Ti rich precipitate evolution in vanadium based alloys during annealing above 400 °C Accepted Manuscript Ti rich precipitate evolution in vanadium based alloys during annealing above 400 °C A Impagnat[.]
Accepted Manuscript Ti-rich precipitate evolution in vanadium-based alloys during annealing above 400 °C A Impagnatiello, T Toyama, E Jimenez-Melero PII: S0022-3115(16)30808-X DOI: 10.1016/j.jnucmat.2016.12.040 Reference: NUMA 50060 To appear in: Journal of Nuclear Materials Received Date: 22 September 2016 Revised Date: 27 December 2016 Accepted Date: 31 December 2016 Please cite this article as: A Impagnatiello, T Toyama, E Jimenez-Melero, Ti-rich precipitate evolution in vanadium-based alloys during annealing above 400 °C, Journal of Nuclear Materials (2017), doi: 10.1016/j.jnucmat.2016.12.040 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Ti-rich precipitate evolution in vanadium-based alloys during annealing above 400°°C a b RI PT A Impagnatielloa,b,*, T Toyamac, E Jimenez-Melero,a,b School of Materials, The University of Manchester, Manchester M13 9PL, UK Dalton Cumbrian Facility, The University of Manchester, Moor Row CA24 3HA, UK The Oarai Center, Institute for Materials Research, Tohoku University, SC c TE D M AN U Oarai, Ibaraki 311-1313, Japan EP Corresponding author (*): Dalton Cumbrian Facility AC C University of Manchester Westlakes Science & Technology Park Moor Row CA24 3HA United Kingdom Tel.: +44 1946 508860 Email: andrea.impagnatiello@postgrad.manchester.ac.uk ACCEPTED MANUSCRIPT Abstract We have assessed the plate-like TiO precipitate evolution in V-4Ti and V-4Ti-4Cr alloys during isochronal annealing above 400°C, by combining Vickers hardness, positron lifetime RI PT and coincidence Doppler broadening measurements Our results reveal the formation of additional TiO precipitates in both alloys at temperatures of 450-600°C in both alloys The implanted positrons become trapped at the nm-thick TiO/matrix interface, and act as effective SC probes of the concomitant annealing of vacancies taking place inside the TiO precipitates above 550°C in V-4Ti alloy The presence of Cr in the ternary alloy not only retards the M AN U recovery of dislocations, but also enhances the oxygen diffusivity and therefore decreases the vacancy content in the TiO precipitates These results will impact the expected alloy stability and capacity to bind light elements in the operational temperature window of these alloys for fusion reaction applications AC C EP nuclear fusion reactor TE D Keywords: refractory metal, crystalline oxides, positron annihilation, electron microscopy, ACCEPTED MANUSCRIPT Introduction The future realization of magnetically-confined fusion reactors relies on the prior development and testing of reduced-activation engineering alloys for the first wall and divertor components of the reactor [1, 2] Vanadium-based alloys stand out as promising first RI PT wall materials due to their low activation characteristics under fast neutron bombardment [3-6], in combination to their high temperature strength [6-9], void swelling resistance [10, 11] and compatibility with liquid metal coolants [12-14] Cr is added to the material as SC an effective solid-solution strengthener that increases the thermal creep and oxidation resistance, while Ti enhances the void swelling resistance of the alloy [15] The high M AN U temperature limit of ∼700°C for safe operation of V-based alloys in fusion reactor conditions is controlled mainly by the material’s resistance to thermal creep and helium embrittlement On the other hand, the low temperature limit is currently considered to lie at ∼400°C, due to the significant low-temperature hardening and embrittlement caused by the presence of small TE D amounts of O, C, and N and/or radiation-induced lattice defects in the matrix [9, 16, 17] Furthermore, relatively high amounts of Cr + Ti (>10wt.%) also lead to additional embrittlement at low temperatures [18] V-4Cr-4Ti is therefore widely considered as the EP reference vanadium alloy for structural applications in future nuclear fusion reactors [18, 19] AC C The presence of Ti in the material triggers the formation of ultra-fine Ti(C,O,N) precipitates during the thermo-mechanical processing of the material, and therefore improves the material’s resistance to low-temperature hardening and embrittlement, and also the workability and weldability of V-4Cr-4Ti [20, 21] The precipitate density can be maximised by cold working prior to annealing, and the best impact properties are achieved by annealing at 900-950°C [22] Welding V-based plate materials can dissolve part of the Ti-rich precipitates, and