Patent Application
20200384160
Mechanical properties double those of bioabsorbable polymers (which typically have a tensile strength of less than 110 MPa) are required for structural bone fixation, both for surgical insertion and also to insure rigid fixation of the repaired bone array. Requisite strength levels are seen in magnesium (Mg) alloys that are hardened by aluminum (Al), which have strength levels greater than 200 MPa. However, Al is a suspected contributor to dementia, Alzheimer's disease and bone dissolution. Accordingly, an alternate strengthening mechanism is required for bioabsorbable Mg alloys.
In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a strengthened Mg based alloy comprising Mg as a base element and at least two microalloying elements. The microstructure of the Mg based alloy has at least one of dislocations, stacking faults, coherency strains, grain boundaries and dislocation domains decorated (DDD) by segregation of microalloying elements. Additionally, one of the microalloying elements is a large atom element having an atomic size larger than the atomic size of a Mg atom and another of the microalloying elements is a small atom element having an atomic size smaller than the atomic size of the Mg atom.
In another aspect, the Mg based alloy includes an additional microalloying element having separate nanometer-sized 3rd phase particles that inhibit grain growth.
In a further aspect, the additional microalloying element is Mn and the separate nanometer-sized 3rd phase particles are alpha Mn particles.
In an additional aspect, the 3rd phase particles are in the range of 10 to 200 nanometers.
In another aspect, the 3rd phase particles have a solvus at a higher temperature than equilibrium intermetallic compounds of the microstructure.
In a further aspect, the microstructure includes an absence of continuous films of grain boundary intermetallic compounds.
In an additional aspect, the microstructure includes an absence of denuded grain boundaries.
In yet another aspect, atoms of the large atom element have atomic radii of 173 Angstroms or more and atoms of the small atom element have atomic radii of 145 Angstroms or less.
In still a further aspect, the atoms of the large atom element have an electronegativity of 1.1 or less and atoms of the small atoms element have an electronegativity of 1.4 or more.
In an additional aspect, the large atom element is Ca and the small atom element is at least one of Zn and Mn.
In another aspect, the microalloying elements essentially consist of Zn, Ca and Mn in the ranges (weight %) of 0.7 to 1.8 Zn, 0.2 to 0.7 Ca and 0.2 to 0.7 Mn.
In yet a further aspect, the microalloying elements essentially consist of Zn, Ca and Mn in the ranges (weight %) of 0.5 to 2.0 Zn, 0.2 to 1.0 Ca and 0.2 to 1.0 Mn.
In an additional aspect, the base and microalloying elements are human body nutrients and are osteoconductive
In still another aspect, the Mg alloy is provided in the form of a bioabsorbable, animal, particularly human, body implant.
In a further aspect, the Mg alloy is provided in the form of a structural reinforcement device.
In yet an additional aspect, Mg allow retains dislocation and partial dislocation content of greater than 1013/m3.
In another aspect, the Mg alloy has a yield strength of greater than 220 MPa.
In a further aspect, the Mg alloy includes texture below a multiple of a random distribution (MRD) value of 5 as measured by electron diffraction and formability enhanced by an r value of less than 2 (as measured by width and thickness of deformed tensile samples).
In an additional aspect, strength of the Mg alloy is supplemented by intragranular GP zones (which are ordered arrays of big and small atoms on the basal plane of the Mg matrix, see FIGS. 2A, 2B and 3) of less than 100 nm in size and/or intragranular intermetallic particles (e.g, Ca3Mg6Zn2) of less than 200 nm in size.
In a further aspect, strength of the Mg alloy is supplemented by coherency stresses arising from segregation of alloying elements. The high alloy volumes differ in lattice constants from the low alloy volumes, thus generating stresses when these volumes retain lattice coherency.
In another aspect of the invention, a method of processing a bioerodible magnesium alloy containing at least 95 weight percent magnesium in combination with microalloying elements for forming an endoprosthesis device is provided and includes the steps of forming one of an ingot or billet comprised of the magnesium alloy; strengthening the magnesium alloy by applying a deformation and segregation treatment, the treatment forming at one of dislocations, grain boundaries, stacking faults, clusters, GP zones and/or dislocation domains that are decorated by segregation of microalloying elements; at least one element of the microalloying elements is a large atom microalloying element having atoms with an atomic size larger than a magnesium atom; at least another element of the microalloying elements is a small atom microalloying element having atoms with an atomic size smaller than the magnesium atom, and forming in the microstructure separate grain growth inhibiting nanometer-sized 3rd phase particles of an additional microalloying element.
In another aspect, one of a rod, wire, hollow tube and sheet is formed from the ingot or billet.
In a further aspect, the bioerodible magnesium alloy is formed into an endoprosthesis implant in the form of at one of a screw, plate, wire, mesh, scaffold and/or stent.
