metals Article
Characterisation of an Advanced Nickel Based Superalloy Post Cold Work by Swaging Martin R. Bache 1, *, James O’Hanlon 1 , Philip J. Withers 2 , Daniel J. Child 3 and Mark C. Hardy 3 1 2 3
*
Institute of Structural Materials, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, UK; james.o’
[email protected] School of Materials, University of Manchester, Oxford Rd, Manchester M13 9PL, UK;
[email protected] Rolls-Royce plc, P.O. Box 31, Derby DE24 8BJ, UK;
[email protected] (D.J.C.);
[email protected] (M.C.H.) Correspondence:
[email protected]; Tel.: +44-1792-295287
Academic Editor: Johan Moverare Received: 21 December 2015; Accepted: 23 February 2016; Published: 4 March 2016
Abstract: Cylindrical bars of the advanced nickel based superalloy RR1000 were subjected to swaging to induce approximately 30% cold work. Grain size analysis demonstrated a distinct modification to the microstructure whilst electron back scattered diffraction (EBSD) measurements confirmed the evolution of a relatively strong texture parallel with the longitudinal bar axis. Intragranular strain damage was identified. The effects of the swaging on bulk mechanical properties are illustrated across a range of test temperatures. Keywords: RR1000; swaging; microstructure; texture; mechanical properties
1. Introduction Surface treatments such as shot peening or burnishing are often applied to engineering components, in order to resist fatigue crack initiation in particular. The benefits are evident by an increase in the fatigue endurance strength, especially notable in the high cycle fatigue regime. This strength improvement can be assigned to the combined effects of the compressive, residual stress and the degree of cold work (pre-strain) induced at or near surface. The partitioning of the relative effects of these two factors is difficult to discern. However, previous high temperature fatigue studies have indicated that the joint application of high temperature and high strain can cause significant relaxation of shot peened residual stresses. Fatigue testing of shot peened Udimet® 720Li at 350 ˝ C, 650 ˝ C and 700 ˝ C, at a strain range of 1.2% by Evans et al. [1], measured approximately 50% relaxation of residual stresses after the initial fatigue cycle alone. Kirk [2] showed that a small amount of plastic strain applied under static tension could cause significant stress relaxation in shot peened copper and nickel. However, at the same time, both of those studies illustrated that the prevailing thermomechanical conditions did not significantly affect the inherent cold work microstructure. Hasegawa et al. [3] showed increased fatigue life at low temperatures in shot peened 0.5% carbon steel despite relaxation of residual stresses. Life deficits were only found after cold work levels relaxed at higher temperatures. Many aspects of these studies indicate that it is the cold work from shot peening that improves the fatigue performance over and above the effects of the compressive residual stress. In an attempt to isolate the effects of cold work on mechanical behaviour, the current study applied swaging to cylindrical bars of the nickel based superalloy RR1000. Detailed characterisation of the post swaged microstructures will demonstrate the resultant grain size and form, together with the
Metals 2016, 6, 54; doi:10.3390/met6030054
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evolution of microstructural texture. Mechanical assessment included microhardness and monotonic Metals 2016, 6, 2 of 13 tension across a 54 range of temperatures. Metals 2016, 6, 54 2 of 13 the evolution evolution of microstructural microstructural texture. texture. Mechanical Mechanical assessment assessment included included microhardness microhardness and and 2. Materials and Methods the of monotonic tension tension across across aa range range of of temperatures. temperatures. monotonic
2.1. Alloy Preparation
2. Materials and Methods
2. Materials and Methods Utilised for high pressure compressor and turbine discs, RR1000 offers good high temperature strength through an optimised microstructure containing coherent primary, secondary and tertiary γ’ 2.1. Alloy Preparation 2.1. Alloy Preparation precipitates within ahigh disordered γcompressor matrix. The size and form of these offers precipitates together with grain Utilised for for high pressure compressor and turbine turbine discs, discs, RR1000 RR1000 offers good high high temperature temperature Utilised pressure and good size are controlled via precise thermo-mechanical processing. The reader is directed to a previously strength through through an an optimised optimised microstructure microstructure containing containing coherent coherent primary, primary, secondary secondary and and tertiary tertiary strength published paper bywithin Mitchell and co-workers describing the form detailed microstructural evolution of this γ’ precipitates a disordered γ matrix. The size and of these precipitates together with γ’ precipitates within a disordered γ matrix. The size and form of these precipitates together with alloy grain together implications mechanical properties [4]. The nominal composition of RR1000 is grain sizewith are controlled controlled via for precise thermo-mechanical processing. The reader reader is directed directed to aa size are via precise thermo-mechanical processing. The is to previously published paper by Mitchell and co-workers describing the detailed microstructural provided in Table 1. previously published paper by Mitchell and co-workers describing the detailed microstructural evolution of of this this alloy alloy together together with with implications implications for for mechanical mechanical properties properties [4]. [4]. The The nominal nominal evolution Table 1. RR1000 nominal composition (wt. %). composition of RR1000 is provided in Table 1. composition of RR1000 is provided in Table 1.