a post-weld heat treatment is recommended to recover the precipitate distribution and, consequently, the pre-weld values of the ductile-to-brittle transition ACCEPTED MANUSCRIPT temperature [19] Recent work on the low-temperature behaviour of V-4Cr-4Ti reveals the formation of additional Ti(C,O,N) precipitates below 400°C in the vicinity of radiationinduced dislocation loops [23] This radiation-induced precipitation allows the optimal temperature window during annealing prior to irradiation to be extended up to 1000°C [24] RI PT Despite the intensive efforts to optimise the precipitate characteristics for improved properties at low temperatures, limited information is available about the behaviour of those precipitates at intermediate temperatures, i.e within the accepted temperature window SC (∼400-700°C) for use of the V-4Cr-4Ti alloy in fusion reactor applications, and how any M AN U potential changes would affect mechanical properties and radiation resistance of this material In this paper, we performed micro-hardness tests to monitor the microstructural changes above 400°C in this material containing Ti-rich precipitates, in combination with positron annihilation spectroscopy studies, since positrons are reported to present a relatively high affinity for the Ti-rich precipitates present in the V-4Cr-4Ti alloy [23] The potential TE D influence of Cr on the structural stability is assessed by comparing the results for the V-4Ti and V-4Cr-4Ti alloys This study constitutes the stepping stone to assess the precipitate stability at relevant reactor temperatures and radiation doses, and the consequences for the AC C Experimental EP radiation tolerance and structural integrity of these low-activation materials V-4Ti and V-4Cr-4Ti materials were solution annealed at 1100°C for hours in inert atmosphere, followed by water quenching to room temperature The resultant microstructure was characterized by scanning and transmission electron microscopy Specimens for structural analysis were prepared by mechanical pre-thinning, followed by electro-polishing using an electrolyte of 60 vol.% methanol–35 vol.% 2-buthoxyethanol–5 vol.% perchloric acid (60%) at a temperature of −35°C We used the FEI Quanta 250 FEG Scanning Electron microscope, and also the FEI Tecnai G2 F30 (FEG TEM) microscope operating at an ACCEPTED MANUSCRIPT accelerating voltage of 200kV Afterwards, samples of both alloys were isochronally annealed for 1.5 hours at selected temperatures between 200°C and 900°C, and subsequently quenched in water to room temperature We measured the Vickers micro-hardness of the annealed samples using a load of 0.5kg and duration of 5s RI PT Samples annealed at temperatures in the range of 500-700°C of both alloys, together with the reference room temperature condition, were also studied using Positron Annihilation Spectroscopy (PAS) For this experiment, positrons were emitted from a 22Na source having SC an activity of ∼1MBq and sealed in Kapton foil The source was placed between two equivalent vanadium samples The annihilation radiation generated from the positron-electron M AN U pair contains information about the electronic environment around the annihilation site The positron lifetime spectra were obtained using a fast digital oscilloscope and two BaF2 scintillator detectors, with a time resolution of ∼180ps at full width at half maximum For each spectrum we collected ∼2.5-4 × 106 coincidence events The data was corrected for the TE D background and source contributions, and analysed using the PALSfit software package [25] We also recorded the Coincidence Doppler broadening (CDB) spectra of the positron annihilation radiation using two high-resolution Ge detectors in coincidence When there is a EP net centre of mass associated with the annihilating pair, the Doppler broadening corresponds AC C to ∆E = ± (1/2) × pL × c, where pL denotes the longitudinal component of the electronpositron momentum along the direction of the γ-ray emission and c is the speed of light in vacuum The Ge detectors collects simultaneously the upshifted and down-shifted γ-rays The two-detector arrangement enhances the signal to noise ratio by three orders of magnitude, as compared to the single-detector approach We were therefore able to assess, using the high-momentum region of the CDB spectra, the positron annihilation with the core electrons of atoms in the vicinity of the positron trapping site [26] The overall momentum resolution was ∼4 × 10-3 m0c, where m0 denotes the electron/positron rest mass and c is the ACCEPTED MANUSCRIPT speed of light in vacuum The CDB spectra for each sample were taken for 80k s with