the alloy according to the present invention contains at least about 0 . 30 % and preferably at least about 0 . 35 % carbon . carbon contributes to the high strength and hardness capability provided by the alloy . when higher strength and hardness are desired , the alloy preferably contains at least about 0 . 33 % carbon ( e . g ., preferred a ) or at least about 0 . 40 % carbon ( e . g ., preferred b ). carbon is also beneficial to the temper resistance of this alloy . too much carbon adversely affects the toughness provided by the alloy . therefore , carbon is restricted to not more than about 0 . 55 % and better yet to not more than about 0 . 50 %. preferably , the alloy contains not more than about 0 . 45 % carbon for good toughness at higher strength and hardness levels . the inventor has found that when the alloy contains as little as 0 . 33 % carbon , the upper limit for carbon can be restricted to not more than about 0 . 45 % and the alloy can be balanced with respect to its constituents ( e . g ., preferred a ) to provide a tensile strength of at least about 295 ksi . at least about 0 . 6 %, better yet at least about 0 . 7 %, and preferably at least about 0 . 8 % manganese is present in this alloy primarily to deoxidize the alloy . it has been found that manganese also benefits the high strength provided by the alloy . if too much manganese is present , then an undesirable amount of retained austenite may result during hardening and quenching such that the high strength provided by the alloy is adversely affected . therefore , the alloy may contain up to about 1 . 75 % or 1 . 5 % manganese . otherwise , the alloy contains not more than about 1 . 3 % manganese . silicon benefits the hardenability and temper resistance of this alloy . therefore , the alloy contains at least about 0 . 9 % silicon and preferably , at least about 1 . 2 % silicon . at least about 1 . 0 % and preferably at least about 1 . 5 % silicon is present in the alloy when higher hardness and strength are needed . too much silicon adversely affects the hardness , strength , and ductility of the alloy . in order to avoid such adverse effects silicon is restricted to not more than about 2 . 8 % and preferably to not more than about 2 . 7 % in this alloy . the alloy according to this invention contains at least about 0 . 6 % chromium because chromium contributes to the good hardenability , high strength , and temper resistance provided by the alloy . preferably , the alloy contains at least about 0 . 7 % and better yet at least about 1 . 5 % chromium . more than about 2 . 5 % chromium in the alloy adversely affects the impact toughness and ductility provided by the alloy . in the high strength embodiments of this alloy chromium is restricted to not more than about 1 . 8 %. otherwise , chromium is preferably restricted to not more than about 2 . 0 % in this alloy . nickel is beneficial to the good toughness provided by the alloy according to this invention . therefore , the alloy contains at least about 2 . 7 % nickel and preferably at least about 2 . 75 % nickel . a preferred embodiment of the alloy ( e . g ., preferred a ) contains at least about 3 . 0 % nickel . when the alloy is balanced to provide higher strength , it preferably contains at least about 4 . 0 %. the benefit provided by larger amounts of nickel adversely affects the cost of the alloy without providing a significant advantage . in order to limit the upside cost of the alloy , the amount of nickel is restricted to not more than about 7 . 0 %. thus , for the highest strength embodiment of the alloy ( e . g ., preferred b ), up to about 5 . 0 % nickel can be present . in lower strength embodiments ( e . g ., preferred a ) the alloy contains not more than about 4 . 5 % nickel . molybdenum is a carbide former that is beneficial to the temper resistance provided by this alloy . the presence of molybdenum boosts the tempering temperature of the alloy such that a secondary hardening effect is achieved at about 500 ° f . molybdenum also contributes to the strength and fracture toughness provided by the alloy . the benefits provided by molybdenum are realized when the alloy contains at least about 0 . 25 % molybdenum , better yet , at least about 0 . 4 %, and preferably at least about 0 . 5 % molybdenum . for higher strength , the alloy contains at least about 0 . 7 % molybdenum . like nickel , molybdenum does not provide an increasing advantage in properties relative to the significant cost increase of adding larger amounts of molybdenum . for that reason , the alloy contains up to about 1 . 3 % molybdenum , better yet not more than about 1 . 1 % molybdenum , preferably not more than about 0 . 