POWDER HOT ISOSTATIC PRESSING CYCLE

Abstract

A method of fabricating, at least in part, an article from a precursor thereof, the method comprising: providing the precursor, wherein the precursor comprises a metal having a closed pore therein; and hot isostatic pressing, HIPing, the precursor at an Nth temperature of a set of temperatures, at an Nth pressure of a set of pressures and for an Nth duration of a set of durations, thereby fabricating, at least in part, the article; wherein HIPing the precursor comprises regulating the set of temperatures, the set of pressures and/or the set of durations to control, at least in part, a morphology of the closed pore.

Claims

1. A method of fabricating an article from a precursor thereof, the method comprising: providing the precursor, comprising encapsulating a powder of an α + β Ti alloy in a container, wherein the powder is formed by electrode induction gas atomisation; EIGA; cold pressurisation of the precursor by isostatically compressing the precursor at a first pressure, thereby providing a compressed precursor; and hot isostatic pressing the compressed precursor at an Nth pressure in a range from 75 MPa to 150 MPa and at an Nth temperature in a range from 850° C. to 950° C., thereby fabricating the article; wherein a ratio of the first pressure to the Nth pressure is in a range from 1 : 2 to 9 : 10.

2. The method according to claim 1, wherein a ratio of the first pressure to the Nth pressure is in a range from 2 : 3 to 17 : 20.

3. The method according to claim 1, wherein cold pressurisation of the precursor by isostatically compressing the precursor at the first pressure is without applying heating or cooling.

4. The method according to claim 1, wherein particles of the powder comprise entrapped bubbles of argon.

5. The method according to claim 4, wherein the particles of the powder have a dimension of at least 50 .Math.m.

6. The method according to claim 1, comprising depressurising the article isothermally from the Nth pressure towards ambient pressure, and subsequently, cooling the depressurised article from the Nth temperature towards ambient temperature.

7. The method according to claim 6, wherein depressurising the article isothermally from the Nth pressure towards ambient pressure comprises depressurising the article isothermally from the Nth pressure to ambient pressure.

8. The method according to claim 6, wherein cooling the depressurised article from the Nth temperature towards ambient temperature comprises cooling at a first cooling rate and subsequently, cooling at a second cooling rate, wherein the first cooling rate is slower than the second cooling rate.

9. The method according to claim 6, wherein cooling the precursor comprises isobarically cooling the precursor.

10. The method according to claim 9, wherein isobarically cooling the precursor is at ambient pressure.

11. The method according to claim 6, wherein cooling the depressurised article from the Nth temperature towards ambient temperature comprises isobarically cooling the precursor from the Nth temperature to an N+1th temperature, wherein the N+1th temperature is at least 80% of the Nth temperature.

12. The method according to claim 1, wherein cold pressurisation of the precursor is at ambient temperature.

13. The method according to claim 1, wherein the α + β Ti alloy is a Ti-6Al-4V alloy.

14. The method according to claim 1, wherein the Nth pressure is in a range from 90 MPa to 125 MPa and/or the Nth temperature is in a range from 875° C. to 925° C.

15. The method according to claim 1, wherein the article is an aerospace component, a vehicle component, or a medical component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0112] For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

[0113] FIG. 1 schematically depicts isotropic HIPing of a precursor, comprising a metal having a closed pore therein;

[0114] FIG. 2 schematically depicts anisotropic HIPing of a precursor, comprising a metal having a closed pore therein;

[0115] FIG. 3 schematically depicts uniaxial HIPing of a precursor, comprising a metal having a closed pore therein;

[0116] FIG. 4 schematically depicts a method of according to an exemplary embodiment;

[0117] FIG. 5 schematically depicts a method of according to an exemplary embodiment.

[0118] FIG. 6 schematically depicts a conventional method;

[0119] FIG. 7 schematically depicts a method of according to an exemplary embodiment and a conventional method;

[0120] FIG. 8 schematically depicts a method of according to an exemplary embodiment; and

[0121] FIG. 9 schematically depicts the method of FIG. 8, in more detail.

DETAILED DESCRIPTION OF DRAWINGS

[0122] FIG. 1 schematically depicts isotropic HIPing of a precursor P, comprising a metal M having a closed pore p therein. Particularly, FIG. 1 shows schematically the isostatic collapse of a spherical bubble (i.e. the closed pore p) from 0.05 mm diameter to 0.005 mm.