a total number of counts of ∼800k, whereas the counting time for reference Ti and SiO2 materials was 420k s, with a total number of counts of ∼40M From the recorded CDB spectra, we derived the line shape parameters, namely the S- and W-parameter, by taking the ratio of the RI PT low momentum (|pL| < × 10-3 m0c) and high momentum (10 × 10-3 m0c < |pL| < 18 × 10-3 m0c) region of the spectrum to the total region, respectively The S-parameter corresponds to positron annihilation with low-momentum valence electrons, and is therefore sensitive to low SC electron density areas in the material such as vacancies or pores The W-parameter yields M AN U information about the characteristics of the positron annihilation with high-momentum core electrons of the atoms in the vicinity of the positron trapping site [27] Since the core electrons retain their atomic character in the solid, the W-parameter is sensitive to the chemical surrounding at the annihilation site [26] The S-W plot reveals information about the chemical environment of the potential positron annihilation sites We also obtained the TE D CDB ratio curves by normalizing the momentum distribution of each spectrum to that of a reference sample of either well-annealed Ti or SiO2 The CDB ratios help us to reveal small differences in the high-energy (i.e high-momentum) region of the spectrum, since the EP recorded CDB spectra span several orders of magnitude [26, 28] SiO2 is used as a reference AC C for O presence at the positron site, since implanted positrons in SiO2 are expected to annihilate mainly with the electrons of the oxygen atoms [29, 30] The high momentum region of the CDB ratio curves with respect to Ti and SiO2 manifests the presence of either Ti or O close to the positron trapping site [26, 31, 32] Results 3.1 Initial microstructure The room-temperature microstructure of both V-4Ti and V-4Cr-4Ti materials, after annealing at 1100°C for hours, contains a relatively high density of precipitates Fig 1a ACCEPTED MANUSCRIPT shows an SEM image of plate-like TiO precipitates present inside the grains of the V matrix The interface between those precipitates and the matrix is characterised by a superstructure (Fig 1b), which in some cases even extends through the full thickness of the precipitate (Fig 1c) Our recent aberration-corrected STEM/EELS results revealed the atomic ordering RI PT of the V and Ti at the precipitate/matrix interface (Fig 1d) This superstructure was interpreted in terms of the intergrowth of the TiO fcc and V bcc structures [33] In addition to the plate-like precipitates, we have also observed the presence of a high density of cuboidal- SC shaped precipitates, which are mainly decorating the grain boundaries of the V matrix (Fig 1e) and present a size distribution in the range of 200-700nm (Fig 1f) The chemical M AN U analysis of those precipitates confirmed the TiO nature of these GB precipitates, with small amounts of C and N Cuboidal-shaped precipitates were also observed inside the grains in the V matrix, but mainly in the vicinity of grain boundaries 3.2 Behaviour during annealing TE D Fig shows the hardness values of both V-4Cr-4Ti and V-4Ti as a function of the annealing temperature At room temperature, in the ‘as-quenched’ condition after the initial annealing at 1100°C for hours, the hardness of the ternary alloy is higher due to the EP presence of Cr atoms in the matrix The hardness remains constant up to 300°C in both AC C materials At higher temperature, the hardness in V-4Ti decreases, and only recovers the room temperature value when 500°C is reached On the contrary, the hardness in V-4Cr-4Ti remains constant until 400°C, and above that temperature the hardness gradually increases and attains a maximum at 600°C The V-4Ti alloy also presents a maximum in hardness at 600°C However, this alloy presents a smaller increase in hardness with respect to the room temperature value, as compared to the ternary alloy At temperatures higher than 600°C, the hardness in both materials gradually decreases, reaching values at 900°C even lower than at room temperature This decrease in hardness is especially noticeable in the V-4Ti alloy ACCEPTED MANUSCRIPT In order to gain further insight into the processes taking place in both alloys during annealing in the temperature range of 500-700°C, we performed positron lifetime and CDB measurements at selected temperatures in that range, and also at room temperature for both alloys and pure V as reference Fig shows the values of the lifetime (τ) and line shape RI PT S-parameter for the studies samples The lifetime values of the alloy samples lie in the range of 