Background to the Invention Heat treatment for strengthening by age hardening is applicable to alloys in which the solid solubility of at least one alloying element decreases with decreasing temperature. Relevant magnesium alloys include those based on additions of aluminium, zinc, calcium, silver, copper, thorium, tin, and many of the rare-earth series of elements such as neodymium and yttrium for example.

Alloys within the Mg-Zn system display a maximum solid solubility of zinc in magnesium of 6.2 wt. % or 2.4 at. % at 342°C. When such alloys are solution treated at appropriate elevated temperatures, the additive elements are dissolved into solid solution, and the magnesium grains contain their maximum solubility of zinc in magnesium at the given temperature. For such alloys, after solution treatment and quenching, artificial age strengthening is then typically conducted at temperatures ranging from close to ambient temperature (25°C) up to temperatures such as 300°C. The purpose of this lower temperature heat treatment is to facilitate the progressive precipitation of fine dispersions of precipitates from a supersaturated solid solution, that form as an equilibration response to thermal exposure at the lower temperature. The strengthening that occurs from such precipitation affects the mechanical properties of these alloys by increasing the ability of the material to resist deformation by the process of slip.

The sequence of precipitates that typically form between magnesium and zinc within magnesium alloys containing zinc as a major alloying element are: SSSS GP zones (coherent discs 11 {0001} Mg) MgZn2 (coherent rods-L {0001} g) MgZn2 (semi-coherent discs// {0001} Mg) < Mg2Zn3 (incoherent trigonal particles) Minor or trace amounts of certain elements cause significant and marked changes to the microstructure and properties of many alloy systems, in particular aluminium alloys. For magnesium alloys, minor or trace element additions are

typically aimed at improving the as-cast grain structure of alloys, such as microporosity. The effects of such additions on hardening precipitation are not known to have been examined as closely for magnesium alloys.

Summary of the Invention The present invention is directed to providing age-hardenable magnesium alloys, containing zinc as a major alloying element, in which hardening precipitation processes are modified by selected alloy additions.

We have found that, with magnesium alloys containing zinc as a major alloying element, a selected alloy addition of at least one of calcium and strontium enables modification of precipitation kinetics and precipitate structure in these alloys. This modification is found to enable enhancement of the ageing response of the alloys, with prospects for improved mechanical properties.

Thus, according to the present invention, there is provided an age- hardenable magnesium-zinc alloy, wherein the alloy contains: - from about 3 wt. % to about 6 wt. % of zinc as a major alloying element, and - a modifier selected from the group consisting of calcium, strontium and mixtures of calcium and strontium; wherein the alloy optionally contains: - aluminium up to a level not exceeding about 10 wt. % of the content of said modifier, and -alloying element additions typically present in magnesium-zinc age- hardenable wrought and casting alloys ; wherein the alloy, apart from impurities and incidental elements, has a balance comprising magnesium; and wherein the modifier is present at a level whereby the alloy is age- hardenable to an enhanced ageing response compared with an alloy which, apart from not containing modifier, is of the same composition.

The invention also provides an age hardened magnesium-zinc alloy, wherein the alloy contains: from about 3 wt. % to about 6 wt. % of zinc, and a modifier selected from the group consisting of calcium, strontium and mixtures of calcium and strontium; wherein the alloy optionally contains:

-aluminium up to a level not exceeding about 10 wt. % of the content of said modifier, and -alloying element additions typically present in magnesium-zinc age- hardenable wrought and casting alloys ; wherein the alloy, apart from impurities and incidental elements, has a balance comprising magnesium; and wherein the modifier is present at a level of from about 0.01 wt. % to about 1.0 wt. % whereby the alloy exhibits an enhanced ageing response compared with an alloy which, apart from not containing modifier, is of the same composition.

Moreover, the invention provides a method of producing an age hardened magnesium-zinc alloy, wherein the method includes the steps of: (a) solution treating, within a suitable elevated temperature range or ranges, an age-hardenable magnesium-zinc alloy according to the present invention, for a time or times sufficient to allow the elements active in the precipitation reaction to be dissolved into solid solution; (b) quenching the solution treated alloy from the temperature cycle for step (a) whereby the dissolved elements are retained in a supersaturated solid solution; and (c) treating the quenched alloy from step (b) to an artificial age strengthening treatment at a temperature and for a period of time sufficient to enable the alloy to develop an enhanced ageing response compared with an alloy which, apart from not containing modifier, is of the same composition and which is subjected to the same method.