RR1000
Co
Element RR1000 18.5 RR1000
Element Element
Cr 15
Table 1. 1.TiRR1000 RR1000 nominal nominal composition composition (wt. %). %).Zr MoTable Al Ta Hf(wt. 5 Co Cr 3.6 Mo 3Ti Al 2 Ta 0.5 Hf Zr0.06
Co 18.5 18.5
Cr 15 15
Mo 55
Ti 3.6 3.6
Al 33
Ta 22
Hf 0.5 0.5
Zr 0.06 0.06
C
B
Ni
Balance C0.027 B B 0.015 Ni Ni C 0.027 0.015 Balance 0.027 0.015 Balance
Six cylindrical bars of RR1000, approximately 300 mm length and original diameter 27 mm, Six cylindrical cylindricalatbars bars of RR1000, RR1000, approximately 300 mm length and original diameter 27 mm, mm, were Six of approximately length diameter 27 were rotary swaged room temperature aiming300 to mm achieve a and finaloriginal nominal diameter of were 20.9 mm rotary swaged at room temperature aiming to achieve a final nominal diameter of 20.9 mm (i.e., 40% rotary swaged at room temperature aiming to achieve a final nominal diameter of 20.9 mm (i.e., (i.e., 40% area reduction). The source of these bars was a proprietary disc forging of the “fine40% grained area reduction). The source of these bars was a proprietary disc forging of the “fine grained RR1000” area variant. reduction). The source of these bars was a proprietary disc forging of the “fine grained RR1000” RR1000” variant. variant. The final diameter after swaging at 20 20mm mmintervals intervals along length of each The final final diameter after swagingwas wasmeasured measured at at along thethe length of each each bar bar The diameter after swaging was measured 20 mm intervals along the length of bar (e.g., (e.g., as shown in Figure 1), using calibrated vernier callipers. All diameter measurements areare plotted as shown in Figure 1), using calibrated vernier callipers. All diameter measurements (e.g., as shown in Figure 1), using calibrated vernier callipers. All diameter measurements are in Figure 2. The measured bar diameters were used to determine the local area reduction and plotted in in Figure Figure 2. 2. The The measured measured bar bar diameters diameters were were used used to to determine determine the the local local area area reduction reduction and and the plotted the predicted predicted coldachieved, work achieved, achieved, Figure 3. predicted cold work FigureFigure 3. 3. the cold work
Figure 1. Post-swaged RR1000 bar.
Figure1.1.Post-swaged Post-swaged RR1000 Figure RR1000bar. bar.
Figure 2. Combined post swage bar diameter measurements.
Figure Combinedpost post swage swage bar Figure 2. 2. Combined bardiameter diametermeasurements. measurements.