[0123] Firstly, the spherical bubbles (i.e. closed pores) may simply compress in a fully symmetrical manner (as illustrated schematically by the radial arrows, of equal length, indicating compression of the closed pore p by the isostatic HIP pressure applied on the precursor P) and simply become bubbles of much smaller diameter but still of spherical form. Typically, for a very large EIGA bubble of approximately 0.05 mm diameter wholly contained inside the largest EIGA particle in the specified PSD (i.e. a granule with diameter of 0.106 mm) undergoing HIP at 100 MPa pressure at 920° C. is expected to compress such that the internal pressure approaches the externally applied HIP pressure and the expectation is that such bubble will reduce in diameter by about an order of magnitude (i.e. from D=0.05 mm at A to D=0.005 mm at B). On removal of the HIP pressure there is a likelihood that some swelling will occur, particularly if the pressure is removed whilst still at very high temperature. However, the collapse of the bubble during HIP will also result in the surrounding surfaces diffusion bonding in the closed geometric condition and so any such swelling is expected to be relatively minor. This sort of bubble compression with only limited or negligible post-HIP swelling is seen as the most favourable outcome.

[0124] Secondly, FIG. 2 schematically depicts anisotropic HIPing of a precursor P, comprising a metal M having a closed pore p therein (the longer arrows depicting preferential direction of collapse). Particularly, FIG. 2 shows schematically the partially-isostatic collapse of a spherical bubble from 0.05 mm diameter (at A) (as illustrated schematically by the radial arrows, of non-equal length, indicating directional compression of the closed pore p by the anisotropic HIP pressure applied on the precursor P) to an ellipsoidal bubble (i.e. pressure is not uniformly applied in all directions) (at B).

[0125] So, the spherical bubble may compress in a partially asymmetrical manner to a squashed sphere with an ellipsoidal form. This behaviour is likely to occur if the pressure transmission through the surrounding grains is not perfectly uniform. The degree of asymmetrical collapse may, potentially, result in a quite exaggerated ellipsoidal form. This is of some concern since the internal gas pressure inside the ellipsoid after removal of the external HIP pressure will generate a higher stress concentration than for the purely spherical bubble. In a severely squashed ellipsoid, there is the possibility that the combination of internal pressure and geometrical stress concentration may cause the material to tear or cleave apart and for significant cracks to form at the oblately squashed radii.

[0126] Thirdly, FIG. 3 schematically depicts a more extreme case of anisotropic bubble compression with uniaxial pressurisation occurring during the “HIPing process” of a precursor P, comprising a metal M having a closed pore p therein. Particularly, FIG. 3 shows schematically the uniaxial collapse of a spherical bubble from 0.05 mm diameter (at A) (as illustrated schematically by the parallel arrows, of equal length, indicating uniaxial compression of the closed pore p by the uniaxial HIP pressure applied on the precursor P) to a toroidal double bubble of doughnut form with a central diffusion bonded zone (at C via B).

[0127] So, if the transmission of HIP pressure is wholly or nearly-wholly uniaxial then the bubble may squash into a quasi 2-dimensional platelet where the opposing internal surfaces are brought into extremely close proximity. The HIP pressures involved are likely to result in the very central regions touching and diffusion bonding together and so a toroidal form is expected to result. This is considered the least desirable outcome and one which is expected to be of real harm to the material integrity.

[0128] The first approach is to “encourage” all particles to be compressed isostatically from the onset (i.e. before significant heating) and so when there is a significant temperature increase there is also limited opportunity for sintering to commence without all surfaces also being in intimate contact ensuring an uninterrupted pressure pathway throughout the powder system.

[0129] Hence, the method according to the first aspect may be better served by applying a very high level of “cold-pressurisation” before the onset of heating rather than the standard approach used in HIP of pre-charging the pressure vessel with argon to only a relatively low initial gas pressure and then using the increase in temperature to effect the majority of the required pressure increase. The latter approach is generally used in the HIP industry where the initial cold-pressurisation is capped and would not exceed 25% of the final target pressure. For titanium Powder HIP, the final target pressure will be at least 100 MPa and so the initial pressure used will be no more than 25 MPa. This pressure cap ensures that argon gas is not wasted since any cold pressurisation (i.e. ambient) above 25% of final target pressure will necessitate subsequent venting as the pressure builds during the heating phase. For example, and according to the Universal Gas Equation, if the final target temperatures and pressures are 920° C. and 100 MPa then any cold-pressurisation (i.e. at, say 20° C.) above a pressure of 24.56 MPa would be expected to either result in excess pressure or a need to vent gas in order to stay within the target pressure range. In contrast, the method according to the first aspect may be to cold pressurise to approximately 75% of the target pressure (e.g. 75 MPa) and then heat up to 920° C. with the expectation that as the temperature reaches approximately 118° C. the pressure will approach the final target pressure of 100 MPa and that venting will then be required for the remainder of the heat-up phase. In so doing it is anticipated that as the titanium starts to deform plastically and intra-granular bubbles start to collapse then there will be uniform isostatic pressure across the powder system and asymmetric bubble collapse will not occur to any significant level.