The enhanced ageing response may comprise one of an enhanced level of peak hardness, an enhanced level of strength and a combination of enhanced levels of peak hardness and strength. The alloy may be age-hardenable or age hardened to enhanced levels of peak hardness and strength which are up to 10% greater compared with an alloy which, apart from not including calcium or strontium, is of the same composition. The enhanced ageing response usually is able to be attained in a shorter period or artificial ageing, at a given temperature, compared with the alloy of the same composition but not containing modifier.

Also, an age-hardened alloy according to the present invention also is able to exhibit improved creep resistance, particularly in the peak aged condition.

The age-hardenable alloys of the present invention have a zinc content ranging up to the maximum solubility limit. Zinc may be present at from about 3 wt. % up to 6 wt. %. Within this general context, the invention is applicable to the binary Mg-Zn system, as well as to ternary and higher systems. However alloy elements additional to zinc most preferably are those typically present in age- hardenable wrought and casting magnesium alloys. These may include, but are not necessarily restricted to, silicon, the rare earth elements (with atomic numbers from 57 to 70), yttrium, copper, beryllium and manganese. The alloy may also have a grain refining addition, most preferably zirconium at for example 0.1 wt. % to 1 wt. %. Additionally, aluminium also can be present, but only at low levels which do not exceed about 10% of the total level of addition of at least one of calcium and strontium.

The present invention provides age-hardenable zinc-containing alloys suitable as casting alloys, as well as wrought alloys. Other than zinc, the alloy elements detailed above may be present within the usual ranges for casting or wrought alloys.

The level of selected alloy addition of at least one of calcium and strontium generally does not exceed about 1 wt. %. That is, the addition of either one of those elements or the two of them in total, generally does not exceed about 1 wt. %. The lower level of addition of at least one of calcium and strontium generally is in excess of about 0.01 wt. %, but preferably is in excess of about 0.05 wt. % and more preferably is in excess of 0.07 wt. % such as in excess of 0.1 wt. %.

The extreme levels of 0.01 wt. % and 1 wt. % for addition of at least one of calcium and strontium, particularly the upper level, can vary to a degree with other alloy additions. However, in general, additions of strontium and/or calcium below about 0.01 wt. % do not achieve a useful level of microstructure modification.

Also, additions above about 1 wt. % either achieve no further beneficial effect or can lead to a reduction in the level of microstructure modification or other adverse effects.

A suitable selected alloy addition of at least one of calcium and strontium can modify the microstructure, from that otherwise existing, in the following ways: 1. The addition is able to stabilise the average grain size of the as-cast microstructure. The addition thereby enables retention of a finer grain

structure in solution treated material heat treated at a suitable elevated temperature, such as above 500°C.

2. The precipitation reaction that exists within the alloys is modified, so that the kinetics of the ageing process (and precipitation that normally occurs) is altered. Typically in the alloys there are three main effects that dominate.

These are: (a) The kinetics of the ageing process are affected, in that the time required to reach peak strengthening of each alloy at a set ageing temperature is reduced and the time to reach an over aged microstructure is notably increased.

(b) Each alloy, after an appropriate solution heat treatment (ST), will typically strengthen to a level up to approximately 10% greater in the fully strengthened condition than that of an alloy differing only in not containing the calcium and/or strontium addition and fully strengthened.

(c) The precipitate morphology is modified during ageing. The distribution of particles exhibits a much more uniform and finer distribution of precipitates, as well as precipitates that exist on growth planes that are not well represented in the alloys without ) addition of calcium and/or strontium.

These microstructure modifications and effects suggest that the alloys of the invention are able to retain a modified, yet highly stable microstructure for extended periods at elevated temperatures.

The present indications are that a positive influence on the precipitation behaviour (and therefore the mechanical properties) is able to be exhibited in all magnesium age-hardenable alloys containing zinc both in the grain refined and un-refined conditions, except for those having Al in excess of about 10% of the level of addition of Ca and/or Sr. This has been confirmed for binary Mg-Zn alloys (from 3 wt. % to 6 wt. % Zn). It also has been confirmed for the commercial alloys Mg-Zn-Zr (ZK alloys), Mg-Zn-RE-Zr (specifically ZE41 type alloys) and Mg-Zn-Cu (specifically ZC63), as well as other alloy systems such as Mg-Zn-Y. That is, we have confirmed that the effect is achieved in all of the above alloy systems.