90 % molybdenum in the higher strength forms of the alloy ( preferred a and preferred b ). tungsten may be substituted for some or all of the molybdenum in this alloy . when present , tungsten is substituted for molybdenum on a 2 : 1 basis . this alloy preferably contains at least about 0 . 30 % copper which contributes to the hardenability and impact toughness of the alloy . when higher strength is desired , the alloy contains at least about 0 . 35 % copper . too much copper can result in precipitation of an undesirable amount of free copper in the alloy matrix and adversely affect the fracture toughness of the alloy . therefore , not more than about 1 . 25 % and preferably not more than about 1 . 2 % copper is present in this alloy . vanadium contributes to the high strength and good hardenability provided by this alloy . vanadium is also a carbide former and promotes the formation of carbides that help provide grain refinement in the alloy and that benefit the temper resistance and secondary hardening of the alloy . for those reasons , the alloy preferably contains at least about 0 . 10 % and preferably at least about 0 . 25 % vanadium . too much vanadium adversely affects the strength of the alloy because of the formation of larger amounts of carbides in the alloy which depletes carbon from the alloy matrix material . accordingly , the alloy may contain up to about 1 . 0 % vanadium , but preferably contains not more than about 0 . 40 % vanadium . niobium can be substituted for some or all of the vanadium in this alloy because like vanadium , niobium combines with carbon to form m 4 c 3 carbides that benefit the temper resistance and hardenability of the alloy . when present , niobium is substituted for vanadium on 1 . 8 : 1 basis . this alloy may also contain a small amount of calcium up to about 0 . 005 % retained from additions during melting of the alloy to help remove sulfur and thereby benefit the fracture toughness provided by the alloy . preferably , the alloy contains not more than about 0 . 002 % or 0 . 001 % calcium . silicon , copper , vanadium , and when present , niobium are preferably balanced within their above - described weight percent ranges to benefit the novel combination of strength and toughness that characterize this alloy . more specifically , the ratio (% si +% cu )/(% v +( 5 / 9 )×% nb ) is about 2 to 34 . the ratio is preferably about 6 - 12 for strength levels below about 290 ksi . for strength levels of 290 ksi and above , the alloy is balanced such that the ratio is about 14 . 5 up to about 34 . it is believed that when the amounts of silicon , copper , and vanadium present in the alloy are balanced in accordance with the ratio , the grain boundaries of the alloy are strengthened by preventing brittle phases and tramp elements from forming on the grain boundaries . the alloy according to this invention contains a small amount of magnesium , yttrium , or a combination thereof . the magnesium and / or yttrium is added during primary melting to deoxidize the steel alloy . magnesium and yttrium also benefit the strength and toughness of the new steel by aiding in grain refinement of the alloy during processing . magnesium is added in sufficient quantities to result in a retained amount of about 0 . 0001 to 0 . 01 %, preferably about 0 . 0001 to 0 . 006 %. yttrium is added in an amount sufficient to yield a retained amount of about 0 . 001 to 0 . 025 %, preferably about 0 . 002 - 0 . 020 %. the balance of the alloy is essentially iron and the usual impurities found in commercial grades of similar alloys and steels . in this regard , the alloy preferably contains not more than about 0 . 01 %, better yet , not more than about 0 . 005 % phosphorus and not more than about 0 . 001 %, better yet not more than about 0 . 0005 % sulfur . the alloy preferably contains not more than about 0 . 01 % cobalt . titanium may be present at a residual level of up to about 0 . 01 % from deoxidation additions during melting and is preferably restricted to not more than about 0 . 005 %. up to about 0 . 