A new strategy to strength-toughen metals: Tailoring disorder

Minqing Jing , , , Lnhong Di , ,

a State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China

b School of Engineering Science, University of Chinese Academy of Sciences, Beijing 101408, China

Keywords: Amorphous alloys High entropy alloys Strength-toughening Disorder tailoring Plasticity and fracture

ABSTRACT Metals have been mankind’s most essential materials for thousands of years.In recent years, however, innovation-driven development of major national security strategy and core areas of the national econ- omy is highly impeded by a shortage of advanced higher-strength-toughness metals.One of the main reasons is that metals inherently exhibit the inverted-relationship of strength-toughness.The emergence of two types of disordered metals: amorphous alloys and high entropy alloys, provides a fully-fresh strat- egy for strength-toughening by tailoring the topological and/or chemical disorder.In this paper, we first briefly review the history of strength-toughening of metals, and summarize the development route-map.We then introduce amorphous alloys and high entropy alloys, as well as some case studies in tailoring disorder to successfully achieve coexisting high strength and high ductility/toughness.Relevant challenges that await further research are summarized in concluding remarks.

1.Introduction

What types of materials are massively in service is an impor- tant sign of the progress of human civilization.For example, the development of steels is considered to be one of the driver forces to the first industrial revolution marked by the steam engine.From then on, various metallic materials have been widely developed and applied in high-tech fields such as transportation, manufactur- ing, defense, and aviation.Nowadays metals occupy a dominated niche in engineering structural materials, and they are irreplace- able in many fields [1] .The national strategy of energy saving, emission reduction and sustainable development urges stronger metals with higher toughness that can serve in demanding envi- ronments.In the past decades, a series of approaches to strength- toughen metals have been proposed based on concepts of hi- erarchical, multi-scale structure/phase construction [2–8] .Among them, amorphous alloys (AAs) [9–11] and high entropy alloys (HEAs) [ 12 , 13 ] have attracted worldwide attention because of their record-high strength and ductility/toughness.Due to inherent topological/chemical disorder, the two types of disordered alloys contain trans-scale emergence and evolution of non-equilibrium disordered structures under external loading.Their plastic flow therefore becomes spatio-temporally complicated and diverse, sub- verting the classical lattice-defect-mediated plastic mechanisms.Importantly, the research on plastic flow and fracture of disor- dered alloys provides a new way to crack the natural inverted- relationship between the strength and toughness [14] .Such fun- damental researches will also greatly promote the applications of disordered alloys as a new generation materials for kinetic energy weapon, nuclear energy and spacecraft shielding [15–19] .

2.Route-map for strength-toughening

Throughout the development history of metals for thousands of years, this type of materials is of two basic attributes.One is that their chemical compositions always contain one or two principal elements.The other is that their topological structures are packing mainly with ordered lattices.In this sense, traditional metals can be defined as ordered alloys dominated by chemical/topological order.Metallic materials are of two most important mechanical properties: strength and toughness.In this review, toughness is a loosely-defined property associated with cracking resistance, plas- tic deformation in compression, and uniform elongation (ductility) in tension.For ordered crystalline metals, the two properties de- pends not only on the types of “order”, but also largely on the “disorders”, i.e., lattice defects, in the “order”background.