Calcium additions ideally are between 0.01 wt. % and 1 wt. % in order to have a positive effect on the age-hardenable response of Mg-Zn alloys. Strontium

also has the same effect over the same range, but barium does not appear to have a positive effect.

The effect of the additions of Ca and/or Sr only works in conjunction with heat treatment. Moreover it would appear that the best results are obtained for elevated solution treatment temperatures (preferably between about 450°C and 530°C, such as from about 500°C to 530°C) after some initial time at a lower temperature, such as about 320°C to 370°C, preferably about 345°C. It is apparently not necessary to dwell at the elevated temperature for any length of time, as just reaching the elevated temperature has been found to be adequate to induce the desired response in a modified alloy. A low solution temperature equivalent (such as about 345°C) shows little benefit from the Ca and/or Sr addition above the level of about 0.3 wt. % unless it is followed by the elevated temperature treatment. However, there is a positive effect on the ageing response after solution treatment at 345°C for a Ca and/or Sr addition up to about the 0.3 wt. % level. The time at the low temperature prior to heating to the final elevated temperature can be between 0 to 14h with no marked influence on the resulting enhanced ageing response. The low temperature start avoids incipient melting problems that may result from segregation in the alloy from the original casting process but, with an appropriate starting material, it may not be necessary to start the solution treatment cycle at such a low temperature.

A positive effect on the ageing response has been established for elevated solution treatment temperatures of 345°C, 400°C, 450°C, 500°C and 530°C.

Indications are that all solution treatment temperatures in between these extremes will also display a positive response. It is apparent that the most optimum solution treatment temperature is composition dependent.

In order that the invention may be more readily understood, description now is directed at the accompanying drawings, in which: Figure 1 shows the microstructure for solution treated Mg-4Zn+XCa alloys for which X=0 wt. % (a), 0.088 wt. % (b), 0.35 wt. % (c) and 0.70 wt. % (d); Figure 2 shows typical artificial ageing curves for Mg-4Zn and Mg-4Zn- 0.5Zr alloys each compared with a respective similar alloy according to the invention, after a common solution treatment of 12 hours at 530°C ;

Figure 3 shows ageing curves for alloys similar to Figure 2, but contrasting the performance of Ca and Sr with Ba, with a solution treatment of 8 hours at 345°C plus a ramp to 530°C over 2 hours.

Figure 4 is similar to Figure 2, but shows the artificial ageing curves after an alternative solution treatment; Figures 5 to 7 are similar to Figure 3, but show variants on the alternative solution treatment; Figures 8 to 10 are similar to Figures 5 to 7 and differ only in that Sr was substituted for Ca; Figure 11 is similar to Figure 3, but shows variants on the zinc content of the alloy, from 3 wt. % to 6 wt. % Zn; Figure 12 is similar to Figure 11 and differs only in that Sr was substituted for Ca; Figure 13 shows a respective photomicrograph taken perpendicular to the c-axis ( (a) and (b) ) and the ai-axis ( (c) and (d) ) of the matrix, using TEM, for each of the alloys Mg-4Zn and Mg-4Zn-0.35 Ca in a peak aged condition; Figure 14 is similar to Figure 13, but pertains to the same alloys after extended ageing; Figure 15 shows schematic representations of precipitate morphology without and with, respectively, the selected alloy additions required by the present invention; Figures 16 to 19 show respective constant load creep curves for each of the alloys Mg-4Zn and Mg-4Zn-0. 1 Ca, Mg-4Zn-0.07 Sr and Mg-4Zn-0. 3 Sr in a peak aged condition under various testing conditions; and Figure 20 shows a comparison of the hardening response of Mg-4Zn-X (Zr without and with Ca.