In an additional aspect, the large atom microalloying element is one or more of Ca, Sr, Ba, Na, K, RE and Y and the small atom microalloying element is of one or more of Zn, Mn, Sn, V, Cr, P, B, Si, Ag and Al.
In still another aspect, deformation is by at least one of cold drawing, cold stamping, cold stretching, cold swaging, cold spinning or cold rolling.
In yet a further aspect, deformation is by at least one of hot extrusion, hot rolling, hot pressing, hot swaging, hot spinning or hot forging wherein the rolls or dies are heated to 150 to 400° C. and the deformation is greater than 30%.
In an additional aspect, the forming step includes forming decorated dislocations of grain sizes less than 5 μm and decorated dislocation domains of less than 50 nanometers (see
In yet another aspect, the product of percent deformation×time (minutes)×temperature (° K) in the segregation treatment is between 5×104 and 5.6×105 for hot deformation and between 8×104 and 21×105 for cold deformation.
In still a further aspect, the deformation and segregation treatment includes cold working producing adiabatic heating resulting in segregation.
In an additional aspect, the additional microalloying element is Mn and the process of deformation and segregation treatment in the presence of the nanometer-sized 3rd phase particles is of a speed avoiding precipitation of intermetallic compounds of CaxMgyZnz and recrystallization.
Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after review of the following description, including the claim, with reference to the drawings that are appended to and form a part of this specification.
In developing a new combination of alloying elements to fortify the mechanical properties of the Mg base, the search was narrowed first to elements that are established nutrients to the body, and then Quantum Mechanics First Principles were used to select and optimize ternary additions of odd-sized elements, both positive and negative sizes to the Mg atom and to each other. At the same time, microalloying principles and transition microstructures were practiced in order to capture synergisms amongst the alloying elements at low levels, while avoiding the detrimental effects on corrosion and ductility of equilibrium phases that are introduced by excessive alloying and faulty processing.
Based on the +/− atomic misfits and strengthening potencies seen below in Table I, the selected ternary alloying elements were zinc (Zn), manganese (Mn) and calcium (Ca). Zn, Mn and Ca achieve significant +/− oddness to Mg, respectively at −17, −14 and +23% in size, with a 32% oddness between Ca and Zn. Changing the equilibrium volume in opposite directions from the Mg matrix fosters co-segregation of Ca (which is positive to Mg) with Zn (which is negative to Mg). The mixing enthalpy between Ca and Zn is negatively large at −22 kJ/mol, an order of magnitude larger than Mg—Ca. Thus, there is a strong attractive interaction between Ca and Zn in the Mg matrix, which results in the forming clusters as seen in
Mg has poor ductility and formability, but ternary microalloying of Mg with both Ca and Zn improves both properties, more so than binary additions of Ca or Zn. This is related to reduced basal texture and enhanced non-basal slip.
Indeed, strengthening and ductilizing is the case wherein clusters (as seen in
Further to their benefit to mechanical properties of Mg, microalloying with Zn, Mn and Ca benefits the corrosion resistance of Mg. However, excessive additions of each element, or excessive combinations, are detrimental.
If one attempts to increase the strength of annealed/recrystallized BioMg 250 Grade 1 by extending the aging time to 20-50 hours, grain boundary films of CaxMgyZnz intermetallics develop, as seen in
The strength level of BioMg 250 Grade 1 is insufficient for various applications, such as self-tapping screws, rigid plates and devices that compete with titanium (Ti) and stainless steel (SS) implants. Accordingly, innovation was required in order to boost the strength level of the base BioMg 250 composition.
As discussed herein, novel thermomechanical processing routes have been discovered that utilize deformation induced decorated dislocation domain (DDD) nanostructures to boost yield strength above 220 MPa, while controlling the degradation rate in body fluids. As further discussed herein, the generation of dislocations and segregation/decoration can be simultaneous under specific strain rates and temperatures of hot deformation and post thermal treatment. Alternatively, dislocations can be generated by specific cold deformation followed by thermally activated segregation/decoration. Both of the above are herein referred to as “deformation and segregation treatments.”
Introduced in the resultant post processing Mg alloy material are nanometer-sized DDD's comprising a) dense line and screw dislocations and stacking faults, b) dislocation arrays at the domain boundaries, c) multi-atom clusters on those dislocations (see
In addition to its role in solid solution hardening, Mn additions insert spherical nm size alpha Mn particles that refine the grain size and amplify the Hall-Petch strengthening (see
As shown in
The formed transition phases resist dislocation movement; thus increasing strength as clouds on individual dislocations of line, screw and partial types. But also, as seen above, the 20 nanometer-sized decorated dislocation domains are disoriented from the mother grains, as enabled by dislocation walls at their boundary. This disorientation resists easy dislocation slip on the basal plane of Mg, activating additional slip on pyramidal and/or prismatic planes. This results in ductility and formability being improved, while at the same time increasing strength.
The following examples utilize BioMg 250, whose composition by weight % is Mg Base, 1.33 Zn, 0.39 Ca and 0.46 Mn.
