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НОВЫЕ ТЕХНОЛОГИИ ОБРАБОТКИ СОВРЕМЕННЫХ МАТЕРИАЛОВ С ИЗМЕЛЬЧЕНИЕМ ЗЕРЕН
© Теренс Лэнгдон,
почетный доктор наук РАН, иностранный член АН РБ, Исследовательская группа новых материалов,
факультет инженерии и окружающей среды, Саутгемптонский университет, Саутгемптон, SO17 1BJ, Великобритания, эл. почта: langdon@soton.ac.uk
В статье приводятся основные положения доклада, представленного на заседании Академии наук Республики Башкортостан в Уфе 20 июня 2016 г В настоящее время общепризнанным фактом является возможность достичь значительного улучшения свойств объемных металлов путем измельчения зерен до субмикрометрического или нано-метрического уровня. Этого можно добиться, если подвергнуть объемные твердые материалы пластической деформации, используя методы обработки, которые приводят к развитию многочисленных дислокаций с образованием новых границ зерен и уменьшением их размеров и при этом не оказывают существенного влияния на линейные размеры образцов. В статье описываются два новых подхода. Это, во-первых, кручение под высоким давлением (КВД) в производстве нанокомпо-зитов на основе металлической матрицы. Во-вторых, равноканальное угловое прессование (РКУП) в сочетании с холодной прокаткой при изготовлении химически чистого титана для зубных имплантатов. Повышение предела усталостной прочности с применением изгибных циклических нагрузок играет важную роль в улучшении общих качественных характеристик, полученных при обработке методом РКУП. Показано, что оба подхода являются перспективными при использовании промышленных металлических сплавов для синтеза улучшенных высокопрочных материалов, обладающих исключительными свойствами.
Ключевые слова: медицинские имплантаты, равнока-нальное угловое прессование, кручение под высоким давлением, композиты на основе металлической матрицы, интенсивная пластическая деформация
Terence G. Langdon
NEW PROCEDURES FOR PROCESSING ADVANCED MATERIALS USING GRAIN REFINEMENT
Materials Research Group,
Faculty of Engineering and the Environment,
University of Southampton,
Southampton SO17 1BJ, U.K.,
e-mail: langdon@soton.ac.uk
This report summarizes a presentation given to the Academy of Sciences of the Republic of Bashkortostan in Ufa on 20 June, 2016. It is now recognized that it is possible to achieve significant improvements in the properties of bulk metals by introducing grain refinement to the submicrometer or the nanometer level. This refinement may be achieved by subjecting bulk solids to severe plastic deformation using processing techniques that introduce large numbers of dislocations into the materials so that these dislocations can re-arrange to form grain boundaries and thereby reduce the grain size without producing any significant changes in the overall dimensions of the work-pieces. Two new approaches are described in this report. The first approach implies using high-pressure torsion (HPT) for the fabrication of metal matrix nano-composites. The second relies on combining equal-channel angular pressing (ECAP) with cold rolling in the fabrication of commercial purity titanium for use in dental implants. The increase in fatigue life when using cyclic bending loads is significant because it attests to the overall superior behavior introduced by ECAP processing. Both approaches are shown to have the potential for using with commercial metallic alloys in order to synthesize advanced materials having exceptional strength and properties
Key words: medical implants; equal-channel angular pressing; high-pressure torsion; metal matrix composites; severe plastic deformation
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Introduction. Advanced materials are now attracting much attention around the world. This is because they outperform conventional materials due to their superior properties such as hardness, toughness, durability and elasticity. Some advanced materials also have novel properties including an ability to memorize shapes or to respond directly to environmental changes. Therefore, the development of advanced materials provides the potential for designing and introducing new products in a wide range of commercial sectors.
In order to develop new and advanced materials, it is first necessary to identify the specific parameter (or parameters) that require optimization. In practice, the most important parameter is the grain size in all polycrystalline materials. Thus, the grain size dictates the strength through the well-known Hall-Petch relationship wherein higher strengths are achieved with smaller grain sizes and, if the grains are thermally stable at elevated temperatures, very small grains provide an opportunity for using superplastic forming in the fabrication of complex curved parts at high temperatures. These effects are well known in industry and thermo-mechanical processing is routinely conducted to introduce smaller grain sizes. But generally the smallest grain size attained in this way is of the order of a few micrometers and it has proven impossible to use conventional processing to reduce the grain sizes of bulk solids into the submicrometer regime.