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Figure 3. Associated cold work statistics. Figure 3. 3. Associated Associated cold cold work work statistics. statistics. Figure
Swaged bar diameters ranged between 21.4 mm and 23.1 mm, which constituted an area Swaged bardiameters diameters ranged between 21.4 mm31.3%, andmm, 23.1 mm,lower which constituted an area Swaged ranged between 21.4 mmwas and 23.1 which constituted an area reduction reduction of bar 26.6% to 37.9%. The mean cold work slightly than originally desired reduction 26.6% to considerable 37.9%.cold Thework mean cold workslightly wasstrength 31.3%, lower than originally desired of toof 37.9%. mean was 31.3%, lowerslightly than desired but a reflection but26.6% a reflection of The the room temperature of theoriginally alloy. but a reflection of the considerable room temperature strength of the alloy. of the considerable room temperature strength of the alloy. 2.2. Metallography 2.2. Metallography Metallography 2.2. A series of metallographic based inspection techniques were applied to the pre and post A series ofofmetallographic based inspection techniques werewere applied to theto pre andpre post swaged A series metallographic based inspection techniques applied swaged material to characterise the microstructure in various orientations. Figurethe 4 showsand the post axis material to characterise the microstructure in various orientations. Figure 4 shows the axis labelling swaged material to characterise the microstructure in various orientations. Figure 4 shows the axis labelling system employed to describe the results of X-ray diffraction (XRD) and EBSD studies. system employed to describe resultsthe of results X-ray diffraction (XRD) and(XRD) EBSDand studies. labelling system employed to the describe of X-ray diffraction EBSD studies.
Figure 4. Reference system used to describe bar microstructures. Figure Figure 4. 4. Reference Reference system system used used to to describe describe bar bar microstructures. microstructures.
Grain size analysis was performed before and after swaging in the radial (X and Y) and axial (Z) Grain size analysis was performed beforewere and after swaging in the radial (X and Y)with and axial (Z) directions. Standard metallographic sections including final polishing Grain size analysis was performed before and prepared, after swaging in the radial (X and Y) and colloidal axial (Z) directions. Standard metallographic sections were prepared, including final polishing with colloidal silica media. Images of the grain structure were captured by a Field Emission Gun Scanning Electron directions. Standard metallographic sections were prepared, including final polishing with colloidal silica media.(FEG-SEM, Images of the grain structureThe were captured by The a Field Emission Scanning Microscope FEI, Eindhoven, Netherlands). average and Gun Feret diameterElectron (i.e., the silica media. Images of the grain structure were captured by a Field Emission Gun Scanning Electron Microscope (FEG-SEM, FEI, Eindhoven, The Netherlands). The average and Feret diameter (i.e., the greatest distance between two points on a grain boundary) was measured using (i.e., ImageJ Microscope (FEG-SEM, FEI, any Eindhoven, The Netherlands). The average and Feret diameter the greatest distance between any two points on a York, grain NY, boundary) was measured using ImageJ software (1.4.0., National Institutes of Health, New USA). The ratio between the Feret and greatest distance between any two points on a grain boundary) was measured using ImageJ software software (1.4.0.,size National Institutes of Health, York, NY, USA). The ratio between the were Feret also and averageNational grain was used to indicate theNew typical grain form. sizes (1.4.0., Institutes of Health, New York, NY, USA). The ratioAverage between grain the Feret and average average grain size was used to indicatesize thedata typical grain form. Average grain sizes were also converted to ASTM are presented Table 2. were grain size was used classifications. to indicate theGrain typical grain form. Average in grain sizes also converted to converted to ASTM classifications. Grain size data are presented in Table 2. ASTM classifications. Grain size data are presented in Table 2. Sample Sample PreSample Swage Swage PostPre Swage radial Pre Swage Post PostSwage Swageradial axial PostSwage Swage axial radial Post Post Swage axial
Table 2. Pre and post swaging grain size data. Table 2. Pre and post swaging grain size data. Table 2. PreAverage and postGrain swaging grain size data. No. of Grains ASTM Feret Feret Diameter (μm) No. of Grains Average Grain ASTM Feret Measured Size (μm) Class Average Feret Diameter (μm) No. of Grains Average Feret Diameter Measured Size 8Grain (μm) 191 8 ASTM ClassClass 11.1 Feret Average 1.03 Average Measured Size (µm) (µm) 191 8 8 11.1 1.03 240 6 6 12.0 1.07 191 86 8 11.1 1.031.07 240 6 12.0 200 9 11 10.7 1.19 240 69 6 12.0 1.071.19 200 11 10.7 200
9
11
10.7
1.19
Typical examples of the pre and post swaged microstructures viewed on the radial X-Y plane Typical examples of therespectively. pre and post microstructures viewed on the are illustrated in Figure 5a,b, Allswaged images relate to areas near the central axisradial of theX-Y bars.plane are illustrated in Figure 5a,b, respectively. All images relate to areas near the central axis of the bars.
