Method of producing a nano-twinned titanium material by casting

Abstract

A method of producing a nano twinned commercially pure titanium material includes the step of casting a commercially pure titanium material, that apart from titanium, contains not more than 0.05 wt % N; not more than 0.08 wt % C; not more than 0.015 wt % H; not more than 0.50 wt % Fe; not more than 0.40 wt % O; and not more than 0.40 wt % residuals. The material is brought to a temperature at or below 0 C. and plastic deformation is imparted to the material at that temperature to such a degree that nano twins are formed in the material.

Claims

1. A method of producing a nano twinned commercially pure titanium material, comprising the steps of: casting a commercially pure titanium material that apart from titanium contains not more than 0.05 wt % N, not more than 0.08 wt % C, not more than 0.015 wt % H, not more than 0.50 wt % Fe, not more than 0.40 wt % O, and not more than 0.40 wt % residuals; bringing the casted material to a temperature at or below 0 C.; and subsequently imparting plastic deformation to the material at the temperature and a rate of less than 2% per second such that nano twins are formed in the material, the material having a yield strength of above 700 MPa and a tensile strength strength of above 750 MPa.

2. The method according to claim 1, wherein the deformation is imparted to the material at a rate of less than 1.5% per second.

3. The method according to claim 1, wherein the deformation is imparted to the material at a rate of less than 1% per second.

4. The method according to claim 1, wherein the material is brought to a temperature below 50 C. and that the plastic deformation is imparted to the material at that temperature.

5. The method according to claim 1, wherein the material is brought to a temperature below 100 C. and that the plastic deformation is imparted to the material at that temperature.

6. The method according to claim 1, wherein the material is cooled to a temperature of 196 C. and that the plastic deformation is imparted to the material at that temperature.

7. The method according to claim 1, wherein the plastic deformation is imparted to the material by compression.

8. The method according to claim 1, wherein the plastic deformation comprises straining imparted to the material by drawing.

9. The method according to claim 1, wherein the material is plastically deformed to an extent that corresponds to a plastic deformation of at least 10%.

10. The method according to claim 9, wherein the plastic deformation is imparted to the material intermittently with less than 10% per deformation.

11. The method according to claim 9, wherein the plastic deformation is imparted to the material intermittently with less than 6% per deformation.

12. The method according to claim 9, wherein the plastic deformation is imparted to the material intermittently with less than 4% per deformation.

13. The method according claim 1, wherein the deformation is imparted to the material at a rate of more than 0.2% per second.

14. The method according to claim 13, wherein the deformation is imparted to the material at a rate of more than 0.4% per second.

15. The method according to claim 13, wherein the deformation is imparted to the material at a rate of more than 0.6% per second.

16. The method according to claim 1, wherein the casted commercially pure titanium material does not contain more than 0.35 wt % O.

17. The method according to claim 1, wherein the material is plastically deformed to an extent that corresponds to a plastic deformation of at least 20%.

18. The method according to claim 1, wherein the material is plastically deformed to an extent that corresponds to a plastic deformation of at least 30%.

19. The method according to claim 1, wherein the casted commercially pure titanium material does not contain more than 0.30 wt % O.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) Below the invention will be described in detail with reference to the accompanying figures, of which:

(2) FIG. 1 shows a logic flow diagram illustrating the method according to the invention;

(3) FIG. 2 shows a diagram illustrating the tensile stress to strain for a CP titanium material at different temperatures;

(4) FIG. 3 shows a microscope view of a nano twinned CP Ti-material produced in accordance with the invention;

(5) FIG. 4 shows a TEM-study of a nano twinned CP Ti-material produced in accordance with the invention;

(6) FIG. 5 shows an X-ray diffraction pattern of a nano twinned CP Ti-material produced in accordance with the invention; and

(7) FIG. 6 shows a measurement of misorientation mapping in a nano twinned material produced in accordance with the invention.

DETAILED DESCRIPTION

(8) The present invention provides an improvement for commercially pure titanium materials and specifically to a method of producing such materials.

(9) Titanium exists in a number of grades of varying composition. Titanium of composition that corresponds to either of the grades 1 to 4 is generally denoted as commercially pure. Titanium with a composition of grade 5 is generally known as Ti-6Al-4V and is today the most widely used titanium material due to its very good mechanical properties.

(10) The composition of the titanium materials of grades 1-5 are presented below in table 1. Values indicate maximum wt % unless an interval is given.

(11) TABLE-US-00001 TABLE 1 Composition of different grades of titanium. (wt %) Re- O N C H Fe Al V siduals Grade 1 0.18 0.03 0.08 0.015 0.2 0.4 Grade 2 0.25 0.03 0.08 0.015 0.3 0.4 Grade 3 0.35 0.05 0.08 0.015 0.30 0.4 Grade 4 0.40 0.05 0.08 0.015 0.50 0.4 Grade 5 0.20 0.05 0.08 0.015 0.40 5.5-6.75 3.5-4.5 0.4

(12) As indicated above the commercially pure titanium materials are very attractive in some application such as e.g. in the medical field, because they contain no or only very small amounts of the allergenic metal vanadium. A specific object of the invention is to find a method of improving the mechanical properties, especially the yield strength, of a titanium material of a composition within grades 1-4 such that they correspond to the mechanical properties a titanium material of a composition within grade 5.