In 1934, three scientists: Orowan [20] , Polanyi [21] and Taylor [22] , independently proposed the dislocation mechanism of crys- tal plasticity.Dislocations are one type of line defects of lattice, and their activation and motion determine plastic yield and sub- sequent deformation.Traditionally, the core idea of strengthening metals is to introduce various forms of “disorder”through compo- sition regulation and structural manipulation.These “disorder”will effectively slow-down or hinder the movement of dislocations, so that plastic deformation has to surmount higher barrier, thus in- creasing strength significantly.As illustrated in Fig.1 , there are two main routes to achieve such strengthening.The first route is to add some alloying elements to base metals.For example, high-strength steel or stainless steel can be obtained by adding minor C, Cr ele- ments to pure Fe.Such alloying technique can give rise to solid so- lution strengthening, precipitation strengthening, phase transition strengthening, etc.The second route is to reduce the grain sizes of bulk metals down into ultra-fine (≤1 μm) or even nanostructured (≤100 nm) regime.One of the motivations for such push for the very small grain sizes stems from the well-known Hall-Petch rela- tionship that predicts a continuous rise of strength with decreasing grain size [ 23 , 24 ].In the ultra-fine or nanostructured metals, dis- locations are limited within small grains and their movements are highly impeded by plenty of grain boundaries, thus contributing to unprecedented mechanical strength.

Every coin has two sides.Existence of immovable dislocations will inevitably weaken the ability of materials to deform plasti- cally, ultimately sacrificing ductility or toughness and particularly uniform elongation in tension.Therefore, the strength of metals is usually irreconcilable with their toughness or ductility, an effect referred to as the strength-toughness trade-off[14] , which seems an insurmountable hurdle.However, if there are multiple or excess plastic mechanisms in addition to dislocations that can be stimu- lated and act synergistically during the external loading process, it is possible to achieve a certain optimal balance between strength and toughness.Based on this idea, a few highly-strong-tough met- als have been developed in the past decades [2–8] .These metals are usually constructed by hierarchical, multi-scale structures or phases, so that multiple plastic mechanisms such as slips, twin- ing, phase transition, and grain boundary diffusion could be acti- vated synergistically upon loading.Nevertheless, these heteroge- neous metals still belong to chemically or topologically ordered systems, and their strength-toughening (S-Ting) level is gradually reaching the upper boundary.A question naturally arises.Is it pos- sible to break through such traditional routes, and to realize a new strategy for S-Ting which is dominated and regulated by the topo- logical and/or chemical disorder?

3.A new strategy: case studies

In recent years, some new types of high-strength-toughness metals represented by AAs and HEAs continue to emerge [25–32] , signifying the possibility of this new strategy.AAs and HEAs subvert traditional metals that have existed in an orderly form for thousands of years.They inherently are of topological and/or chemical disorder, and therefore belong to the disordered al- loys.AAs are formed upon cooling high-temperature melts fast enough to avoid crystal nucleation, but by deep supercooling to the glass transition temperature where the liquid structure is sud- denly “frozen”to a solid.This new type of glassy solid resides in thermodynamically metastable states, and the atomic packing in AAs is only of short-range-order [33–35] , but lacking in long-range order and transitional symmetry [36–38] .Such unique features en- dow AAs with inherent structural heterogeneities on nanoscale.In a sense, AAs can be regarded as the topological disorder limit of nano-structuring of metals, see the “topological structure”axis of Fig.1 .HEAs break through the traditional concept of alloy de- sign using one or two dominant metal elements.Instead, HEAs are multi-principal-element metallic systems, which are formed based on the idea of entropy-stabilized random solid solution [ 12 , 13 ].They contain high concentration (20%-25%) of multiple ele- ments with different crystal structures, but can crystallize as a single phase.Recently, the phase composition of HEAs tends to be multiphase or eutectic [ 19 , 28 , 39-41 ], which refreshes the precipitated-phase strengthening, the second-phase strengthening or other strengthening mechanisms.Equiatomic or near-equiatomic HEAs are the chemical disorder limit of element-devising of met- als, see the “chemical composition”axis of Fig.1 .For HEAs, there are four core effects [ 42 , 43 ]: (i) high-entropy effect in thermo- dynamics; (ii) sluggish effect in kinetics; (iii) severe lattice distor- tion in structures; and (iv) cocktail effect in properties.Next, we present some case studies about the new S-Ting strategies of the two types of disordered alloys: both AAs and HEAs.

Case 1: Shear-banding-mediated S-Ting .