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Typical examples of the pre and post swaged microstructures viewed on the radial X-Y plane are illustrated in Figure 5a,b, respectively. All images relate to areas near the central axis of the bars.4 of 13 Metals 2016, 6, 54 Metals 2016, 6, 54
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Figure 5. Typical microstructures from (a) pre swaged RR1000 (X-Y plane, optical microscopy); (b) Figure 5. Typical microstructures from (a) pre swaged RR1000 (X-Y plane, optical microscopy); (b) post Figure 5. Typical from (a) pre swaged (X-Y plane, optical microscopy); (b) post swaging (X-Ymicrostructures plane, secondary electron SEM) and RR1000 (c) post swaging (along Z axis, SEM). swaging (X-Y plane, secondary electron SEM) and (c) post swaging (along Z axis, SEM). post swaging (X-Y plane, secondary electron SEM) and (c) post swaging (along Z axis, SEM).
Cumulative distribution function (%) Cumulative distribution function (%)
The grain size prior to swaging was within proprietary specifications for the fine grained (FG) The grain sizeand prior tograin swaging was within within proprietary specifications for the the fine grained (FG) The size prior to swaging was proprietary specifications for grained (FG) variant ofgrain RR1000 the morphology was close to equiaxed. However, thefine swaging process variant of RR1000 and the grain morphology was close to equiaxed. However, the swaging process has variant of RR1000 and the grain equiaxed. the swaging process has reduced the average grainmorphology size from 8was μmclose to 6toμm in the However, radial direction, alongside an reduced the average grain size from 8 µm to 6 µm in the radial direction, alongside an elongation to has reduced the average grain size from 8 μm to 6 μm in the radial direction, alongside an elongation to 9 μm in the axial sense. Each Feret diameter was plotted on a cumulative distribution 9function µm in the axial sense. Each Feret diameter was plotted on a cumulative distribution function (CDF) elongation to 9 μm in the axial sense. Each Feret diameter was plotted on a cumulative distribution (CDF) graph to emphasise this effect, Figure 6. graph to (CDF) emphasise this Figure 6. effect, Figure 6. function graph to effect, emphasise this 99.9 99.9 99 99 90 90 70 70 50 50 30 30 10 10 1 1 0.1 0.1 1 1
Swg Radial Swg Swg Radial Axial Swg Axial FG RR1000 FG RR1000 10
10 Grain size (um) Grain size (um) Figure 6. Cumulative distribution plot of Feret diameters, pre and post swaging. Figure Figure 6. 6. Cumulative Cumulative distribution distribution plot plot of of Feret Feret diameters, diameters, pre pre and and post post swaging. swaging.
As well as affecting grain size, the swaging process has imparted strain deformation within As as grain size, the process strain deformation within individual grains. This was most evident when viewed theimparted X-Y radial planes. Intersecting slip As well well as affecting affecting grain size, the swaging swaging processonhas has imparted strain deformation within individual grains. This was most evident when viewed on the X-Y radial planes. Intersecting slip lines were visible within many grains (Figure 7a), with others indicating the presence of individual grains. This was most evident when viewed on the X-Y radial planes. Intersecting slip lines lines were visible within many grains (Figure 7a), with others indicating the presence of intra-granular twin boundaries (Figure 7b). For reference, similar deformation features were found intra-granular (Figure 7b). For were found in conventionaltwin shotboundaries peened RR1000 material butreference, at a muchsimilar lower deformation severity thanfeatures the swaged material in conventional shot peened RR1000 material but attoa much lower severity swaged material and isolated to grains immediately adjacent the surface (Figurethan 7c).theWhen inspecting and isolated to grains immediately adjacent to the surface (Figure 7c). When inspecting microstructures on the axial plane the elongated form of individual grains was evident, Figure 8. microstructures on the axial plane the elongated form of individual grains was evident, Figure 8.