116-128ps, in close vicinity of the measured τ value for V of 117ps Both the τ and S-parameter for the room temperature, 500°C and 550°C samples in the V-4Ti alloy are SC significantly higher than for the V-4Cr-4Ti samples, and shift to lower values when the M AN U annealing temperature is 600°C or higher In contrary to the behaviour observed for the binary alloy, the values of both positron parameters remain closer to the values for V, and not reveal any clear trend with temperature We have extracted additional information about the positron local environment from the CDB data, by deriving the W-parameter from the high-momentum region of the 511keV peak and the ratio curves with respect to the reference TE D materials Ti and SiO2, see Fig and respectively The data collected in both figures reveal that the values of the W-parameter and the CDB ratios lie closer to Ti and SiO2 in the case of temperature AC C Discussion EP V-4Ti, and shift toward the ternary alloy and the V sample with increasing annealing V-4Ti and V-4Cr-4Ti alloys present an equivalent microstructure at room temperature, after the initial solution annealing at 1100°C for hours Their microstructure contains a fine dispersion of Ti-rich precipitates, with two types of morphologies and spatial distributions: cuboidal-shaped precipitates [34] decorating or in the vicinity of the grain boundaries of the V matrix, and plate-like precipitates [20, 35, 36] distributed within the grains of the V matrix The interface of the latter precipitates with the matrix is characterised by an intergrowth of the bcc V and fcc TiO structures [33] Both types of precipitates are ACCEPTED MANUSCRIPT primarily TiO, but contain small amounts of C and N These precipitates may have formed during cooling from 1100°C down to room temperature, by the transformation of the high temperature β-TiO to α-TiO [37] Despite the similarities in the starting precipitate characteristics and distribution of RI PT both alloys at room temperature, the hardness dependence on the annealing temperature up to 900°C shows significant differences The hardness in the V-4Ti alloy decreases in the temperature range of 300-450°C This reduction in hardness is not observed in the V-4Cr-4Ti SC alloy Previous work on the recovery and recrystallization of V samples with varying nitrogen M AN U and oxygen levels showed that the dislocation-impurity interaction in vanadium is overcome at 300°C or higher temperatures [38] Therefore the softening observed in V-4Ti can be attributed to an additional recovery of pre-existing dislocations at 300-450°C However, in V-4Ti-4Cr, the Cr atoms present in the matrix exert an effective pinning effect on the movement of dislocations [39, 40], and can therefore delay the recovery and the TE D corresponding alloy softening at those temperatures Furthermore, the presence of extra O atoms in the V matrix, due to O intake during the material’s processing or from the Li-based EP coolant during reaction operation, is expected to precipitate as plate-like TiO [35], and therefore induce additional hardening to these alloys The diffusion of O interstitials in the AC C V matrix is relatively fast; with a reported value for activation energy of 119.6-122.5 kJ mol-1 [41-43] The presence of Cr in the matrix reduces the activation energy for oxygen diffusion by 10-15% [43] Ti atoms tend to decorate dislocations in V-Ti alloys, and therefore experience an enhanced mobility due to pipe diffusion along dislocations, prior to the formation of Ti-rich precipitates [23, 37] The increase in hardness in both alloys at temperatures of ∼400-600°C can therefore be attributed to the additional formation of Ti-rich precipitates Further increase in temperature above 600°C leads to the precipitate coarsening, destabilization and subsequent dissolution ACCEPTED MANUSCRIPT Once implanted in the sample and thermalized, the positron can become trapped by existing open volume lattice defects The reduced electron density at the trapping site increases the positron lifetime with respect to the bulk lifetime Not only vacancy-like defects form attractive potentials for positrons but also metallic clusters or precipitates, as long as RI PT their positron affinity for those is higher than for the surrounding material In the case of the V-Cr-Ti system, the positron affinity for Cr is lower than for either V or Ti, and has recently been reported to be relatively high for the Ti-rich precipitates [23, 44] In our study, the SC measured positron lifetime in each of the studied samples remain close to the value for the V sample of τ = 117ps [45] We have only detected one component in the