The photomicrographs of Figure 1 exhibit the respective solution treated microstructure of the Mg-4Zn alloy (a) and of three Mg-4Zn+XCa alloys, where X = 0.088 wt. % (b), 0.35 wt. % (c) and 0.70 wt. % (d), after solution treatment to 530°C. Each alloy was solution treated for 8h at 345°C and 2h to 530°C, followed by a cold water quench. The microstructures illustrate the differences in grain size between the alloy without a Ca addition and those with a Ca addition. Sr additions instead of Ca have been found to behave in a similar manner to those of Ca.

As shown by Figure 1, the initial effect of trace additions of Ca to Mg alloys containing Zn as a major alloying element is to stabilise the grain size in the solution treated condition. In Figure 1, the unmodified alloy displays an average grain size of many hundreds of microns and the modified alloy less than one hundred microns, after solution treatment at an elevated temperature of 530°C.

Figure 2 exhibits typical artificial ageing curves at 177°C for Mg-4Zn alloy, as compared to Mg-4Zn-0.35Ca alloy, and for Mg-4Zn-0.5Zr alloy compared to Mg-4Zn-0.5Zr-0. 35Ca alloy, in each case after solution treatment for a prolonged time (12h) at 530°C. Thus, Figure 2 shows comparative age strengthening curves of Mg-Zn alloys, in the unrefined and grain-refined condition, with and without trace additions of Ca. It can be seen that the Ca containing alloys exhibit faster ageing kinetics in the initial part of age strengthening reaching peak hardness in 32ks (-9h) as opposed to 350ks (-100h) for the unmodified alloy. In addition, the time to overage the Ca modified alloy is increased by an order of magnitude being greater than 15500ks (-4300h) as compared with 1650ks (-460h) for the unmodified alloy, for a 5% reduction in peak hardness.

Figure 3 shows the hardening response of Mg-4 wt. % Zn (Zr), contrasted with the response of that alloy with a respective addition (near equal atomic percentages) of Ca, Sr or Ba. In each case, the alloy was solution treated for 8 hr at 345°C, for 2 hr to 530°C and then cold water quenched, before ageing at 177°C. As shown by Figure 3, Sr has substantially the same effect as Ca, but Ba has not achieved a similar effect. Indeed, Ba exhibits a reduction in peak hardness compared with the Mg-4 wt. % Zn (Zr) alloy without Ca or Sr.

Figure 4 exhibits typical artificial ageing curves at 177°C comparing the same alloys as Figure 2. However, in this instance the curves were obtained after solution treatment for a short time (1 min) at 530°C (attained after solution treatment for 14h at 345°C and ramping over 2h to 530°C), followed by cold water quenching. Figure 4 shows similar age-strengthening curves to Figure 2. Also, Figure 4 shows that by optimising the solution treatment cycle prior to ageing, the peak hardness attained can be of the order of 10% greater for the alloy modified by the Ca addition than the values derived from the alloy that has not been modified.

Figures 5 to 7 each show a comparison of the hardening response of Mg- 4wt. % Zn (Zr) without Ca or Sr and with a respective addition of 0.01 wt. %, 0.025

wt. %, 0.088 wt. %, 0.175 wt. %, 0.35 wt. % and 0.70 wt. % of Ca. In each case the alloys were solution treated for 8h at 345°C, followed by ramping over 2 hr to a respective elevated solution treatment temperature and cold water quenching, and then ageing at 177°C. For Figures 5 to 7, the respective elevated solution treatment temperature was 530°C, 450°C and 345°C. As is evident from Figures 5 to 7, depending on the alloy composition, a positive effect on the ageing response is established for the elevated solution treatment temperatures over the range of 345°C to 530°C. However, only additions of from 0.05 wt. % Ca to 0.70 wt. % Ca retain the additional benefit of a pronounced delay in over ageing.

Figures 8 to 10 each show a comparison of the hardening response of Mg- 4wt. % Zn (Zr) without Ca or Sr and with a respective addition of 0.025 wt. %, 0.1 wt. %, 0.175 wt. %, 0.35 wt. %, 0.70 wt. % and 1.50 wt. % of Sr and having undergone the same heat treatment cycle as that for the alloys represented in Figures 5 to 7. As is evident from Figures 8 to 10, depending on the alloy composition, a positive effect on the ageing response is established for the elevated solution treatment temperatures over the range of 345°C to 530°C.