This problem was resolved over twenty years ago with the demonstration, conducted by Valiev and his associates in Ufa, that it is possible to refine the grains of bulk metallic alloys into the submicrometer range through the application of severe plastic deformation (SPD) during processing [1]. This work was conducted on an Al-Cu-Zr alloy and it was shown that the grain size was reduced to ~0.3 m and the material then exhibited superplastic properties. This early report stimulated an interest in making use of SPD processing and over the last twenty-five years this interest has spread to many laboratories around the world [2].
Several different SPD processing techniques are currently available and they rely on introducing large numbers of dislocations
into the materials so that these dislocations can re-arrange to form grain boundaries and thereby reduce the grain size [3]. All SPD processes are designed to introduce substantial strain but without changing the overall dimensions of the work-pieces. Two SPD procedures are now receiving significant attention and these are equal-channel angular pressing (ECAP) where a billet is pressed through a die constrained within a channel that is bent through a sharp angle [4] and high-pressure torsion (HPT) where a disk is subjected to a high pressure and concurrent torsional straining [5].
These techniques, and the results obtained through SPD processing, were described in an earlier report appearing in this journal which was designed specifically to mark the 20th anniversary of the Institute of Physics of Advanced Materials at Ufa State Aviation Technical University [6]. More recently, new approaches have been developed for the fabrication of advanced materials using SPD processing and two of these approaches are described briefly in the following sections.
The fabrication of metal matrix nano-composites using HPT. Processing by HPT usually involves the application of torsional straining to a disk of a pure metal or metallic alloy but recently attempts were undertaken to examine the potential for inducing solid-state reactions through the simultaneous application of HPT to two different metals. For example, semi-circular half-disks of Al and Cu were bonded by applying HPT for up to 100 turns at ambient temperature [7] and four quarter-disks of pure Cu and an aluminum 6061 alloy were bonded by HPT in a single turn at room temperature [8]. Later, this approach was further developed by using disks of a commercial purity aluminum 1050 alloy and a ZK60 magnesium alloy, stacking three disks in an HPT facility in an Al/Mg/Al sequence and then processing these disks by HPT at room temperature using an applied pressure of 6.0 GPa for up to 10 turns [9-12].
Examination after HPT showed a multi-layered structure at the disk center with thin layers having thicknesses of ~30 nm and with evidence at the edges of the disk for the formation of an intermetallic compound of
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-Al12Mg17 within the Al matrix to give a metal matrix nanocomposite (MMNC). The presence of an MMNC led to exceptional strengthening and this is demonstrated by the hardness profiles across the disks as shown in Fig. 1 where values of the Vickers microhardness are plotted against the distance from the disk center and the datum points relate to measurements taken along linear traverses after 1, 5 and 10 turns [11]. The two horizontal dashed lines in Fig. 1 denote the hardness values of ~65 and ~105 for the Al-1050 alloy [13] and the ZK60 alloy [14], respectively. Thus, after 1 turn there are variations in hardness within the range of ~60-100 where these variations are due to the measurement locations which lie on different phases of Al or Mg, after 5 turns there is evidence for higher hardness values at radii >4 mm on both sides of the disk and after 10 turns there are exceptionally high hardness values at radii >3 mm with hardness up to ~270.
comparison with the conventional aluminum and magnesium alloys [11]. There is no upper limit on the MMNC data in Fig. 2 because information is not yet available on the attainable mechanical properties but nevertheless these results demonstrate the potential for using HPT with commercial metallic alloys in order to synthesize advanced materials having exceptional strength and properties.