(13) Generally, for the commercially pure titanium materials the strength of the material will increase proportionally to an increased oxygen content. In table 2 some typical mechanical properties of titanium grades 1-5 and grade 23 are shown, where Rp0.2 corresponds to the Yield strength at a plastic deformation of 0.2%, Rm corresponds to the tensile strength, A corresponds to the elongation (ultimate strain) and E corresponds to Young's modulus.

(14) TABLE-US-00002 TABLE 2 Typical mechanical properties of different grades of titanium. Rp0.2 Rm A E (MPa) (MPa) (%) (GPa) Ti Grade 1 170 240 24 102.7 Ti Grade 2 275 345 20 102.7 Ti Grade 3 380 450 18 103.4 Ti Grade 4 483 550 15 104.1 Ti Grade 5 828 895 10 110-114 Ti Grade 23 775 948 16.4

(15) In accordance with the invention it has been shown that nano-twins may be introduced in commercially pure titanium material. This will be shown below in four examples from which an inventive generalisation is possible.

(16) The compositions of the four exemplary samples are shown in table 3.

(17) TABLE-US-00003 TABLE 3 Composition of the four exemplary samples. (max wt %) Composition N C H Fe O Al Others CP Ti #1 0.03 0.06 0.01 0.1 0.19 CP Ti #2, #3 0.05 0.06 0.01 0.2 0.225 CP Ti #4 0.01 0.01 0.01 0.4 0.28

(18) From table 3 it can be concluded that the first sample, i.e. CP Ti #1, has a composition that belongs to titanium grade 2, and that the second and third samples, i.e. CP Ti #2 and #3, have a composition that belongs to titanium grade 3, due the higher content of Nitrogen. The fourth sample belongs to grade 4 due the higher content of Iron.

(19) In the 4 examples below the samples were subjected to intermittent drawing. For the scope of this application the stepwise or intermittent drawing implies that the stress is momentarily lowered to below 90%, or preferably to below 80% or 70% of the momentarily stress for a short period of time, e.g. 5 to 10 seconds, before the drawing is resumed.

(20) The intermittent plastic deformation has proven to be an effective way of increasing the total tolerance to deformation, such that a higher total deformation may be achieved than for a continuous deformation.

(21) Further in order to avoid a temperature increase during the drawing, the material was continuously cooled throughout the whole drawing process.

(22) The start material for the examples below is a bar material that is produced in a conventional metallurgical method including melting, casting, forging/hot rolling and extrusion into the bar material.

(23) Hence, the inventive method may be performed on an otherwise finalised product.

Example 1

(24) In the first example, the sample CP Ti #1 was cooled to a temperature below 100 C. and was subsequently plastically deformed at this temperature.

(25) The sample, which had an initial total length of 50 mm, was plastically deformed by tension at a rate of 20 mm/min (0.67% per second) to a total deformation of 35%. The deformation was made in intervals of 2% at a time.

Example 2

(26) In the second example, the sample CP Ti #2 was cooled to a temperature below 100 C. and was subsequently plastically deformed at this temperature.

(27) The sample, which had an initial total length of 50 mm, was plastically deformed by tension at a rate of 30 mm/min (1% per second) to a total deformation of 35%. The deformation was made in intervals of 2% at a time.

Example 3

(28) In the third example, the sample CP Ti #3 was cooled to a temperature below 100 C. and was subsequently plastically deformed at this temperature.

(29) The sample, which had an initial total length of 50 mm, was plastically deformed by tension at a rate of 20 mm/min (0.67% per second) to a total deformation of 40%. The deformation was made in intervals of 2% at a time.

Example 4

(30) In the fourth example, the sample CP Ti #4 was cooled to a temperature below 100 C. and was subsequently plastically deformed at this temperature.

(31) The sample, which had an initial total length of 50 mm, was plastically deformed by tension at a rate of 30 mm/min (1% per second) to a total deformation of 25%. The deformation was made in intervals of 2% at a time.

(32) After concluded pretension at the indicated temperatures the samples #1-4 were left in room temperature for subsequent testing of mechanical properties in room temperature.

(33) The observed mechanical properties of the samples are represented in table 4.

(34) From table 4 it is apparent that both the yield strength and the tensile strength have increased markedly for all four samples with respect to the corresponding reference values for titanium materials of grade 2 and 3. This increase of the strengths is due to the formation of nano twins in the structure of the materials, which are induced by the pre-straining at low temperature, such that they correspond to or even exceed the properties of the reference materials, e.g. titanium grade 5 and grade 23.