However, only additions of from 0.05 wt. % to 0.70 wt. % Sr retain the additional benefit of a pronounced delay in over ageing. It is also apparent in comparing Figures 5 to 7 with Figures 8 to 10 that single additions of Ca are most effective for solution treatment temperatures of at least 450°C whilst single additions of Sr are most effective for solution treatment temperatures up to about 500°C. It is also evident from Figures 8 and 9 that the combined addition of both Ca and Sr has a beneficial effect on the age hardening response over and above that of similar levels of single additions of either element.

Figure 11 exhibits typical artificial ageing curves at 200°C of Mg-XZn-0.1 wt. % Ca alloys (all grain refined with an addition of-0. 5 wt. % Zr), where X=3 wt. %, 4 wt. %, 5 wt. % and 6 wt. %. Each of the curves was obtained after solution treatment for 8h at 345°C and ramping over 2h to 500°C, followed by a cold water quench. It is evident that the addition of 0.1 wt. % Ca has a positive effect on the ageing response for all zinc contents between 3 wt. % and 6 wt. %. Similarly positive results have been obtained for Ca additions between 0.05 wt. % and 0.175 wt. %. Also, while less positive, similar beneficial effects are obtained with Ca additions below 0.05 wt. % down to 0.01 wt. % and above 0.175 wt. % up to 1.0 wt. %.

Figure 12 exhibits similar sets of ageing curves to Figure 11, with the addition of 0.1 wt. % Ca being replaced by an addition of 0.175 wt. % of Sr. As for Figure 11 it is evident that the addition of 0.175 wt. % Sr has a positive effect on the ageing response for all zinc contents between 3 wt. % and 6 wt. %. Again, similar beneficial effects are obtainable with Sr additions over the range of 0.01 wt. % to 1.0 wt. %.

Figure 13 provides a microstructural comparison, using TEM, in the peak aged condition at 177°C (indicated by the position arrow 1 in Figure 2) between the Mg-4Zn alloy (Figures 13 (a) and 13 (c) ) and Mg-4Zn-0.35Ca alloy (Figures 13 (b) and (d)). In each of Figure 13 (a) and (b), the c-axis of the matrix is perpendicular to the plane of the micrograph. In each of Figure 13 (c) and (d), the ai-axis of the matrix is perpendicular to the plane of the micrograph.

In the contrasting microstructures of Mg-4Zn and Mg-4Zn-0. 35Ca shown in Figure 13, the microstructural differences are manifest in the size, distribution and morphology of precipitates present within the alloy. It can be seen that the precipitates are smaller, more numerous and more uniformly distributed in the Ca modified alloy than they are in the unmodified alloy.

Figure 14 exhibits a microstructural comparison, using TEM, of the alloys of Mg-4Zn shown in Figures 14 (a) and (c) and Mg-4Zn-0.35Ca shown in Figures 14 (b) and (d). Each of the alloys was aged for extended periods at 177°C (indicated by position arrow 2 in Figure 2 at 9500ks or-2600h). Again the c-axis and ai-axis of the matrix are shown perpendicular to the plane of the micrograph, respectively. From the contrasting microstructures shown in Figure 14, it can be seen that the unmodified alloy of Figures 14 (a) and (c) has undergone significant coarsening whereas the modified alloy has undergone significantly less coarsening during the prolonged period at 177°C. Moreover, in the modified alloy two additional"families"of rod-like precipitates, which are not present in the unmodified alloy, can be seen to have stabilized. One of these sets extends along <112 0> type directions in the basal plane of the Mg matrix, while the other set extends along <1012> type directions in the {1011} pyramidal plane of the Mg matrix.

The microstructure illustrated in Figure 14 for Mg-4Zn accords with the published literature on binary Mg-Zn alloys. This accord is indicating that the

precipitates taking part in the age hardening have the same composition but occur in two different morphologies with respective habits. However, microstructural observations indicate that the presence of an addition of at least one of Ca and Sr had a dramatic influence on the nucleation of the precipitating phase (s) which results in a more refined and more homogeneous distribution of the precipitates when compared with the unmodified alloy, such as shown in Figure 14. It would appear, from a qualitative inspection of the micrographs in Figures 14 (b) and (d), that a great deal more precipitation activity has been induced on the basal plane, in the form of very thin coherent discs, in the Mg-4wt. % Zn (Zr) + 0.35 wt. % Ca alloy than has occurred in the straight binary alloy in the peak aged condition (400 ks at 177°C). The refined microstructure of the modified alloy appears to be extremely stable and resists extensive coarsening for prolonged periods at temperature (9600 ks at 177°C), which is not the case for the unmodified alloy.

