劇烈塑性變形：不僅是晶粒細(xì)化

夏克農(nóng)(墨爾本大學(xué)機械工程學(xué)院，澳大利亞維多利亞3010)

劇烈塑性變形：不僅是晶粒細(xì)化

夏克農(nóng)
(墨爾本大學(xué)機械工程學(xué)院，澳大利亞維多利亞3010)

劇烈塑性變形在生產(chǎn)超細(xì)晶材料方面已經(jīng)顯示了巨大的潛力.雖然大量的研究集中在晶粒細(xì)化上，等通道轉(zhuǎn)角擠壓和高壓扭轉(zhuǎn)等劇烈塑性變形過程正在越來越多地被應(yīng)用于其他領(lǐng)域諸如粉末固結(jié)，利用變形引發(fā)相變制作新穎結(jié)構(gòu)與成分，獲得多相及多尺度材料以及固態(tài)回收技術(shù).本文引用我們近十年來的研究成果以介紹劇烈塑性變形在晶粒細(xì)化之外的諸多應(yīng)用.特別是利用劇烈塑性變形顆粒固結(jié)來制取大塊鋁、鈦、鋁/鈦雙相合金和鋁基納米復(fù)合材料，利用機械激活和強制合金化來獲得包括面心立方在內(nèi)的新型鈦結(jié)構(gòu)，利用劇烈塑性變形引發(fā)的相變來制備納米晶beta鈦合金，以及利用等通道轉(zhuǎn)角擠壓來固態(tài)回收鈦合金切削料.最后，對存在的挑戰(zhàn)和機會進(jìn)行了探討.

劇烈塑性變形;等通道轉(zhuǎn)角擠壓;高壓扭轉(zhuǎn);粉末固結(jié);相變

Severe plastic deformation (SPD) refers to unusually large deformation, especially that resulting from some particular processes. On the recommendation of the International SPD Steering Committee, SPD specifically refers to high cumulative strains achieved without significant changes in the overall dimensions of the workpiece and with the application of a considerable hydrostatic pressure[1]. Some of the best known SPD processes include equal channel angular pressing (ECAP), high pressure torsion (HPT) and accumulative roll bonding (ARB). The role played by SPD in refining grains and microstructures of bulk metallic materials is very well known[1]. However, the large amount of plastic deformation can also be used to achieve other objectives. In particular, SPD consolidation of particles offers opportunities to produce bulk materials from powder at relatively low temperatures and multi-phase and/or multi-scale alloys and composites[2], and far-off-equilibrium structures and compositions can be synthesised by SPD (e.g.[3]). This article uses a number of examples from our research in the last decade to illustrate the applications of SPD beyond grain refinement. The reader is referred to the original works cited for figures which are not reproduced and for more comprehensive references.

1 SPD for powder consolidation and creation of nanostructured alloys and composites

There are many reasons for using powder as starting material. First, if the material is not deformable and cannot be melted (e.g. ceramics), powder processing is the only method to shape it. Second, many desirable but non-equilibrium structures and compositions such as nanocrystalline, amorphous, extended solubility and immiscible alloys can be more easily obtained in particles than in bulk form. More importantly, it is most flexible to start with powders to produce composite and hybrid materials since a large number of phases of different structures and properties can be mixed readily.

1.1 Powder consolidation by SPD[2]

Unless the particles can be used directly as in the case of catalysts or fuels, it is necessary to consolidate the particles into bulk material for many applications. Conventionally, metal powders are sintered at high temperatures over long times to achieve bonding between particles. The process is energy intensive and often produces bulk materials with residual pores. Alternatively, particles can be consolidated by plastic deformation which can be achieved at much lower temperatures and shorter times with little porosity.

1.1.1 Principles

Metallic particles do not bond to each other since there exists a surface layer of oxide. In conventional sintering, bonding is achieved through diffusion at high temperatures. In the case of SPD consolidation, individual particles are forced to undergo shape change upon shearing. As the ceramic shells on the particles are not able to deform, they would be disrupted, leading to direct contact between naked metals. It has been demonstrated that two clean metal surfaces will bond spontaneously with each other, needing little activation[4-5].

