High Nitrogen Steels. Preparation of Amorphous High Nitrogen Iron Alloys with Ternary Additions by Mechanical Alloying.

Abstract

In order to prepare amorphous high nitrogen iron alloys, mechanical alloying (MA) of elemental powder mixtures with Fe-9.85mass%N alloy powder of Fe(100-y)-xAyNx(at%) (A=Mn, Cr, Mo, W, V, Nb, Ta, Co or Ni; x=0-20; y=0-25) was performed in an argon atmosphere using a planetary ball mill. Effect of the ternary additives A on MA of the Fe-N materials was investigated in terms of the interaction parameter WAN of the additives A. The interaction parameters WAN represents the difference in bonding energy of A-N atomic pair (UAN) and that of Fe-N pair (UFeN) in the ternary Fe-A-N solution, i.e. WAN=UAN-UFeN, and the values of WAN are calculated using thermodynamic data at 1273 K on austenite. The value of WAN is clearly dependent on the position of the elements A in the periodic table and a similar tendency in the dependencies exists in any liquid or solid solutions of Fe-A-N. Thus, this is also expected to hold for Fe-A-N materials peocessed by MA. In the MA processing, the ternary additives A with a moderately negative WAN value of -330 to -980kJ/mol, such as Cr, Nb and Ta, markedly promote solid-state reactions, leading to formation of amorphous high nitrogen iron alloys having x=8.5-20 after 720 ks of processing by adding about 13-15 at% of these additives to Fe-N materials, in contrast to the additives Co and Ni with the positive WAN value. However, for the MA sample with the additive Ti, the formation of TiN was seen during MA, before progress of the intermixing of atomic species due to its extremely negative WAN value (-4110 kJ/mol). In addition, the additive Mn with the less negative value of WAN (-104 kJ/mol) did noteffectively act on MA of Fe-N materials. Similar effects of such additive were also obtained in MA materials with austenitic stainless steel compositions.