015 % aluminum may also be present in the alloy from deoxidation additions during melting . the alloys according to preferred compositions a and b are balanced to provide very high strength and toughness in the hardened and tempered condition . in this regard , the preferred a composition is balanced to provide a tensile strength of at least about 295 ksi in combination with good toughness as indicated by a charpy v - notch impact strength of at least about 16 ft - lbs and a k ic fracture toughness of at least about 70 ksi √ in . in addition , the preferred b composition is balanced to provide a tensile strength of at least about 310 ksi in combination with a k ic fracture toughness of at least about 50 ksi √ in for applications that require higher strength and good toughness . no special melting techniques are needed to make the alloy according to this invention . primary melting of the alloy is preferably accomplished with vacuum induction melting ( vim ). when desired , as for critical applications , the alloy can be refined using vacuum arc remelting ( var ). primary melting may also be performed by arc melting in air ( arc ) if desired . after arc melting , the alloy may be refined by electroslag remelting ( esr ) or var . the alloy of this invention is preferably hot worked from a temperature of up to about 2100 ° f ., preferably at about 1800 ° f ., to form various intermediate product forms such as billets and bars . the alloy is preferably heat treated by austenitizing at about 1585 ° f . to about 1735 ° f . for about 1 - 2 hours . the alloy is then air cooled or oil quenched from the austenitizing temperature . when desired , the alloy can be vacuum heat treated and gas quenched . the alloy is preferably deep chilled to either − 100 ° f . or − 320 ° f . for about 1 - 8 hours and then warmed in air . the alloy is preferably tempered at about 500 ° f . for about 2 - 3 hours and then air cooled . the alloy may be tempered at up to 600 ° f . when an optimum combination of strength and toughness is not required . the alloy of the present invention is useful in a wide range of applications . the very high strength and good fracture toughness of the alloy makes it useful for machine tool components and also in structural components for aircraft , including landing gear . the alloy of this invention is also useful for automotive components including , but not limited to , structural members , drive shafts , springs , and crankshafts . it is believed that the alloy also has utility in armor plate , sheet , and bars . in order to demonstrate the novel combination of strength and toughness provided by the alloy according to this invention , six 35 - 1b . heats having the weight percent compositions set forth in table 1 below were vacuum induction melted and cast into 4 - inch square ingots . prior to casting , the heats were desulfurized with calcium by means of a 0 . 025 weight percent addition of nickel - calcium . the balance of each heat was iron and usual impurities . heats 1 to 4 are embodiments of the alloy according to the present invention . heats a and b are comparative heats . heats 1 to 4 differ from heats a and b with respect to the retained amounts magnesium . the 4 - inch square ingots of each of heats 1 - 4 , a , and b were homogenized at 2300 ° f . for 6 hours and then hot forged from a starting temperature of 1800 ° f . to 2¼ - inch square billet . a 12 - inch long piece was cut from the x - end of each billet and then hot forged from 1800 ° f . to 1½ - inch square bar . the 1½ - inch bars were cut into three equal - length pieces . each of the three pieces was then forged from 1800 ° f . to ⅝ - inch square bar . the ⅝ - inch bars were cooled in air to room temperature . thereafter the bars were annealed at 1250 ° f . for 8 hours and then air cooled to room temperature . duplicate , standard longitudinal test samples for tensile , toughness , and fracture toughness testing were cut from the annealed ⅝ - inch bars and machined to finish size . a first set of the samples were heated in vacuum at 1685 ° f . for 1 . 5 hours and then quenched with a positive pressure of inert gas . ( heat treatment a .) a second set of the samples were heated in vacuum at 1735 ° f . for 2 hours and then quenched with a positive pressure of the inert gas . ( heat treatment b .) after quenching , the samples were chilled at − 100 ° f . for 8 hours and then warmed in air to room temperature . following the cold treatment , the samples were tempered by heating at 500 ° f . for 2 hours and then cooled in air to room temperature . set forth in tables 2a and 2b are the results of room temperature mechanical testing of the duplicate samples from each heat including the 0 . 2 % offset yield strength ( y . s .) and the ultimate tensile strength ( u . t . s .) in ksi , the percent elongation (% el . ), the percent reduction in area (% r . a . ), the charpy v - notch impact energy ( cvn ) in foot - pounds ( ft .