In 2004, Schroes and Johnson [25] reported a monolithic Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 bulk AA with both 1.4 GPa strength and pro- nounced global plasticity.Under quasistatic compression, this high- strength AA shows a very large plastic strain of 20%, which is as- cribed to multiplication of shear bands during loading, as shown in Fig.2 a.Mediated by multiple shear bands; see Fig.2 b, this Pt- based alloy also shows permanent deformation and a strain ex- ceeding 3% before failure during bending.In 2011, Demetriou et al.[26] reported a Pd 79 Ag 3.5 P 6 Si 9.5 Ge 2 bulk AA with the yield strength close to 1.5 GPa in tension.This Pd-rich metal-metalloid alloy has a very high fracture toughness in terms of a stress intensity,KJ, of ～200 MPa ·m1/2, measured by the fracture resistance curve in Fig.2 c.Again, such remarkable damage-tolerance results from mul- tiplication of shear bands at the tip of an opening crack, and the extensive shear-band sliding promotes significant crack-tip blunt- ing, as shown in Fig.2 d.

Shear-banding in AAs results from stress-driven structural dis- ordering via an avalanche of shear-transformations (STs) [44–46] .Homogenous nucleation and stable propagation of shear bands could effectively balance strength and plasticity of alloys.Usu- ally, a shear band will rapidly evolve into an opening crack, par- ticularly in disordered systems.However, according to a recent study [47] , introducing chemical inhomogeneity/disorder will lead to more difficult cracking nucleation.Obviously, the shear-banding- mediated S-Ting of the two AAs mentioned above is closely as- sociated with unique topological and chemical disorder in these materials.

Case 2: Rejuvenation-induced S-Ting.

Metastable AAs tend to physical ageing, which is detrimen tal to the plastic deformation ability, known as ageing-induced brittle- ness [48] .Recent studies [ 27 , 49-52 ] have shown that either aged or as-cast AAs can be effectively rejuvenated into more disordered high-enthalpy states.The glass rejuvenation is not spontaneous, but requires external energy injection to ‘shake up’ the frozen-in disordered structure [53] .Without loss of high strength, the reju- venated AAs can show enhanced deformability [ 27 , 31 , 54-57 ].In 2015, Ketov et al.[27] reported that, after repeated cryogenic cy- cling between 338 K and 77 K, rejuvenated Zr-based AA rods with ～1.68 GPa strength show an increase in plastic strain under uniax- ial compression, as shown in Fig.3 .Further studies [58–60] have revealed that such rejuvenation-induced S-Ting results from ther- mally activated disordering of topological structures, pointing to- wards heterogeneities in the fictive temperature.

In addition to thermal activation, glass rejuvenation can be achieved by mechanical energy working to directly introduce non- affine strain in disordered structures [ 49 , 50 , 54 , 61-63 ].Pan et al.[49] have performed uniaxial compression on a Zr-based AA rod with a circumferential notch, and they found that the notch re- gion can experience an extreme rejuvenation without any shear banding.Furthermore, Pan et al.[31] cut a rod from the rejuve- nated notch region, as shown in Fig.4 a.Upon uniaxial compres- sion, this rejuvenated AA rod shows extensive strain-hardening (in- dicate as an increase of yield strength with successive loading- unloading) and homogeneous flow, as shown in Fig.4 b.It is re- vealed that these intriguing properties result from a gradual de- crease of topological disorder (i.e., free volume) induced by com- pressive plastic strain applied.Such ordering or relaxation mech- anism leads to strain-hardening and suppression of shear band- ing, which is consistent with the theoretical analyses by Jiang et al [64] .

Case 3: S-Ting via metastability engineering.