lifetime spectrum, M AN U which could be the average value of positrons annihilating in the bulk V matrix and in open volume defects The lifetime of positrons trapped in vacancy clusters is higher than in the bulk, and its value increases with the number of vacancies in the cluster [32] Our results yield values for the positron lifetime that are in all cases higher, but relatively close to the TE D value for vanadium This means that a significant fraction of the positrons are annihilating at mono-vacancies This is confirmed by the higher values of the S-parameter encountered in the samples of both V-4Ti and V-4Cr-4Ti The fcc structure of TiO-type precipitates can host EP up to approx 15% of vacancies [46, 47] and also small amounts of C and N [48] Therefore, a AC C significant fraction of the implanted positrons will be annihilating at vacancy-type defects present mainly in the TiO layer of the precipitate/matrix interface, but potentially also inside the precipitates themselves The decrease in both the positron lifetime and the S-parameter in V-4Ti when the temperature is higher than 550°C can be attributed to the annealing of vacancies associated primarily with the TiO layer at the precipitate interface [37] The decrease in the S-parameter takes place simultaneously with an increase in the W-parameter that characterises the high-momentum region of the CDB spectrum This region of the spectrum, together with the CDB ratio with respect to reference materials such as Ti or 10 ACCEPTED MANUSCRIPT SiO2, contains additional information about the nature of the atoms surrounding the positron trapping side [31, 49] The results in Fig and reveal that the local environment of the positron trapping site has a higher fraction of Ti and O in the binary alloy, and shifts towards a higher presence of V atoms with increasing temperature The values derived for the ternary RI PT alloys at the studied annealing temperatures are all closer to the values of the V sample The relative positron affinity for the mono-vacancies in the fcc TiO layer and for the bcc V-type layer, both forming the intergrowth in the plate-like precipitates, lead to the observed trends SC in the W-parameter and CDB ratios As the amount of vacancies in the TiO-type layer decreases, the fraction of positrons trapped in the V-layer forming the precipitate matrix M AN U interface increases The vacancy content in TiO depends on the annealing temperature, and also on the O diffusivity that is enhanced in the presence of Cr atoms in the V matrix [37] The decrease in the S-parameter and the concomitant increase in the W-parameter observed in the V-Ti alloy with increasing annealing temperature is not observed in the V-4Cr-4Ti TE D This difference is most likely due to the enhanced O diffusivity and therefore lower vacancy content in the TiO precipitates formed in the ternary alloy A linear trajectory in the S-W plot is normally associated with the variation of the fraction of positron annihilation at different EP competing sites [50] The linear trend observed in the S-W plot in our study reflects the AC C competing affinity of the TiO and V interface layers for positron trapping, and implies a higher fraction of positrons trapped in the V layer in the ternary alloy as compared to the binary alloy whose precipitates seem to contain a higher amount of vacancies We have used hcp Ti as reference material for the Ti presence at the trapping site in fcc TiO-type precipitates The screening of the core electrons of the local host atoms by valence electrons, and consequently the penetration of the positron wave function and overlap with the core electron wave function, also depends on the spatial arrangement of the local atoms at the positron site [51] In our study, the different lattice arrangement of the Ti atoms in the Ti 11 ACCEPTED MANUSCRIPT reference material and in TiO precipitates may lead to small errors in the analysis of the highmomentum region of the CDB spectrum, and may not allow us to establish definitive conclusions about the predominance of Ti or O atoms surrounding the positron trapping site in the TiO-type layer of the interface structure However, despite the similar characteristics of RI PT the core electrons in the three main elements of these materials, namely V, Ti and Cr, our high-momentum CDB data is able to reveal small differences in the chemical environment around the positron trapping site, and therefore in the vacancy content at the SC precipitate/matrix interface The similarities in the PAS results at the higher annealing temperatures in both alloys confirm the stability of the TiO/V interfacial intergrowth after