- lbs . ), the rising step load fracture toughness ( k ic ) in ksi √ in , and the rockwell c - scale hardness ( hrc ). the tested samples were also metallographically examined for grain size and the astm grain size number ( grain size ) for each heat is also shown in table 2 . table 2a contains the results for the samples given heat treatment a and table 2b contains the results for the samples given heat treatment b . set forth in table 3 are the weight percent compositions of four additional 35 - lb . heats that were vacuum induction melted and cast in the same manner as the heats described in example 1 above . the balance of each heat was iron and usual impurities . heats 5 to 8 are embodiments of the alloy according to the present invention . heat a is the comparative heat . heats 5 - 6 differ from heat a with respect to the retained amounts of yttrium . heats 5 - 8 and a were processed and tested similarly to the heats in example 1 . set forth in tables 4a and 4b are the results of room temperature mechanical testing of the duplicate samples from each heat including the 0 . 2 % offset yield strength ( y . s .) and the ultimate tensile strength ( u . t . s . ), the percent elongation (% el . ), the percent reduction in area (% r . a . ), the charpy v - notch impact energy ( cvn ) in foot - pounds ( ft .- lbs . ), the rising step load fracture toughness ( k ic ) in ksi √ in , the rockwell c - scale hardness ( hrc ). the tested samples were also metallographically examined for grain size and the astm grain size number for each heat is also shown in table 3 . table 4a contains the results for the samples given heat treatment a and table 4b contains the results for the samples given heat treatment b . in order to demonstrate the novel combination of strength and toughness provided by the alloy according to this invention , six additional 35 - lb . heats having the weight percent compositions set forth in table 5 below were vacuum induction melted and cast into 4 - inch square ingots . the heats were processed similar to heats 1 - 4 , a , and b during melting . the balance of each heat was iron and usual impurities . heats 9 to 13 are embodiments of the alloy according to the present invention . heat c is a comparative heat . heats 9 - 13 differ from heat c with respect to the retained amounts magnesium . heats 5 - 8 and c were processed and tested similarly to the heats in example 1 . set forth in tables 6a and 6b are the results of room temperature mechanical testing of the duplicate samples from each heat including the 0 . 2 % offset yield strength ( y . s .) and the ultimate tensile strength ( u . t . s . ), the percent elongation (% el . ), the percent reduction in area (% r . a . ), the charpy v - notch impact energy ( cvn ) in foot - pounds ( ft .- lbs . ), the rising step load fracture toughness ( k ic ) in ksi √ in , the rockwell c - scale hardness ( hrc ). the tested samples were also metallographically examined for grain size and the astm grain size number for each heat is also shown in tables 6a and 6b . table 6a contains the results for the samples given heat treatment a and table 6b contains the results for the samples given heat treatment b . 1 tensile testing performed on only one specimen for this heat and heat treatment . set forth in table 7 are the weight percent compositions of four additional 35 - lb . heats that were vacuum induction melted and cast in the same manner as the heats described in example 1 above . the balance of each heat was iron and usual impurities . heats 14 to 17 are embodiments of the alloy according to the present invention . heat c is the comparative heat . heats 14 - 17 differ from heat c with respect to the retained amounts of yttrium . heats 14 - 17 and c were processed and tested similarly to the heats in example 1 . set forth in tables 8a and 8b are the results of room temperature mechanical testing of the duplicate samples from each heat including the 0 . 2 % offset yield strength ( y . s .) and the ultimate tensile strength ( u . t . s . ), the percent elongation (% el . ), the percent reduction in area (% r . a . ), the charpy v - notch impact energy ( cvn ) in foot - pounds ( ft .- lbs . ), the rising step load fracture toughness ( k ic ) in ksi √ in , the rockwell c - scale hardness ( hrc ). the tested samples were also metallographically examined for grain size and the astm grain size number for each heat is also shown in tables 8a and 8b . table 8a contains the results for the samples given heat treatment a and table 8b contains the results for the samples given heat treatment b . the data presented in examples 1 to 4 show that the heats of the alloy according to the present invention provide significantly better combinations of strength and toughness than the comparative heats representing the known alloys . the terms and expressions which are employed herein are used as terms of description and not of limitation . there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof . it is recognized that various modifications are possible within the invention described and claimed herein .