(35) TABLE-US-00004 TABLE 4 Mechanical properties of the samples in comparison to references. Rp0.2 Rm A .sub.f Z E (MPa) (MPa) (%) (%) (%) (GPa) nano twinned CP Ti #1 813 829 19.4 13-15 55 120 nano twinned CP Ti #2 803 818 19 12-14 56 116 nano twinned CP Ti #3 912 1170 52 nano twinned CP Ti #4 747 829 12.5 107 Ti-6Al-4V 828 895 10 6-7 110-114 (Ti Grade 5) Ti Grade 23 775 948 16.4 57

(36) From the examples represented above an inventive method may be generalised. In the following part of this detailed description a logic flow diagram of a method of producing commercially pure titanium material according to the invention is described, with reference to FIG. 1.

(37) In a first step a commercially pure titanium material is provided. In accordance with the invention the provided material is casted and is not produced by a powder method, such as e.g. sintering and/or hot isostatic pressing (HIP).

(38) The casted titanium material is cooled to a temperature below room temperature. As a general rule, the lower the temperature, the bigger the effect of the nano twins will be.

(39) In FIG. 2, a diagram is shown over a tensile test of a titanium grade 2 material. In this diagram a sudden drop of the stress followed by portion of serrated curves may be observed. These serrated curves indicate that twinning has occurred. Further, the diagram in FIG. 2 reveals that the temperature at which the tensile tests are performed has a strong influence on the strength of the material, but also on the strain at which the sudden drop of the stress occurs. The lower the temperature the less strain is needed to provoke the sudden drop of the stress and thus to start the formation of twins.

(40) From the diagram it is also apparent that twins may be formed from a temperature of 0 C. and below, although the formation of twins does only occur above a strain of about 9% at 0 C.

(41) In step 4 of the logic flow diagram the material is imparted to a plastic deformation until a nano twinning occur in the material. The plastic deformation should be upheld until a nano twinning of a certain density or nano scale twin spacing is achieved in the material. This is described more closely below.

(42) In view of the shown examples, there is a wide composition span in which a nano twinned material with satisfactory mechanical properties may be obtained by means of the plastic deformation at low temperature. Specifically it appears that the oxygen content, which governs the strength of CP titanium material without nano twins, does not have to be high in order for nano twins to be formed. In sample CP Ti #1 the oxygen content is as low as 0.19 wt %, which is borderline to the definition of titanium grade 1 (not more than 0.18%).

(43) In order to verify the theory that the samples CP Ti #1-4 actually contain nano twins, their respective microstructure was studied both in a low magnification microscope and in a TEM study.

(44) Nano-twinned pure titanium materials have a microstructure full of needles or lath-shaped patterns. These needles or lathes are shown at a relatively low magnification in FIG. 3. As is visible the needles or lathes have similar crystal orientations within a specific cluster, but each cluster has a specific orientation, which is independent of the neighbouring clusters.

(45) The density of the nano-twins can be very high, as is visible in the TEM study in FIG. 4. In this case it is higher than 72%. The so-called nano-scale twin spacing for the material is below 1000 nm. For most of the twins the nano-scale twin spacing is below 500 nm, and especially below 300 nm. Further, most of the twins have a nano-scale twin spacing above 50 nm.

(46) The twin domains do not extend throughout a whole grain, but are rather divided into shorter segments. The misorientations between the grains are large, with entirely different crystallographic orientations of neighbouring domains. From the X-ray diffraction pattern shown in FIG. 5 small complementary dots appear close to most dots that constitute the characteristic HCP-structure of the titanium. These complementary dots indicate the presence of twins.

(47) FIG. 6 shows a measurement of a misorientation mapping in the nano twinned CP titanium material. In this figure, the uncorrelated peaks are denoted with reference numeral 1, wherein the correlated peaks are denoted with reference numeral 2. The correlated peaks 2 follow the random or theoretical line, which is denoted with reference numeral 3. There are several uncorrelated peaks at about 9, 29, 63 and 69, 83 and 89. These misorientations are different from those of normal CP titanium material, where there are only two misorientations located at 60 and 85. The misorientation at 60 is formed by compressive twinning, and the misorientation at 85 is formed by tensile twinning. The misorientation at 32 is usually formed by 27 twinning. The misorientations that are smaller than 10 to 20 are formed by special low angle grain boundaries, which do not represent twins.

(48) One speculation that can be made concerning the nano twinned materials is that the misorientations at 63 and 69 can belong to one group (compressive twinning) and the misorientations at 83 and 89 can belong to another group (tensile twinning).

(49) From the TEM-study it may however be concluded that twins are present, and that most of the twin domains are of such a size, at least smaller than 1000 nm, that they should be referred to as nano twins.

(50) In this description four examples are represented. Other examples of similar characteristics have however also been performed that support the represented examples and the achieved mechanical properties. The invention is thus not limited by the represented examples, but by the following claims.