M AN U annealing, and allow us to assign the observed changes in the annealing behaviour of both alloys to the oxygen diffusion and vacancy content at the precipitate/matrix interface In future work we intend to study the spatial distribution of the vacancies at the precipitate/matrix interface, and their influence on the local bonding and symmetry, using TE D atomic-resolution electron energy loss spectroscopy and the strain contrast in the annular dark field (ADF-TEM) data We also aim to investigate the structure stability and the oxygen/vacancy flux at the precipitate/matrix interface in simulated nuclear reactor EP environments, and therefore evaluate if these precipitates with the TiO/V interface AC C intergrowth can act as an effective sink for radiation-induced lattice defects and enhance the radiation tolerance of the V-4Cr-4Ti alloy, as advanced candidate material for future nuclear fusion reactors Conclusions We have studied the evolution of the Ti-rich precipitates present in V-4Ti and V-4Cr-4Ti alloys during isochronal annealing, by combining Vickers hardness, positron lifetime and coincidence Doppler broadening measurements Our results reveal that the local atom probe characteristics of the implanted positrons are suitable to monitor the annealing of vacancies in 12 ACCEPTED MANUSCRIPT plate-like TiO precipitates, esp at the interface with the surrounding matrix The presence of Cr in the material both retards the recovery of pre-existing dislocations in V-4Cr-4Ti and enhances the O diffusivity, and consequently increases the oxygen content in the precipitates Additional precipitate formation takes place in both alloys materials above 400°C, whereas RI PT the annealing of vacancies, present in the TiO precipitates, above 550°C is observed only in the V-4Ti alloy Acknowledgements SC We acknowledge the Engineering and Physical Sciences Research Council (EPRSC) for providing funding for this project via the Centre for Doctoral Training in the Science and M AN U Technology of Fusion Energy (http://www.fusion-cdt.ac.uk/) The work described was supported in part by the Dalton Cumbrian Facility Project, a joint initiative of The University of Manchester and the Nuclear Decommissioning Authority A.I would also like to thank the TE D Tohoku University for the travel grant to 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1996 [49] K.G Lynn, A.N Goland Solid State Commun 18 (1976) 1549 [50] M Clement, J.M.M de Nijs, P Balk, H Schut, A van Veen J Appl Phys 81 (1997) SC 1943 [51] M Alatalo, P Asoka-Kumar, V.J Ghosh, B Nielsen, K.G Lynn, et al J Phys Chem AC C EP TE D M AN U Solids 59 (1998) 55 16 ACCEPTED MANUSCRIPT Figure captions Fig (a) SEM image of the plate-like precipitate distribution in V-4Cr-4Ti alloy, TEM HAADF image of (b) superstructure and uniform regions inside the precipitates, and (c) superstructure region extending through the thickness of the precipitate, (d) composite image RI PT constructed from the EELS maps generated by integrating the L2,3 edge intensity of V (red) and Ti (blue), (e) SEM image of the distribution of cuboidal-shaped precipitates at the grain boundaries of the V matrix with (f) their size distribution Data in (b)-(d) taken from ref [33] SC Fig Variation of the Vickers hardness with annealing temperature for both V-4Cr-4Ti and V-4Ti alloys M AN U Fig CDB line shape S-parameter as a function of the positron lifetime (τ) for V-4Cr-4Ti and V-4Ti alloys annealed at different temperatures, together with the room-temperature values for a reference V sample Fig (a) W-S plot containing data for V-4Cr-4Ti and V-4Ti alloys annealed at different TE D temperatures, together with data for reference V, Ti, Cr and SiO2 samples, (b) enlarged view of the V-containing samples Fig CDB ratio curves for V-4Cr-4Ti and V-4Ti alloys, both at room temperature and after EP annealing at 700°C for 1.5 hours, taken as a reference material (a) pure Ti and (b) SiO2 The AC C inset show the high-momentum region of the curves 17 TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP Fig 18 ...AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Ti- rich precipitate evolution in vanadium- based alloys during annealing above 400? ?? ?C a b RI PT A Impagnatielloa,b,*,... plate-like TiO precipitate evolution in V- 4Ti and V- 4Ti- 4Cr alloys during isochronal annealing above 400? ?C, by combining Vickers hardness, positron lifetime RI PT and coincidence Doppler broadening... future nuclear fusion reactors Conclusions We have studied the evolution of the Ti- rich precipitates present in V- 4Ti and V-4Cr- 4Ti alloys during isochronal annealing, by combining Vickers hardness,