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were visible within many grains (Figure 7a), with others indicating the presence of intra-granular twin boundaries (Figure 7b). For reference, similar deformation features were found in conventional shot peened RR1000 material but at a much lower severity than the swaged material and isolated to grains immediately adjacent to the surface (Figure 7c). When inspecting microstructures on the axial plane the elongated form of individual grains was evident, Figure 8. Metals 2016, 6, 54 5 of 13 Metals 2016, 6, 54
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Figure 7. Intra-granular strain deformation in swaged material (a) intersecting slip; (b) slip defining Figure Figure 7. 7. Intra-granular Intra-granular strain strain deformation deformation in in swaged swaged material material (a) (a) intersecting intersecting slip; slip; (b) (b) slip slip defining defining twinning; (c) within isolated grains beneath a shot peened surface. twinning; twinning; (c) (c) within within isolated isolated grains grains beneath beneath aa shot shotpeened peenedsurface. surface.
Z Z
Figure 8. Intra-granular slip and elongated grain structure viewed on the axial plane. Figure 8. 8. Intra-granular Intra-granular slip slip and and elongated elongated grain grain structure structure viewed viewed on on the the axial axial plane. plane. Figure
2.3. Micro-Texture Analysis 2.3. Micro-Texture Micro-TextureAnalysis Analysis 2.3. A radial section of swaged RR1000 was examined by electron backscatter diffraction (EBSD) to A radial radial section section of of swaged swaged RR1000 RR1000 was was examined examined by by electron electron backscatter backscatter diffraction diffraction (EBSD) (EBSD) to to studyAgrain orientation, the propensity for slip and localised misorientation. Samples were mounted study grain orientation, the propensity for slip and localised misorientation. Samples were mounted study grainand orientation, propensity for slip and were mounted in Bakelite polished the with 0.06 μm colloidal silicalocalised solutionmisorientation. to produce the Samples highly polished surface in Bakelite and polished with 0.06 μm colloidal silica solution to produce the highly polished surface in Bakelite and polished with 0.06 µm colloidal silica solution to produce the highly polished surface required for high quality EBSD mapping. required for high quality EBSD mapping. required high quality EBSD mapping. Lowfor and high magnification EBSD maps were obtained. Pole and inverse pole figures at low Low and and high high magnification magnification EBSD EBSD maps maps were were obtained. obtained. Pole Pole and and inverse inverse pole figures figures at at low low Low magnification, shown in Figure 9, indicate considerable evidence of texture in pole the {111} direction magnification, shown in in Figure 9, 9, indicate indicate considerable considerable evidence evidence of of texture in in the the {111} {111} direction direction magnification, parallel to the Zshown axis (axialFigure direction). The high exposure densities (redtexture regions) shown at the centre parallel to the Z axis (axial direction). The high exposure densities (red regions) shown at the centre of the {111} pole figure (Figure 9a) and the {111} corner of Z inverse pole figure (Figure 9b), indicate a of the {111} pole figure (Figure 9a) and the {111} corner of Z inverse pole figure (Figure 9b), indicate a strong fibre-like texture. strong fibre-like texture.
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parallel to the Z axis (axial direction). The high exposure densities (red regions) shown at the centre of the {111} pole figure (Figure 9a) and the {111} corner of Z inverse pole figure (Figure 9b), indicate a strong Metalsfibre-like 2016,6,6,54 54 texture. of13 13 Metals 2016, 66of
(a) (a)
(b) (b)
Figure (a) The poleand and(b) (b)inverse inversepole polefigures figures measured measured by by electron back scattered diffraction Figure The pole and (b) inverse pole figures diffraction Figure 9. 9.9. (a)(a) The pole measured byelectron electronback backscattered scattered diffraction (EBSD) on swaged RR1000. Intensity (times random orientation) indicated by the colour keys inkeys each in (EBSD) swagedRR1000. RR1000. Intensity random orientation) indicated by theby colour keys in each (EBSD) ononswaged Intensity(times (times random orientation) indicated the colour case. case. each case.
