image and FFT patterns showing that the B2 nanoparticles are coherent with the BCC matrix.

Fig. 3. TEM bight-field image (a) and the corresponding dark-field image and (b) of the aged alloy at 773 K for 48 h, in which the particle size of spherical B2 nanoprecipitates is about 3 ~ 5 nm in diameter.

Fig. 4. Mechanical properties of the current alloy at both solutionized and aged states. (a): Variation tendency of microhardness of the current alloy with aging time at 773 K. (b): Room-temperature engineering stress-strain tensile curves of the current alloy at different heat-treated states, in which PH 13-8 Mo [8] and Custom 465 [5] MSSs are also involved. (c): True stress-strain curves and the fitted curves (the inset) of these alloys. (d): Normalized strain hardening rate-true strain curves of the current alloy at the solutionized and 8 h-aged states, as well as PH 13-8 Mo [8] and Custom 465 [5] MSSs.

Table 1. Mechanical properties of the current stainless steel alloy at different heat-treated states, including microhardness (HV), yield strength (σYS), ultimate tensile strength (σUTS), uniform elongation (UE), and elongation to fracture (El). The fitted strain hardening coefficient (n) and the constants (K and εe) from the equation σ (MPa) = K × (ε - εe)n are also listed. Related data of PH 13-8 Mo and Custom 465 MSSs are listed for comparison.

States	HV	σ_YS (MPa)	σ_UTS (MPa)	UE (%)	Εl (%)	N	K (MPa)	ε_e (%)
Solutionized	315±9	841	1032	1.2	5.3	0.081	1515	0.65
8 h-aged	584±9	1872	2028	4.2	6.7	0.035	2390	1.12
12 h-aged	586±7	1870	2017	5.1	6.4	0.036	2382	1.03
24 h-aged	577±11	1863	1986	4.5	6.6	0.031	2302	1.05
48 h-aged	564±10	1835	1944	4.5	8.0	0.030	2248	0.97
PH 13-8 Mo¹	~	1428	1510	1.2	9.3	0.022	1691	0.91
Custom 465²	~	1711	1791	2.0	8.5	0.022	2007	0.92

Fig. 5. TEM characterization of the current alloy after tension. (a): TEM BF image of the solutionized alloy showing the elongated dislocation cells (marked with cyan arrows). (b): TEM BF image of the 48 h-aged alloy, in which the corresponding TEM DF image and SAED pattern along the [110] axis are also shown. (c, d): TEM DF images of aged alloy observed under two-beam conditions with g = 22?2 and g = 2?00 diffraction vectors (white arrows in the insets) showing the dislocation structures, i.e., homogeneous planar slip mode by dislocations cutting through B2 nanoprecipitates. (e): TEM DF image of Custom 465 MSS before tension, in which the corresponding TEM DF image and SAED pattern are also shown. (f): TEM BF images of Custom 465 MSS after tension, in which the Ni3Ti particles are in a rod shape, and dislocation loops and pile-up on lath boundaries after tension are marked with blue circles and red arrows, respectively. (g, h): Schematic maps of the interactions between dislocations and nanoprecipitates after tension in the current aged alloy and Custom 465 MSS, respectively.

Fig. 6. SEM observations of fracture morphologies of the solutionized (a) and 8h-aged (b) alloy samples after tension.

Fig. 7. Potentiodynamic polarization curves of the solutioned and 12 h-aged alloy samples, together with those of 304 ASS and FV520B MSS for comparison.

Table 2. Corrosion potential (Ecorr) and current density (Icorr) of the designed alloy in 3.5 wt.% NaCl solution at room temperature, in which those of 304 ASS and FV520B MSS are also measured for comparison.

Alloy	E_corr (V)	I_corr (μA⋅cm^-2)
The solutionized alloy	-0.328	2.108
The 12 h-aged alloy	-0.423	4.223
304 ASS¹	-0.331	1.749
FV520B²	-0.373	1.057
Custom 465³	-0.352	1.032

Fig. 8. Calculated strength increments from different strengthening mechanisms in solutionized (ST) and 8 h-aged samples, in which the measured yield strength values are also presented with dark star symbols for comparison.

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