The single-f.c.c.-phase Fe 20 Mn 20 Ni 20 Co 20 Cr 20 , i.e., the so-called Cantor alloy [13] , is a case in point, and this alloy is the most successful HEA so far.In 2014, Gludovatz et al.[65] reported a grain-refined Cantor alloy with 6 μm grains.When the temper- ature decreases from 293 K to 77 K, this alloy shows a syn- chronous increase in both strength and ductility, as shown in Fig.5 a.But its fracture toughness (～220 MPa ·m1/2) remains un- changed, which is ascribed to a synergy of deformation mecha- nisms.In 2016, Li et al.[28] modified the equiatomic Cantor al- loy to the non-equiatomic Fe 50 Mn 30 Co 10 Cr 10 .The latter consists of the f.c.c.matrix (of ～45 μm grain size) and the h.c.p.phase lam- inate layers (ranging from several nanometers to 10 μm in thick- ness).Despite of distinct topological structures, the two phases are of the same composition distribution even at phase boundaries.Such metastable, dual-phase HEAs exhibit a mechanical behavior almost identical to that of grain-refined Cantor alloy [65] , as shown in Fig.5 b.More importantly, on grain refinement to ～4.5 μm, both strength and ductility of this alloy will further increase remark- ably.The excellent combination of strength and ductility stems from the existence of metastable f.c.c.phase where a large num- ber of stacking faults are present.When subjected to external load- ing, these stacking faults act as the nuclei of martensitic transfor- mation (f.c.c.→ h.c.p.).This metastable phase transformation can significantly accommodate plastic deformation, and at the same time, effectively activate dislocation slip and mechanical twining due to increased phase boundary density, as illustrated in Fig.5 c.Recently, Huang et al.[29] have confirmed that such metastability engineering still works well in brittle b.c.c.HEAs.The metastability engineering combines the chemical compositions adjustment with spontaneously changes of the topological disorder upon loading.This strategy suggests that the near-infinite compositional space of HEAs offers a broader room for S-Ting via topological/chemical dis- order regulation.

Case 4: S-Ting via chemical short-range order.

In the beginning, researchers take it for granted that the con- stituent elements of HEAs occupy the lattice sites completely ran- domly, although their packing structures still keep solid-solution crystals.However, recent studies by simulations and experiments have shown that chemical short-range order (CSRO) inherently ex- ists in HEAs or medium-entropy alloys (MEAs) [66–70] .In other words, some elements due to enthalpic interactions will preferen- tially form locally clustered domains, usually on nanoscale.These local CSRO domains could roughen dislocation pathways and thus affect their selection in slip, faulting and twinning [ 67 , 69 ].This provides a new avenue for S-Ting of MEAs and HEAs.

Recently, such CSRO-induced S-Ting has been successfully achieved in a TiZrHfNb HEA doped with 2 at% oxygen [30] .It is surprising to find that addition of 2.0 at% oxygen to the TiZrHfNb HEA simultaneously improves strength and ductility, as shown in Fig.6 a.However, doping this HEA with 2 at% nitrogen still presents the usual strength-ductility trade-off.Structural characterizations reveal that the S-Ting of the oxygen-doped HEA is due to the ex- istence of ordered oxygen complexes (OOCs) that are promoted by CSRO.More specifically, oxygen tends to occupy interstitial posi- tions adjacent to Zr and/or Ti-enriched lattice sites in an orderly manner, as shown in the inset of Fig.6 a.These nanoscale oxygen- containing complexes severely distort the local lattice, leading to a large strain field around them.On the other hand, during de- formation, these OOCs interact with dislocations, via pinning and promotion of dislocation double cross-slip, as shown in Fig.6 b.Or- dered oxygen interstitial complexes change the dislocation shear mode from planar slip to wavy slip, ultimately leading to disloca- tion multiplication.This is a new S-Ting mechanism, different from either conventional interstitial strengthening or nano-precipitate strengthening [8] .

4.Concluding remarks

From the four case studies about S-Ting of AAs or HEAs, we sug- gest that topological/chemical disordering of alloys provides near- infinite space where the disorder-property relationship can be ef- fectively tailored.These strategies are beyond the simple modifi- cation of lattice defects and conventional microalloying, which is thus expected to break the long-standing strength-ductility trade- offin metals.It seems to be exciting and intriguing, but some fun- damental mechanics problems should be further clarified to pave the path.We summarize these problems as follows.