The low low magnification magnification inverse inverse pole pole figure figure (IPF) (IPF) orientation orientation maps, maps, Figure Figure 10a,b, 10a,b, re-emphasise re-emphasise The The low magnification inverse pole in figure (IPF) orientation maps, Figureby10a,b, re-emphasise the heterogeneous distribution of grains the X and Y directions, as indicated the random grain the heterogeneous distribution of grains in the X and Y directions, as indicated by the random grain thecolours. heterogeneous distribution of grains in the X and Y directions, as indicated by the random grain The {111} {111} texture texture parallel parallel to to the the ZZ direction direction isis then then noted noted from from the the domination domination of of blue blue colours. The colours. The {111} texture parallel to the Z direction is then noted from the domination of blue coloured colouredgrains grainsin inthe theFigure Figure10c. 10c.ItItwould wouldappear appearthat thatmost mostof ofthe the{101} {101}orientated orientatedgrains grainsparallel parallelto to coloured grains indirection the Figure 10c. It would appeartowards that most of theorientation {101} orientated grains parallel to the Z the Z have been re-orientated the {111} by the swaging operation, as the Z direction have been re-orientated towards the {111} orientation by the swaging operation, as direction have re-orientated towards the {111}grains orientation by the swaging operation, as images indicated indicated bybeen the low low number of of green coloured coloured grains in Figure Figure 10c. High magnification magnification images indicated by the number green in 10c. High byshow the low number of green coloured grains in Figure 10c. High magnification images show evidence show evidence of the highly strained swaged microstructure as orientation varies within individual evidence of the highly strained swaged microstructure as orientation varies within individual grains in Figure Figure 11.swaged Grains demonstrating demonstrating this wereoften often surrounded surrounded byindividual smaller grains, grains, most likely11. of grains the highly strained microstructurethis as orientation varies within grainsmost in Figure in 11. Grains were by smaller likely to be γ’ precipitates. Grains demonstrating this were often surrounded by smaller grains, most likely to be γ’ precipitates. to be γ’ precipitates.
Figure 10. 10. Inverse Inverse pole pole figure figure maps maps in in the the (a) (a) XX direction; direction; (b) (b) YY direction direction and and (c) (c) ZZ direction. direction. Figure Figure 10. Inverse pole figure maps in the (a) X direction; (b) Y direction and (c) Z direction. Orientation Orientation colour key indicated. Orientation colour key indicated. colour key indicated.
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Figure 11. High magnification inverse pole figure (IPF) map taken from the axial plane indicating Figure 11. High magnification inverse pole figure (IPF) map taken from the axial plane indicating Figure 11. High magnification inverse polecircled figureregions). (IPF) map taken from the axial plane indicating intra-grain (examples within intra-grain misorientation misorientation (examples within circled regions). intra-grain misorientation (examples within circled regions).
Schmid factor maps illustrated a majority of grains with a Schmid factor between 0.4 and 0.5 in Schmid factor a majority majority of of grains with withaaSchmid Schmidfactor factor between0.4 0.4and and0.5 0.5 factor maps mapsasillustrated illustrated the X Schmid and Y directions, shown inaFigure 12a,b.grains The {111} texture in the Z between direction provided ain in theXXand andYYdirections, directions,as asshown shownin in Figure Figure 12a,b. 12a,b. The The {111} {111} texture texture in the ZZ direction provided a the in the direction provided significantly lower number of {111} planes favourably orientated at ˝45° to the Z direction in Figurea significantly lower number of {111} planes favourably orientated at 45 to the Z direction in Figure 12c, significantly of propensity {111} planesfor favourably at 45° to the (Z). Z direction in Figure 12c, therefore lower givingnumber a reduced slip in theorientated fibre texture direction The distribution therefore giving a reduced propensity for slip in the fibre texture direction (Z). (Z). TheThe distribution in 12c, therefore giving a reduced propensity for slip in the fibre texture direction distribution in Schmid factors measured in the three directions is plotted in Figure 12d. Schmid factors measured in the three directions is plotted in Figure 12d.12d. in Schmid factors measured in the three directions is plotted in Figure
Figure 12. Schmid factor maps in the (a) X, (b) Y and (c) Z directions. Distribution data representing Figure 12. Schmid factor maps the (a) X, (b) Y and (c) Z directions. Distribution data representing each direction is plotted (d). ininthe Figure 12. Schmid factorin maps (a) X, (b) Y and (c) Z directions. Distribution data representing each direction is plotted in (d). each direction is plotted in (d).