[0130] The second approach is to amend the way in which the pressure and temperature is reduced once the HIP-dwell at maximum pressure and temperature is concluded. In the prior art the norm is to reduce temperature and pressure concurrently. As pressure is vented so there will an expansion of the HIP argon gas and temperature will reduce. This will aid cooling. In addition, “gas quenching” may also applied (i.e. injecting cold gas) to accelerate the cooling process. The proposed novel approach is to reduce the HIP pressure whilst maintaining the temperature at a high level in order to encourage re-spherodisation of any ellipsoidal bubbles that may be present. This may be done either substantially isothermally, for example without applying heating or cooling, at the full HIP temperature (e.g. typically 920° C. for titanium alloys) or, alternatively, a level of pre-cooling to an intermediate temperature whilst remaining at full HIP pressure may be applied before then substantially isothermally, for example without applying heating or cooling, (i.e. at the intermediate reduced temperature) lowering the HIP pressure. The latter approach will provide an increase in the flow stress of the titanium and provide a degree of resilience in order to better withstand the ensuing positive pressure differential inside the entrapped argon bubbles once HIP de-pressurisation commences. Otherwise, there will be a risk of damage occurring. Either way, the rate of HIP vessel de-pressurisation is to be controlled such that the initial rate of pressure decay is more limited and is allowed to progressively increase according to a prescribed process. Only once the HIP vessel pressure has been substantially reduced will the full reduction in temperature be implemented.

[0131] The rationale for the modified de-pressurisation to a substantially reduced level (either at the full HIP temperature or at an intermediate and partially reduced temperature) followed by the main cooling phase is to enable the spherodisation of any ellipsoidal gas bubbles that may have been formed despite the use of the cold-pressurisation step. The controlled de-pressurisation of the HIP vessel is therefore a precautionary measure but is one that could also be used in the absence of a cold-pressurisation step in order to mitigate ellipsoidal gas bubble re-inflation occurring in a damaging manner (see below). The concern, otherwise, would be that without the precautionary progressive de-pressurisation at high temperature any sudden decrease in HIP pressure that occurs during the cooling phase could give rise to an alarmingly high positive pressure differential between the inside of the gas bubble and the HIPed component. This could then create a tendency for tearing or cleavage rather than the more desirable slow re-inflation of the bubble and the formation of cracks or at the least for the preservation of non-spherodised gas bubbles that could then act as sites for premature fatigue crack initiation in the component during service.

[0132] FIG. 4 schematically depicts a method of according to an exemplary embodiment. Particularly, FIG. 4 shows a HIP Temperature & Pressure Curve for a cycle in Table 3 (“Cold Charge” to 75% of final target pressure (75 MPa) and 40° C. of Isobaric cooling prior to a controlled (progressively increasing rate of de-pressurisation). It should be noted that the Stabilisation Hold times in Table 3 can be extended in order to provide a margin of safety and allow gas bubbles to re-inflate to the optimum level before the next stage of de-pressurisation commences.