Moreover, as indicated, the modified alloy shows two additional"families"of needle-like precipitates which appear to have been significantly stabilized. These two additional precipitate habits are not known to have been reported previously in the literature.

Figure 15 shows a schematic representation of the major differences in the precipitate morphology and habit with respect to the magnesium matrix between the unmodified alloys and Ca and/or Sr modified alloys, at least as far as they are understood at present. In the case of the modified alloy, the occurrence of significant numbers of refined precipitates on a variety of planes within the hexagonal crystal structure, especially on the basal plane (shown as small discs), would suggest superior mechanical properties when compared with the straight binary. In particular, combined with the long-term thermal stability of this modified microstructure, such a structure is indicative of excellent creep resistance at elevated temperatures between 150°C and 200°C.

Table 1 contains a comparison of the tensile properties, in the peak aged condition at 177°C after a suitable solution treatment, between the Mg-4Zn alloy and Mg-4Zn-0. 1Ca alloy (both grain refined with an addition of-0. 5 wt. % Zr) tested in tension at respective temperatures of room temperature, 150°C and 177°C. It can be seen that for all test conditions, the 0.2% proof strength, the ultimate tensile strength and the elongation to failure are increased for the alloy modified with the addition of Ca over those of the unmodified binary alloy.

Table 1 Mg RT 150°C 177°C Alloy 0.2% UTS Elong. 0.2% UTS Elong. 0.2% UTS Elong. Proof (MPa) % Proof (MPa) % Proof (MPa) % (MPa) (MPa) (MPa) 4Zn 129 253 11 101 139 12 100 136 14 4Zn 135 263 11 117 176 20 115 155 16 +0.1 Ca

Table 2 contains a comparison of the steady-state creep rates for Mg-4Zn, Mg-4Zn-0. 1Ca, Mg-4Zn-0.07Sr and Mg-4Zn-0.3Sr alloys (all grain refined with an addition of-0. 5 wt. % Zr), in the peak aged condition at 177°C after a suitable solution treatment. The creep rates were measured for each alloy at temperatures of 150°C and 177°C and under constant loads of 35 MPa and 70 MPa.

Additionally, Figures 16 to 19 each show a comparison of the constant load creep curves for the Mg-4Zn, Mg-4Zn-0. 1Ca, Mg-4Zn-0.07Sr and Mg-4Zn-0. 3Sr alloys at the temperatures and under the constant loads detailed in Table 2. It can be seen that additions of Ca or Sr to an alloy containing 4 wt. % Zn improve the creep resistance over that of the unmodified alloy under all the conditions tested.

Table 2 Alloy Steady-State Creep Rate (s-i) 150°C 35MPa 150°C 70MPa 177°C 35MPa 177°C 70MPa Mg-4Zn 2. 9 x 10-11. 3 x 10'3. 1 x 10'23. 0 x 10- Mg-4Zn-0. 1 Ca 2. 17 x 10-3. 6 x 10-1. 8 x 10-2. 7 x 10- Mg-4Zn-0.07Sr 2.78 x 10-7. 8 x 10-4. 4 x 10-11. 0 x 10-8 Mg-4Zn-0.3Sr 2. 48 x 10-lu 2. 8 x 10-Y 2. 6 x 10-y 16. 0 x 10-8 Figure 20 shows a comparison of the hardening response of Mg-4 wt. % Zn- 1 wt. % X (Zr), where X is one of either Ce, Nd or Y, without and with a respective addition of 0.35 wt. %, 0.175 wt. % and 0.175 wt% of Ca. In each case the alloys were solution treated for 8h at 345°C, followed by ramping over 2 hr to 500°C and cold water quenching, and then ageing at 177°C. It is evident from Figure 20 that a positive effect on the ageing response is established for all the ternary alloys containing a Ca addition. It has also been determined that combinations of the ternary alloying additions Ce, Nd and Y behave in a similar manner in alloys having a Ca addition. Similar results have been obtained for Sr additions in place of Ca.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.