It is critical, however, that the particles actually undergo shearing rather than sliding against each other. This is demonstrated clearly by comparing the cases of consolidation by ECAP of micro-sized, irregular shaped pure Al particles and that of nano-sized spherical ones. After just one pass at a low temperature of 100 ℃ and a low back pressure of 50 MPa, the micro-sized Al powder was easily consolidated into fully dense bulk material[6-7]whereas even at a much higher temperature of 400 ℃ and back pressure of 200 MPa, the individual nanoparticles maintained their round shape with pores between them[8]. It is obvious that factors favouring particle deformation rather than sliding would facilitate consolidation, i.e. particles that are soft, large and with irregular shape and ECAP at higher temperatures and with higher back pressures[2].

1.1.2 Pure metals

Aluminium: Pure aluminium was selected early to demonstrate the capability of SPD consolidation, in particular ECAP consolidation. The soft, micro-sized particles with irregular shapes were easily deformed and fully dense bulk material was obtained after the first pass[6]. Tensile tests on the samples processed by ECAP at 100 ℃ for 4 passes via route Bc show that the bulk Al produced is much stronger with reduced but still good ductility, and the behaviour is consistent with that observed in solid Al subjected to ECAP[7]. The amount of oxide from the particle surfaces turns out to be insignificant, being too little in quantity and too small in size.

Titanium: Commercially pure Ti (grade 2 with oxygen content (mass fraction)<0.25%) particles need to be consolidated at higher temperatures in order to give it sufficient ductility for ECAP deformation. Following two initial passes at 600 ℃ and two subsequent ones at 420 ℃ via route C, the bulk Ti displayed a high yield strength of >450 MPa (Fig.1), compared to 275 MPa for an ingot processed one. The ductility is reduced owing to the strengthening, as expected, but still adequate at >10% elongation.

With an increase in the content of (mass fraction) oxygen to over 1%, the strength can be significantly enhanced[9]. In addition, there is indication that a higher number of ECAP passes would improve plasticity, as shown in Fig.2.

Magnesium: Pure Mg particles were successfully consolidated by ECAP at 200 ℃. The fully dense bulk Mg displays strength comparable to that of extruded ingot Mg with good ductility[10].

1.1.3 Solid-state recycling

One extended application of SPD consolidation is in solid-state recycling. Pure Ti and Ti-6Al-4V machining chips have been successfully recycled by ECAP at moderate temperatures of 500～600 ℃[11-13]. It has been demonstrated that the surface oxide layer on the chips can be easily dissolved but would not cause any significant increase in the content of oxygen in the resulting bulk material. It is, however, worth pointing out that ECAP should be considered as a primary process (i.e. as a "casting" process which produces primary ingots) and conventional secondary processing is needed to achieve desirable mechanical properties. Indeed, tensile properties comparable or superior to those of the virgin commercial alloy have been obtained following ECAP recycling and subsequent standard mill-annealing[13].

1.1.4 Potential in the production of inexpensive Ti alloys

There has been great commercial effort to produce the so-called meltless Ti particles directly from minerals by chemical processing. The value of this emerging technology will only be realised if the resulting powder can be converted into bulk material without melting. SPD consolidation appears promising for such applications.

1.2 Metal matrix nanocomposites

Conventionally, metal matrix composites are those with added ceramic reinforcement. Metal matrix nanocomposites (MMnCs) refer specifically to those containing nano-sized ceramic particles. Although MMnCs can be produced by liquid metallurgy, it is difficult to get rid of clusters of the nanoparticles. Powder metallurgy has much advantage to offer.

1.2.1 In-situ Al nanocomposites consolidated from nano particles

Although the amount of oxide in the pure Al consolidated from micro-sized particles is negligible, the situation is vastly different in the case of nano-sized particles. Despite the inadequate consolidation after the first pass, fully dense bulk materials from ECAP consolidation of the nano Al particles were achieved after 4 passes at 400 ℃ with the application of a back pressure of 200 MPa[8, 14]. Theγ-Al2O3particles are about 5～10 nm in size, together with some amorphous alumina regions. The estimated volume fraction of aluminium oxide is 20%～30%[14]. Consequently, the material is very strong but with little ductility.