TABLE-US-00003 Pressure MPa Temperature °C Temperature K Time Step mins Total Time mins Comments Pressure Ramp Rate MPa/min 0 20 293 0 0 Start 0.000 75 20 293 60 60 Cold Pressurisation to 75% 1.250 100 920 1193 60 120 Heating to full HIP Temperature and Thermal Self Pressurisation 0.417 100 920 1193 120 240 Dwell at full HIP Temperature & Pressure 0.000 100 880 1153 40 280 Isobaric Cooling to 880° C. 0.000 100 880 1153 4 284 4 mins Thermal Stabilisation Hold 0.000 96.75 880 1153 13 297 Isothermal De-pressurisation (Step 1) -0.250 96.75 880 1153 10 307 10 mins Pressure Stabilisation Hold 0.000 93 880 1153 15 322 Isothermal De-pressurisation (Step 2) -0.250 93 880 1153 8 330 8 mins Pressure Stabilisation Hold 0.000 85.5 880 1153 15 345 Isothermal De-pressurisation (Step 3) -0.500 85.5 880 1153 6 351 6 mins Pressure Stabilisation Hold 0.000 74.25 880 1153 15 366 Isothermal De-pressurisation (Step 4) -0.750 74.25 880 1153 4 370 4 mins Pressure Stabilisation Hold 0.000 59.25 880 1153 15 385 Isothermal De-pressurisation (Step 5) -1.000 59.25 880 1153 3 388 3 mins Pressure Stabilisation Hold 0.000 36.75 880 1153 15 403 Isothermal De-pressurisation (Step 6) -1.500 36.75 880 1153 2 405 2 mins Pressure Stabilisation Hold 0.000 6.75 880 1153 15 420 Isothermal De-pressurisation (Step 7) -2.000 6.75 880 1153 2 422 2 mins Pressure Stabilisation Hold 0.000 1 880 1153 2 424 Isothermal De-pressurisation (Step 8) -2.875 1 450 723 50 474 First Stage Cooling to 450° C. 0.000 1 50 323 20 494 Second Stage Cooling to 50° C. 0.000 0 30 303 5 499 Cycle End -0.200

[0133] HIP Cycle using a “Cold Charge” to 75% of final target pressure (75 MPa) and 40° C. of isobaric cooling prior to controlled (progressively increasing) rate of de-pressurisation (75 MPa Cold Charge Pressure Transitional Cooling & Extended Stabilisation Holds).

[0134] FIG. 5 schematically depicts a method of according to an exemplary embodiment. Particularly, FIG. 5 shows a HIP Temperature & Pressure Curve for cycle in Table 4 (“Cold Charge” to 75% of final target pressure (75 MPa and no Isobaric cooling prior to a controlled (progressively increasing) rate of de-pressurisation).

TABLE-US-00004 Pressure MPa Temperature °C Temperature K Time Step mins Total Time mins Comments Pressure Ramp Rate MPa/min 0 20 293 0 0 Start 0.0000 75 20 293 60 60 Cold Pressurisation to 75% 1.2500 100 920 1193 60 120 Heating to full HIP Temperature and Thermal Pressurisation 0.4167 100 920 1193 120 240 Dwell at full HIP Temperature & Pressure 0.0000 96.75 920 1193 13 253 Isothermal De-pressurisation (Step 1) -0.2500 96.75 920 1193 10 263 10 mins Pressure Stabilisation Hold 0.0000 93 920 1193 15 278 Isothermal De-pressurisation (Step 2) -0.2500 93 920 1193 8 286 8 mins Pressure Stabilisation Hold 0.0000 85.50 920 1193 15 301 Isothermal De-pressurisation (Step 3) -0.5000 85.50 920 1193 6 307 6 mins Pressure Stabilisation Hold 0.0000 74.25 920 1193 15 322 Isothermal De-pressurisation (Step 4) -0.7500 74.25 920 1193 4 326 4 mins Pressure Stabilisation Hold 0.0000 59.25 920 1193 15 341 Isothermal De-pressurisation (Step 5) -1.0000 59.25 920 1193 3 344 3 mins Pressure Stabilisation Hold 0.0000 36.75 920 1193 15 359 Isothermal De-pressurisation (Step 6) -1.5000 36.75 920 1193 2 361 2 mins Pressure Stabilisation Hold 0.0000 6.75 920 1193 15 376 Isothermal De-pressurisation (Step 7) -2.0000 6.75 920 1193 2 378 2 mins Pressure Stabilisation Hold 0.0000 1 920 1193 2 380 Isothermal De-pressurisation (Step 8) -2.8750 1 450 723 50 430 First Stage Cooling to 450° C. 0.0000 1 50 323 20 450 Second Stage Cooling to 50° C. 0.0000 0 30 303 5 455 Cycle End -0.2000

[0135] HIP Cycle using a “Cold Charge” to 75% of final target pressure (75 MPa) with no isobaric cooling prior to controlled (progressively increasing) rate of de-pressurisation (75 MPa Cold Charge Pressure Isothermal Cooling & Extended Stabilisation Holds).

[0136] FIG. 6 schematically depicts a conventional method. Particularly, FIG. 6 shows a conventional HIP Cycle using a “Cold Charge” to 25% of final target pressure (25 MPa) with concurrent cooling and de-pressurisation (see Table 5).