Cold working in accordance herewith is very effective in boosting the strength of BioMg 250. This step is introduced prior to the segregation treatment noted above. This room temperature deformation can be applied by stretching, drawing, spinning and/or rolling. Results achieved by cold stretching sheet material with a grain size of 16 microns are reported in Table II. Strength (YS, UTS) increased and elongation (El) decreased as the segregation treatment factor (F) (defined below) increased. In addition, prior thermal treatment to attain finer grain size enhanced the combination of strength and elongation (see Table III). The highest elongation was seen with a pre-cold work Thermal Treatment Factor (TTF) of 10.72, wherein:
TTF=Temperature (° K/1000)×{18+log time (hours)} (1)
The results with cold bar drawing are listed Table IV. The effect of % cold work is listed in Table V, wherein cold work increases strength and decreases work hardening. The segregation treatment is quantified by a factor, F, the product of its key variables, namely: a) % of prior deformation, b) time of segregation treatment in minutes and c) temperature of segregation treatment in ° K. For example, treatment may include a 20% deformation, 30 minutes heat treatment at 500° K, resulting in a segregation treatment F of 3.0×105.
The effect of higher % cold work is presented in Table VI, which shows that hardness increased with an increase in % cold work.
Further practice of cold work was applied to wire of BioMg 250, as seen in Table VII. Fine wire of 100 μm diameter was cold drawn to attain yield strength of 400 MPa, with 3-4% elongation.
As to the mechanism of cold work strengthening, dislocation introduction and decoration with Ca and Zn atoms was confirmed. Atom probe electron microscopy revealed that dislocations had been introduced by cold work and that Ca and Zn atoms had segregated to those dislocations, as seen in
As previously noted, the fine microstructure and low texture favor improved corrosion resistance. The corrosion resistance of BioMg 250 Grade 2 was improved by the cold work/segregation interjection in the processing (see Table VIII), as tested in Synthetic Body Fluid and measured by H2 evolution and by potentiodynamic tests.
Avoidance of grain boundary intermetallics and resultant denuded grain boundaries, seen in
In addition cold deformation, hot deformation plus segregation treatment is effective in increasing strength. Dislocations are introduced and decorated at controlled strain rates and temperatures dependent on rate of dislocation movements and diffusion rates of the segregating elements. Residual dislocations from hot working were retained in Grade 2 material by avoiding recrystallization that occurs during the 1 hour anneal at 400° C. used on Grade 1. Dislocation contents on the order of 1014 to 1015 m−3 were introduced. These dislocations were decorated by Ca—Zn clusters. In addition, grain growth was restricted to retain grain sizes of 2-3 μm, much finer than the 13-18 μm sizes of Grade 1. Thus, the Hall-Petch strengthening mechanism was utilized wherein strength is inversely proportional to the square root of the grain diameter. The results from such residual hot deformation are revealed in Table IX. Yield strengths above 267 MPa with elongations above 9% were gained, as enabled by decorated dislocations and grain sizes of 3 μm or less. Greater hot reduction in the last rolling pass (e.g. 50%) introduced more dislocations and led to higher yield strength.
A two-step segregation treatment, introducing dislocations during hot deformation and then segregating at a lower temperature, was also seen to be effective in reaching a yield strength of 291 MPa with good elongation (see Table X).
The large Ca atoms and small Zn atoms co-segregate to grain boundaries in a strong interaction. The large Ca atoms segregate to the extension region of dislocations in the grain boundaries, while the small Zn atoms segregate to the compression region—both minimizing the elastic strains of the dislocations in the grain boundaries. By this mechanism, grain growth of highly oriented <1120> grains is inhibited; thus randomizing the growth of grains with other orientations. Also contributing is the presence of second phase Mn particles during thermomechanical processing. Rather than domination of basal slip, deformation is shared on prism and pyramid planes. This makes for superior formability compared to conventional Mg alloys, which exhibit higher textures of the multiple of a random distribution, MRD, of about 10 in examination of solid test specimens by electron diffraction.
Ca, Zn and Mn are preferred additions to Mg for microalloying of bioabsorbable Mg alloys. However, the present concept opens the door for structural Mg alloys with alternate combinations of big and small atoms that will segregate on dislocations. In a broad scope of the invention, alternate candidates to supplement or replace Ca and Zn are identified in Table XI.
The above description is meant to be illustrative of some preferred implementations incorporating the principles of the present invention. One skilled in the art will really appreciate that the invention is susceptible to modification, variation and change without departing from the true spirit and fair scope of the invention, as defined in the claims that follow. The terminology used herein is therefore intended to be understood in the nature of words of description and not of limitation.
This application claim priority to U.S. provisional patent application No. 62/632,600, filed Feb. 20, 2018, the entire contents of which are herein incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2019/018545 | 2/19/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62632600 | Feb 2018 | US |