1.2.2 Al-C nanocomposites

Carbon black (CB) is an inexpensive material usually associated with fillers in rubber products such as tires and shoe soles. It is little known that the observed large CB particles are actually agglomerates of nano-sized (30～50 nm) globules. By milling CB and Al powders together, the individual CB nanoparticles can be dispersed in the Al matrix, forming Al-C nanocomposites[15-16]. After ECAP consolidation at 400 ℃ for 4 passes, the strength can reach >300 MPa in compression, considerably higher than that of the pure Al processed under the same conditions. In addition, the lubricating nature of CB may avail such composites to applications requiring low friction.

1.2.3 Ti-TiN nanocomposites

To improve mechanical properties at high temperatures, titanium alloys can be reinforced by forming Ti matrix composites. Although fibres appear to be most effective, discontinuous reinforcement would give rise to more isotropic properties and lower costs. In one example, initially sub-micro-sized TiN particles were mechanically milled with pure Ti ones and then ECAP consolidated at 500 ℃ for 4 passes with a back pressure of 150 MPa[17]. Perfect interfaces are achieved between Ti and TiN, with significantly increased strength but reduced ductility.

1.3 Multi-phase alloys

One of the advantages of SPD consolidation is that any number of phases can be used as long as the matrix phase is plastically deformable. In the context here, multi-phase alloys refer to multiple metallic/intermetallic phases. SPD consolidation is particularly useful if the phases involved are very different in such properties as strength and melting points. One such example is the binary phase alloy of Ti and Al[18-19]. Since Ti is much stronger than Al, mechanical milling is necessary to mix the two phases. Also, as Ti and Al can form intermetallic compounds easily, the processing has to be carried out at relatively low temperatures. In fact, the ECAP consolidation following ball milling to form dual phase particles was conducted at 350 ℃ to avoid the formation of titanium aluminides. The resulting microstructure consists of a hierarchy of the Ti and Al phases from nano to micro scales. Very high yield strength of > 800 MPa with considerable plasticity in compression is obtained in a Ti-47% Al (mole fraction) two phase material that has been ECAP consolidated for 12 passes[19].

2 Phase transformations and structural changes caused or affected by SPD

Phase transformations are most commonly associated with changes in temperature. However, it is well known that mechanical activation can also trigger such events, as in the case of stress induced martensitic transformation[20]. Most notably, mechanical milling which involves SPD can cause the formation of far-off-equilibrium phases such as amorphous and supersaturated solid solution[21].

2.1 Ti-Mg

Ti and Mg are immiscible since they have little mutual solubility in each other. In fact, their enthalpy of mixing is positive. In addition, the boiling point of Mg is lower than the melting point of Ti. It is thus impossible to make Ti-Mg alloys by the usual melting and casting method. When Mg is forced to alloy with Ti by mechanical milling, several metastable phases are observed[22]. In particular, an orthorhombic phase (γ) which is only observed under very high pressure (120 GPa) in pure Ti is found at ambient pressure following milling. A new orthorhombic phase (ε) and an fcc structured phase are identified and associated with local high Mg contents. The fcc-structured Ti is of particular interest from the technological point of view since it might offer better ductility and other novel properties in this class of important engineering alloys. It is also of theoretical importance. It is intriguing to explore the compositions that would lead to the stabilisation of the fcc Ti structure in addition to the familiar hcp and bcc structures.

2.2 Transformation of lamellar structures

Many alloy systems bear a eutectic or eutectoid reaction. Full lamellar or duplex structures are often present in engineering materials including steels andγ-TiAl alloys. However, the presence of a lamellar structure may, in some cases, be detrimental to properties or manufacturing characteristics. The following examples illustrate the role played by SPD in transforming lamellar structures.

2.2.1 Nickel-aluminium-bronze (NAB)

NAB is widely used in marine applications for its good strength and corrosion resistance. The corrosion performance, however, can be improved by eliminating the continuous morphology of the eutectoidκⅢlamellae. It is shown that SPD processes such as ECAP can quickly transform the lamellae by causing fragmentation, buckling and subsequent accelerated spheroidisation[23]as well as give rise to a complex fine grained structure which leads to enhanced strength with good ductility[24-25].