Fig. 1. Hardness variations along the diameters of Al-Mg disks after HPT for 1, 5 and 10 turns [11]; also included are data for ZK60 [14] and Al-1050 [13]
Thus, the development of an MMNC provides an opportunity for attaining exceptionally high strengthening and this may be demonstrated very easily by making use of a toughness-strength diagram which was introduced earlier to delineate the range of fracture toughness and the corresponding strength-to-weight ratio for many metals and materials [15]. This approach is illustrated in Fig. 2 where the region for the MMNC has shifted to the right and covers a wide area by
Fig. 2. Fracture toughness versus strength-to-weight ratio for many metals and materials [15] including the synthesized Al-Mg nano-composite after processing by HPT [11]
Using ECAP to fabricate medical implants from commercial purity titanium. It is now well
established that nanocrystalline commercial purity (CP) titanium exhibits excellent biocompatibility by comparison with its coarsegrained counterpart [16, 17] and there is a direct report of the production of dental implants from CP-Ti processed initially by ECAP and then subjected to thermal mechanical treatments [18].
It is important to recognize that the mechanical response of dental implants under loading in real-life conditions cannot be evaluated adequately using conventional mechanical testing because of the occurrence of a combination of multiple loading conditions and various stress concentrations. To address this specialized problem, a standard was developed by the International Organization of Standardization specifically for the testing of dental implant materials. This standard is designated ISO 14801 and it incorporates the
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inclined loading and bone resorption that is a feature of the fatigue life of real dental implants. Figure 3 provides a schematic illustration of the principles of fatigue testing in accordance with the ISO 14801 standard [19]. In practice, the CP-Ti implant lies at an angle of 30 from the vertical and it is fixed to a steel crown having a hemispherical shape at the most distant point. An alternating load is then applied vertically through the centre of the hemisphere on the crown to correspond to the fatigue environment experienced in real situations.
Fig. 3. Schematic illustration of the method of fatigue testing designed to meet the ISO 14801 standard developed for the testing of dental implants [19]
Experiments of this type were conducted where the load followed a sinusoidal pattern such that the minimum load was equal to 10% of the maximum load and the frequency of application was 10 Hz. Experiments were conducted using CP-Ti both without SPD processing and after processing by ECAP for 4 passes at room temperature. The results demonstrated that processing by ECAP increased the yield stress and the ultimate stress but reduced the strain hardening rate so that the elongation to failure was reduced [19]. The increase in fatigue life when using cyclic bending loads is significant because it attests to the overall superior behavior
introduced by ECAP processing.
Even further strengthening may be introduced by following a procedure used earlier in which the CP-Ti is subjected to a two-step processing route of ECAP followed by cold-rolling [20]. This two-step procedure was conducted by processing with ECAP through up to 6 passes at a temperature of 573 K using a die with a channel angle of 120 , immersing in liquid nitrogen for 10 minutes, removing from the bath and then cold-rolling at a temperature of ~173 K to a reduction of ~70%. The results showed an exceptional strength/ ductility combination even after processing by ECAP through only 2 passes [20]. In practice, the behavior of the CP-Ti exceeded that of a Ti-6Al-4V alloy and the latter alloy is a standard material used regularly in orthopaedic implants [21].
The innovation potential for SPD processing. Sufficient results are now available to firmly establish the superior mechanical and functional properties that may be attained in bulk materials subjected to SPD processing. New avenues of processing are also available as discussed in the preceding two sections. There are now many opportunities for using these techniques to improve the physical and chemical properties of materials, including increasing the electrical conductivity, superconductivity and thermoelectricity in metals and alloys, improving the hydrogen storage capability and optimizing the biocompatibility. These trends, and other new developments, were discussed in a very recent review which provides a summary of the latest developments occurring over the last decade [22].
Summary and conclusions. Processing through the application of severe plastic deformation provides a valuable tool in fabricating advanced materials having superior properties.
Two new approaches are described based on using high-pressure torsion for the fabrication of exceptionally strong metal matrix nanocomposites and the use of a combination of equal-channel angular pressing and cold-rolling for the fabrication of commercial purity titanium for use in dental implants.
Acknowledgements. As a foreign member of the Academy of Sciences of the Republic of
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Bashkortostan since 1994, it was a very great pleasure for me to have an opportunity to make a presentation to the academy at a special meeting in Ufa on 20 June, 2016. I am grateful to Professor Ruslan Z. Valiev of Ufa State Aviation Technical University for arranging this meeting
and for his friendship and collaborations over a period of more than twenty years since my first visit to Ufa in May 1989. The preparation of this lecture was supported by the European Research Council under ERC Grant Agreement No. 267464-SPDMETALS.
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