2.4. Residual Stress Measurements 2.4. Residual Stress Measurements 2.4. Residual Stress Measurements Axial residual stress and cold work measurements were obtained at 2 mm intervals across the Axial residual stress and cold work measurements were obtained at 2 mm intervals across the radialAxial surface by XRD. A and large collimator (2 mm) was used a high of exposures residual stress cold work measurements werewith obtained at number 2 mm intervals across(20) the radial surface by XRD. A large collimator (2 mm) was used with a high number of exposures (20) and long exposure time (5 s) to increase the amount of analysed material and enhance the accuracy radial surface by XRD. A large collimator (2 mm) was used with a high number of exposures (20) and long exposure time work (5 s) to increase the from amount analysed material and enhance the accuracy of XRD results. Thetime cold predicted theofof measured widthand halfenhance maximum and long exposure (5 s) towas increase the amount analysed full material the (FWHM) accuracy of XRD results. The cold work was predicted from the measured full width half maximum (FWHM) diffraction data using the following equation [4]. diffraction data using the following equation [4].
Cold work = 9.9296 × FWHM − 22.6 Cold work = 9.9296 × FWHM − 22.6
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of XRD results. The cold work was predicted from the measured full width half maximum (FWHM) diffraction data using the following equation [4]. Metals Metals 2016, 2016, 6, 6, 54 54
Cold work “ 9.9296 ˆ FWHM ´ 22.6
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Axial Axial residual residual stresses stresses in in the the swaged swaged material material were were relatively relatively symmetrical symmetrical across across the the radial radial Axial residual stresses in the swaged material were relatively symmetrical across the radial surface, with a minimum stress around −667 MPa near the centre of the bar and tensile stresses of surface, withwith a minimum stress the centre centreofofthe the bar and tensile stresses surface, a minimum stressaround around−667 ´667MPa MPa near near the bar and tensile stresses of of +85 approximately 22 mm to 4 mm inboard of the bar surface, Figure 13. The typical level of +85 MPa MPa approximately mm of the thebar barsurface, surface, Figure typical +85 MPa approximately 2 mmtoto4 4mm mminboard inboard of Figure 13. 13. TheThe typical levellevel of of compressive residual stress due to shot peening is indicated for comparison. Compressive residual compressive residual stress due to shot peening is indicated for comparison. Compressive residual compressive residual stress due to shot peening is indicated for comparison. Compressive residual stresses ranged between −459 MPa and −667 MPa in central 10 mm stresses ranged between −459 MPa andand −667 MPa in the the central 1010 mm diameter core of the bar. For stresses ranged between ´459 MPa ´667 MPa in the central mmdiameter diametercore coreof ofthe the bar. bar. For later reference, this was the region sampled by the gauge section of the specimens extracted later reference, thisthe wasregion the region sampled thegauge gauge section section of extracted for for later For reference, this was sampled byby the ofthe thespecimens specimens extracted for subsequent mechanical testing. subsequent mechanical testing. subsequent mechanical testing.
Figure 13. residual stress across the radial plane aa swaged Figure 13. Axial Axial residual stress data measured acrossthe theradial radial plane swaged bar. Figure 13. Axial residual stressdata datameasured measured across plane of of aofswaged bar. bar.