2.2.2 Medium carbon steel

To cold forge medium carbon steels into parts such as bolts and nuts, it is necessary to soften them by annealing to cause spheroidisation of the cementite phase.The standard annealing takes 72 h at 720 ℃ and is energy intensive. By applying ECAP, the annealing time can be reduced to just 30 min, leading potentially to significant savings for the industry[26].

2.3 Nanostructured beta Ti alloys

In addition to the above direct influences on phase and structural transformations, SPD also have effects on transformations that may have positive impact on microstructure and properties, as illustrated by the following examples.

2.3.1 Nanocrystalline metastable beta Ti alloys via SPD induced martensitic transformation

There is a limit to the minimum grain size achievable by SPD[27]. In the case of stableβTi alloys, the minimum grain size appears to be 100-150 nm after HPT for well over 100 of equivalent strain. However, in metastable beta Ti alloys, a martensitic transformation can be induced by plastic deformation, and this has been shown to facilitate grain refinement so thatβgrains of <50 nm are obtained in a Ti-5553 alloy by HPT at a moderate strain of 3-6[28].

2.3.2 Equiaxedαprecipitates in stableβTi alloys

SPD can also affect the formation of precipitates in subsequent heat treatment. In a stable Ti-20Mo alloy, the usual needle-shapedαprecipitates are changed into equiaxed morphology either inside the shear bands after ECAP or throughout the material after HPT, forming an ultrafine duplex structure of equiaxed andβ[29-30].

3 Challenges and opportunities

There are significant challenges before the potential benefits from SPD consolidation and SPD induced phase transformations can be realised. The first is inherent to the use of powder as starting material. In addition to surface oxides which might be turned into good use in forming MMnCs, impurities associated with the particle surfaces are always going to be a concern. Unlike in ingot metallurgy, there is little to do once they have been incorporated. However, SPD might make them distributed uniformly rather than segregated, thus minimising their effect as long as the total amount is small enough.

Although large, soft particles are identified as more amenable to SPD consolidation, the more desirable particles are likely to be hard and fine. Very high back pressure in ECAP might be needed to prevent sliding of particles. HPT is more suitable as the hydrostatic pressure involved is high, but the volume of material produced is limited.

SPD consolidation presents good opportunities for a future of materials by design. In this desirable approach, the user comes up with a series of demands that are difficult to meet by conventional alloying and processing since the diversity of properties would require multiple phases of very different characteristics, dimensions and quantities. The combination of performances would be provided by designing the material from scratch, i.e. by selecting appropriate particles of various structures and compositions. SPD consolidation is then used to realise bulk material with integrity.

4 Summary

Severe plastic deformation is a versatile processing method which can achieve much more than grain refinement. Through examples from our research, it is demonstrated that SPD consolidation of particles and SPD induced phase and structural transformations show great potential to produce novel materials meeting ever sophisticated demands for the future.

Acknowledgments

I am grateful to a large number of students and researchers in my group as well as international collaborators whose names are associated with various publications cited here. The research is supported partially by the Australian Research Council with the ARC Centre of excellence for Design in Light Metals and partially by the Defence Materials Technology Centre.

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Severe plastic deformation:beyond grain refinement

Kenong Xia
(Department of Mechanical Engineering, University of Melbourne, Victoria 3010, Australia)

Severe plastic deformation (SPD) based processes have shown great potential in producing bulk ultrafine grained materials. Although most of the research has been focused on grain refinement, SPD processes such as equal channel angular pressing (ECAP) and high pressure torsion (HPT) are increasingly used to achieve other objectives including powder consolidation, creation of novel structures and compositions through deformation induced phase transformations, production of multi-phase and/or multi-scale materials, and solid-state recycling.

severe plastic deformation; equal channel angular pressing; high pressure torsion; powder consolidation, phase transformation

10.14186/j.cnki.1671-6620.2015.04.001

TG 146.2

A

1671-6620(2015)04-0239-06

Using examples from our research in the past decade, various SPD applications other than pure grain refinement are presented. In particular, bulk pure Al and Ti, Al/Ti alloys, Al nanocomposites are synthesised by SPD consolidation of particles, unusual structures including fcc are created in Ti by mechanical activation and forced alloying, nanocrystalline beta Ti alloys are produced with the assistance of SPD induced phase transformation, and Ti machining chips are recycled in the solid state. Challenges and opportunities are discussed.