TABLE-US-00005 Pressur e MPa Temperature °C Temperature K Time Step mins Total Time mins Comments Pressure Ramp Rate MPa/min 0 20 293 0 0 Start 0.000 25 20 293 40 40 Cold Pressurisation to 25 MPa 0.625 62.5 470 743 45 85 Intermediate Temperature & Pressure 0.833 100 920 1193 35 120 Heating to full HIP Temperature and Thermal Self Pressurisation 1.071 100 920 1193 120 240 Dwell at full HIP Temperature & Pressure 0.000 1 50 323 87 327 Combined Cooling and De-pressurisation -1.138 0 30 303 5 332 Cycle End -0.200

[0137] Conventional HIP Cycle using a “Cold Charge” to 25% of final target pressure (25 MPa) with concurrent cooling and de-pressurisation (see FIG. 6).

[0138] FIG. 7 schematically depicts a method of according to an exemplary embodiment and a conventional method. Particularly, FIG. 7 shows a comparison between the preferred embodiment and the conventional HIP cycle of FIG. 6.

[0139] It will be noted that the conventional HIP cycle is a shorter cycle, see FIG. 7. The preferred embodiment is the longest cycle as this also included the isobaric cooling phase that adds approximately 44 minutes to the cycle without isobaric cooling and is 167 minutes longer than the conventional process described.

[0140] Applying the conventional process to Powder HIP parts made from EIGA powder will potentially result in highly ellipsoidal and even toroidal gas bubbles and the prospect of cracks and/or tears occurring either during the process is during subsequent high temperature heat treatments or service exposure. The enhanced process also has applicability to other processes that use HIP as an ameliorating treatment (e.g. for the elimination of internal porosity in additively manufactured parts). This includes both the Laser and Electron Beam Selective Powder Bed processes. In the case of Laser Powder Bed there is the prospect of argon gas entrapment occurring either through use of a less than optimum set of laser build parameters where the argon process gas that is used becomes entrained in the porosity created by the inferior build process, or through argon entrapment already present in the powder stock. The latter is less likely as the SLM process typically uses 15-45 micron powder grade and is generally considered immune from intra-granular argon bubbles associated with the EIGA route. In contrast, the Electron Beam Powder Bed process typically uses coarser powder and may well contain intra-granular (i.e. within the precursor gas particles) argon gas entrapment. This form of gas entrapment is known to be capable of surviving the EB melting process despite the use of a vacuum environment. This invention therefore has broader applicability than solely for Powder HIP.

[0141] FIG. 8 schematically depicts a method of according to an exemplary embodiment.

[0142] The method is of fabricating, at least in part, an article from a precursor thereof.

[0143] At S801, the method comprises providing the precursor, wherein the precursor comprises a metal having a closed pore therein.

[0144] At S802, the method comprises isostatically compressing the precursor, thereby providing a compressed precursor; and hot isostatic pressing, HIPing, the compressed precursor at an Nth temperature of a set of temperatures, at an Nth pressure of a set of pressures and for an Nth duration of a set of durations, thereby fabricating, at least in part, the article.

[0145] FIG. 9 schematically depicts the method of FIG. 8, in more detail, as summarised in Table 6.

TABLE-US-00006 Time / s Temperature / K Pressure / MPa Duration / s t.sub.o T.sub.0 P.sub.0 t.sub.1 T.sub.1 ≈ T.sub.0 P.sub.0 << P.sub.1 ≈ 0.75P.sub.0 d.sub.1 = t.sub.1 - t.sub.0 t.sub.2 T.sub.2> T.sub.1 P.sub.N d.sub.2 = t.sub.2 - t.sub.1 t.sub.3 T.sub.N >> T.sub.2 P.sub.N d.sub.3 = t.sub.3 - t.sub.2 t.sub.N T.sub.N P.sub.N d.sub.N = t.sub.N - t.sub.3 t.sub.N+1 T.sub.N+1 ≈ 0.95T.sub.N P.sub.N+1 ≈ P.sub.N d.sub.N+1 = t.sub.N+1 - t.sub.N t.sub.N+2 T.sub.N+1 P.sub.N+2 < P.sub.N+1 d.sub.N+2 = t.sub.N+2 - t.sub.N+1 t.sub.N+3 T.sub.N+1 P.sub.N+3 < P.sub.N+2 d.sub.N+3 = t.sub.N+3 - t.sub.N+2 t.sub.N+4 T.sub.N+1 P.sub.0 ≈ P.sub.N+4 << P.sub.N+3 d.sub.N+4 = t.sub.N+4 - t.sub.N+3 t.sub.N+5 T.sub.N+5 < T.sub.N+1 P.sub.0 ≈ P.sub.N+4 d.sub.N+5 = t.sub.N+5 - t.sub.N+4 t.sub.N+6 T.sub.0 ≈ T.sub.N+6 << T.sub.N+5 P.sub.0 ≈ P.sub.N+4 d.sub.N+6 = t.sub.N+6 - t.sub.N+5