The swaged cold work was predicted measurements and plotted in The The swaged cold work was predicted from the FWHM measurements and plotted in Figure Figure 14, swaged cold work was predictedfrom from the the FWHM FWHM measurements and plotted in Figure 14, 14, along with the shot peened surface cold work and the average cold work predicted from the alongalong withwith the average average shot peened surface cold work and the average cold work predicted from the the average shot peened surface cold work and the average cold work predicted from the area reduction achieved by swaging. reduction achieved swaging. area area reduction achieved byby swaging.
Figure 14. work from the full width maximum (FWHM) measurements Figure 14. Cold workpredicted predicted from width half half maximum (FWHM) measurements on a radialon Figure 14. Cold Cold work predicted fromthe thefull full width half maximum (FWHM) measurements on aa section of swaged material compared to average level predicted from bulk areabulk reduction. radial section of material compared to level from area radial section of swaged swaged material compared to average average level predicted predicted from bulk area reduction. reduction.
3. 3. Mechanical Mechanical Properties Properties 3.1. 3.1. Micro-Hardness Micro-Hardness Vickers Vickers micro-hardness micro-hardness measurements measurements of of the the alloy alloy were were performed performed before before and and after after swaging swaging on on
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3. Mechanical Properties 3.1. Micro-Hardness Vickers micro-hardness measurements of the alloy were performed before and after swaging on a radial section of material with a 1 kg load. Hardness indentations were produced at 0.5 mm intervals across the centre line of the swaged radial section and at 1 mm intervals across a sample taken prior to swaging. Metals 2016, The 6, 54 measured micro-hardness values (HV) are shown in Figure 15. 9 of 13 Metals 2016, 6, 54
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Figure swaged bar bar materials. materials. Figure15. 15.Micro-hardness Micro-hardnessmeasurements measurements across across radial radial sections sections of of pre pre and and post post swaged Figure 15. Micro-hardness measurements across radial sections of pre and post swaged bar materials.
The trend in the micro-hardness data for the swaged bar was similar to that illustrated by the The trend trend in micro-hardness data for for the swaged bar was to that illustrated by the the The in the the micro-hardness data the swaged bar was similar similar to that illustrated residual stress and cold work measurements, whereby peak micro-hardness levels (625 HV)by were residual stress and cold work measurements, whereby peak micro-hardness levels (625 H ) were residual stress andcentre cold work measurements, whereby peak micro-hardness levels (625 2Hmm V) were V found near the bar and regions of considerably lower hardness were noted between and found near near the thebar barcentre centreand andregions regions of considerably lower hardness were noted between 2 mm found of considerably lower hardness were noted between 2 mm 4 mm from the bar surface. Average micro-hardness measurements performed prior to swaging (463 and HV) and 4 from mm from thesurface. bar surface. Average micro-hardness measurements performed prior to swaging 4 mm the bar Average micro-hardness measurements performed prior to swaging (463 HV) and after swaging (581 HV) showed a 26% increase in hardness of the swaged material. (463 HV ) and after (581 swaging (581 HV )ashowed a 26%in increase in hardness of thematerial. swaged material. and after swaging HV) showed 26% increase hardness of the swaged 3.2. Monotonic Tension 3.2. Monotonic Monotonic Tension Tension 3.2. Plain cylindrical test specimens, Figure 16, were machined from swaged bars, with their central Plain cylindrical cylindrical test test specimens, specimens, Figure Figure 16, were were machined machined from from swaged bars, bars, with their their central Plain axis coincident with the bar centre line. The16, gauge diameter was 10 swaged mm. Swagedwith test piecescentral were axis coincident coincident with withthe thebar barcentre centreline. line.TheThe gauge diameter was 10 mm. Swaged test pieces axis gauge diameter was 10 mm. Swaged test pieces subjected to the same surface finish as pre-swaged test pieces (i.e., Ra < 0.25 μm achievedwere via were subjected the same surface as pre-swaged test pieces achieved a < 0.25 subjected to polishing). thetosame surface finishfinish as pre-swaged test pieces (i.e., (i.e., Ra