(i) Plastic carry.In AAs, the basic carriers of plastic deforma- tion are dynamic STs [ 64 , 71-73 ], i.e., local irreversible re- arrangements occurring within zones a few to hundreds of atoms.In the background of long-range topological disorder, the structural origin of STs and their tempo-spatial evolution re- main mysterious [74–76] .Although lattice defects (e.g., disloca- tions) can be defined in HEAs, their characterizations face a big challenge.This is because that everything becomes fluctuated, from lattice parameters, Burgers vector, to stacking-fault ener- gies [77–79] .In HEAs, solvent and solute atoms are indistin- guishable, which also greatly challenges the conventional solid- solution concept.

(ii) Constitutive theory.To develop constitutive theory for AAs, the first challenge is the intrinsic interaction between vibra- tion and activation, or elasticity and plasticity.Such an inter- action is highly nonlinear and occurs in mesoscale [74] .There- fore, we must introduce proper state-variables to describe the two (fast and slow) processes [ 80 , 81 ].Moreover, the inher- ent Reynolds dilatancy [ 73 , 82 ] of amorphous deformation must be taken into account.For HEAs, their constitutive theory in principle can be developed from the conventional theories of crystalline plasticity [ 83 , 84 ].However, the challenge is to con- sider the structural fluctuation and the resulting multiple plas- tic mechanisms.In fact, it is necessary to consider the entropy (configurational and vibrational) contribution to deformation in the development of constitutive theory for both AAs and HEAs.

(iii) Flow instability.In either AAs or HEAs, the plastic flow can become tempo-spatially unstable in the form of shear-banding [ 17 , 44 ], cavitation or microvoid [85–87] , necking [ 32 , 88 ] and flow serration [ 89 , 90 ].These flow instabilities, as precursors of cracking, are closely associated with internal transport pro- cesses and external loading conditions.In addition to usual mo- mentum and energy transports, the intrinsic transport from dis- order should be carefully considered and described.More im- portantly, these flow instabilities will compete with each other, in which some effects from nonlocal, rate, and inertia should play key roles.

(iv) Fracture mechanism.In which way AAs and HEAs fracture di- rectly determines their S-Ting levels.For crystalline alloys, the crack propagates either by a certain blunt mechanism such as dislocations, or with a sharp tip to cleavage along a proper crys- tallographic plane.This picture, however, cannot be applied to AAs where any concept based ordered lattices is not well de- fined.Instead, the crack becomes blunt via ST-mediated shear banding, or advances in a brittle manner by a series of nano- cavitation [ 85 , 91 , 92 ].The two mechanisms’ competition highly depends on the degree of locally topological or chemical disor- der at the crack tip.In HEAs, the crack propagation mechanism is also complicated, due to the uncertain selection of crack-tip dislocations via slip, faulting and twinning.

(v) S-Ting strategy.For AAs, the S-Ting strategies include multipli- cation or suppression of shear bands at crack tips.The shear- banding behavior depends closely on the tempo-spatial feature of STs.But the challenging is to determine the relationship be- tween STs and topological/chemical disorder.For HEAs, in ad- dition to CSRO-induced S-Ting mentioned above, some conven- tional strategies such as second-phase [40] , fine-grain [41] , so- lution [19] and nanoprecipitate [93] still work well.But near- infinite compositional space of HEAs provides a huge tailoring room where the disorder-property relationship holds the key of S-Ting.

Declaration of Competing Interest

The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Outstanding Youth Science Fund Project of National Natural Science Foundation of China (NSFC) (No.12125206), the NSFC Basic Science Center for “Multiscale Problems in Nonlinear Mechanics”(No.11988102), and the NSFC (Nos.11972345 and 11790292).