[0146] HIP Cycle using a cold charge to 75% of final target pressure and isobaric cooling prior to controlled (progressively increasing) rate of de-pressurisation.

[0147] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises regulating heating to the Nth temperature of the set of temperatures and/or regulating pressurising to the Nth pressure of the set of pressures, for example by pressurising to substantially the Nth pressure of the set of pressures without heating and subsequently, heating to the Nth temperature of the set of temperatures.

[0148] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises pressurizing the precursor from a zeroth pressure of the set of pressures, for example ambient pressure, to a first pressure of the set of pressures during a first duration of the set of durations.

[0149] In this example, a ratio of the first pressure to the Nth pressure is 3 : 4.

[0150] In this example, pressurizing the precursor from the zeroth pressure of the set of pressures to the first pressure of the set of pressures during the first duration of the set of durations comprises substantially isothermally, for example without applying heating or cooling, pressurizing the precursor from the zeroth pressure of the set of pressures to the first pressure of the set of pressures during the first duration of the set of durations, for example at a first temperature of the set of temperatures, for example ambient temperature.

[0151] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises pressurizing the precursor from the first pressure of the set of pressures to the Nth pressure of the set of pressures during a second duration of the set of durations by heating the precursor to a second temperature of the set of temperatures.

[0152] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises heating the precursor, for example from the second temperature of the set of temperatures, to the Nth temperature of the set of temperatures at the Nth pressure of the set of pressures.

[0153] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises regulating cooling from the Nth temperature of the set of temperatures and/or regulating depressurising from the Nth pressure of the set of pressures, for example by depressurising substantially isothermally from the Nth pressure of the set of pressures, for example to ambient pressure, and subsequently, cooling from the Nth temperature of the set of temperatures, for example towards ambient temperature.

[0154] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises cooling the precursor from the Nth temperature of the set of temperatures to an N+1th temperature of the set of temperatures, optionally wherein the N+1th temperature of the set of temperatures is at least 80%, preferably at least 85%, more preferably at least 90% of the Nth temperature, during an N+1th duration of the set of durations.

[0155] In this example, cooling the precursor from the Nth temperature of the set of temperatures to the N+1th temperature of the set of temperatures comprises isobarically cooling the precursor from the Nth temperature of the set of temperatures to the N+1th temperature of the set of temperatures.

[0156] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises depressurizing the precursor to an N+2nd pressure of the set of pressures during an N+2nd duration of the set of durations at a first depressurizing rate of a set of depressurizing rates and depressurizing the precursor to an N+3rd pressure of the set of pressures during an N+3rd duration of the set of durations at a second depressurizing rate of a set of depressurizing rates, and optionally depressurizing the precursor to an N+4th pressure of the set of pressures during an N+4th duration of the set of durations at a third depressurizing rate of a set of depressurizing rates, wherein the first depressurizing rate is slower than the second depressurizing rate and optionally, wherein the second depressurizing rate is slower than the third depressurizing rate, optionally wherein depressurizing the precursor comprises substantially isothermally, for example without applying heating or cooling, depressurizing the precursor, for example at the Nth temperature of the set of temperatures or the N+1th temperature of the said temperatures.

[0157] In this example, regulating the set of temperatures, the set of pressures and/or the set of durations comprises cooling the precursor to an N+5th temperature of the set of temperatures during an N+5th duration of the set of durations at a first cooling rate of a set of cooling rates and cooling the precursor to an N+6th temperature of the set of temperatures during an N+6th duration of the set of durations at a second cooling rate of the set of cooling rates, wherein the first cooling rate is slower than the second cooling rate, optionally wherein cooling the precursor comprises isobarically cooling the precursor, for example at ambient pressure.

[0158] Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

[0159] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0160] All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

[0161] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0162] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.